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1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4Overview of the Linux Virtual File System
5=========================================
6
7Original author: Richard Gooch <rgooch@atnf.csiro.au>
8
9- Copyright (C) 1999 Richard Gooch
10- Copyright (C) 2005 Pekka Enberg
11
12
13Introduction
14============
15
16The Virtual File System (also known as the Virtual Filesystem Switch) is
17the software layer in the kernel that provides the filesystem interface
18to userspace programs. It also provides an abstraction within the
19kernel which allows different filesystem implementations to coexist.
20
21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22are called from a process context. Filesystem locking is described in
23the document Documentation/filesystems/locking.rst.
24
25
26Directory Entry Cache (dcache)
27------------------------------
28
29The VFS implements the open(2), stat(2), chmod(2), and similar system
30calls. The pathname argument that is passed to them is used by the VFS
31to search through the directory entry cache (also known as the dentry
32cache or dcache). This provides a very fast look-up mechanism to
33translate a pathname (filename) into a specific dentry. Dentries live
34in RAM and are never saved to disc: they exist only for performance.
35
36The dentry cache is meant to be a view into your entire filespace. As
37most computers cannot fit all dentries in the RAM at the same time, some
38bits of the cache are missing. In order to resolve your pathname into a
39dentry, the VFS may have to resort to creating dentries along the way,
40and then loading the inode. This is done by looking up the inode.
41
42
43The Inode Object
44----------------
45
46An individual dentry usually has a pointer to an inode. Inodes are
47filesystem objects such as regular files, directories, FIFOs and other
48beasts. They live either on the disc (for block device filesystems) or
49in the memory (for pseudo filesystems). Inodes that live on the disc
50are copied into the memory when required and changes to the inode are
51written back to disc. A single inode can be pointed to by multiple
52dentries (hard links, for example, do this).
53
54To look up an inode requires that the VFS calls the lookup() method of
55the parent directory inode. This method is installed by the specific
56filesystem implementation that the inode lives in. Once the VFS has the
57required dentry (and hence the inode), we can do all those boring things
58like open(2) the file, or stat(2) it to peek at the inode data. The
59stat(2) operation is fairly simple: once the VFS has the dentry, it
60peeks at the inode data and passes some of it back to userspace.
61
62
63The File Object
64---------------
65
66Opening a file requires another operation: allocation of a file
67structure (this is the kernel-side implementation of file descriptors).
68The freshly allocated file structure is initialized with a pointer to
69the dentry and a set of file operation member functions. These are
70taken from the inode data. The open() file method is then called so the
71specific filesystem implementation can do its work. You can see that
72this is another switch performed by the VFS. The file structure is
73placed into the file descriptor table for the process.
74
75Reading, writing and closing files (and other assorted VFS operations)
76is done by using the userspace file descriptor to grab the appropriate
77file structure, and then calling the required file structure method to
78do whatever is required. For as long as the file is open, it keeps the
79dentry in use, which in turn means that the VFS inode is still in use.
80
81
82Registering and Mounting a Filesystem
83=====================================
84
85To register and unregister a filesystem, use the following API
86functions:
87
88.. code-block:: c
89
90 #include <linux/fs.h>
91
92 extern int register_filesystem(struct file_system_type *);
93 extern int unregister_filesystem(struct file_system_type *);
94
95The passed struct file_system_type describes your filesystem. When a
96request is made to mount a filesystem onto a directory in your
97namespace, the VFS will call the appropriate mount() method for the
98specific filesystem. New vfsmount referring to the tree returned by
99->mount() will be attached to the mountpoint, so that when pathname
100resolution reaches the mountpoint it will jump into the root of that
101vfsmount.
102
103You can see all filesystems that are registered to the kernel in the
104file /proc/filesystems.
105
106
107struct file_system_type
108-----------------------
109
110This describes the filesystem. As of kernel 2.6.39, the following
111members are defined:
112
113.. code-block:: c
114
115 struct file_system_operations {
116 const char *name;
117 int fs_flags;
118 struct dentry *(*mount) (struct file_system_type *, int,
119 const char *, void *);
120 void (*kill_sb) (struct super_block *);
121 struct module *owner;
122 struct file_system_type * next;
123 struct list_head fs_supers;
124 struct lock_class_key s_lock_key;
125 struct lock_class_key s_umount_key;
126 };
127
128``name``
129 the name of the filesystem type, such as "ext2", "iso9660",
130 "msdos" and so on
131
132``fs_flags``
133 various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
134
135``mount``
136 the method to call when a new instance of this filesystem should
137 be mounted
138
139``kill_sb``
140 the method to call when an instance of this filesystem should be
141 shut down
142
143
144``owner``
145 for internal VFS use: you should initialize this to THIS_MODULE
146 in most cases.
147
148``next``
149 for internal VFS use: you should initialize this to NULL
150
151 s_lock_key, s_umount_key: lockdep-specific
152
153The mount() method has the following arguments:
154
155``struct file_system_type *fs_type``
156 describes the filesystem, partly initialized by the specific
157 filesystem code
158
159``int flags``
160 mount flags
161
162``const char *dev_name``
163 the device name we are mounting.
164
165``void *data``
166 arbitrary mount options, usually comes as an ASCII string (see
167 "Mount Options" section)
168
169The mount() method must return the root dentry of the tree requested by
170caller. An active reference to its superblock must be grabbed and the
171superblock must be locked. On failure it should return ERR_PTR(error).
172
173The arguments match those of mount(2) and their interpretation depends
174on filesystem type. E.g. for block filesystems, dev_name is interpreted
175as block device name, that device is opened and if it contains a
176suitable filesystem image the method creates and initializes struct
177super_block accordingly, returning its root dentry to caller.
178
179->mount() may choose to return a subtree of existing filesystem - it
180doesn't have to create a new one. The main result from the caller's
181point of view is a reference to dentry at the root of (sub)tree to be
182attached; creation of new superblock is a common side effect.
183
184The most interesting member of the superblock structure that the mount()
185method fills in is the "s_op" field. This is a pointer to a "struct
186super_operations" which describes the next level of the filesystem
187implementation.
188
189Usually, a filesystem uses one of the generic mount() implementations
190and provides a fill_super() callback instead. The generic variants are:
191
192``mount_bdev``
193 mount a filesystem residing on a block device
194
195``mount_nodev``
196 mount a filesystem that is not backed by a device
197
198``mount_single``
199 mount a filesystem which shares the instance between all mounts
200
201A fill_super() callback implementation has the following arguments:
202
203``struct super_block *sb``
204 the superblock structure. The callback must initialize this
205 properly.
206
207``void *data``
208 arbitrary mount options, usually comes as an ASCII string (see
209 "Mount Options" section)
210
211``int silent``
212 whether or not to be silent on error
213
214
215The Superblock Object
216=====================
217
218A superblock object represents a mounted filesystem.
219
220
221struct super_operations
222-----------------------
223
224This describes how the VFS can manipulate the superblock of your
225filesystem. As of kernel 2.6.22, the following members are defined:
226
227.. code-block:: c
228
229 struct super_operations {
230 struct inode *(*alloc_inode)(struct super_block *sb);
231 void (*destroy_inode)(struct inode *);
232
233 void (*dirty_inode) (struct inode *, int flags);
234 int (*write_inode) (struct inode *, int);
235 void (*drop_inode) (struct inode *);
236 void (*delete_inode) (struct inode *);
237 void (*put_super) (struct super_block *);
238 int (*sync_fs)(struct super_block *sb, int wait);
239 int (*freeze_fs) (struct super_block *);
240 int (*unfreeze_fs) (struct super_block *);
241 int (*statfs) (struct dentry *, struct kstatfs *);
242 int (*remount_fs) (struct super_block *, int *, char *);
243 void (*clear_inode) (struct inode *);
244 void (*umount_begin) (struct super_block *);
245
246 int (*show_options)(struct seq_file *, struct dentry *);
247
248 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
249 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
250 int (*nr_cached_objects)(struct super_block *);
251 void (*free_cached_objects)(struct super_block *, int);
252 };
253
254All methods are called without any locks being held, unless otherwise
255noted. This means that most methods can block safely. All methods are
256only called from a process context (i.e. not from an interrupt handler
257or bottom half).
258
259``alloc_inode``
260 this method is called by alloc_inode() to allocate memory for
261 struct inode and initialize it. If this function is not
262 defined, a simple 'struct inode' is allocated. Normally
263 alloc_inode will be used to allocate a larger structure which
264 contains a 'struct inode' embedded within it.
265
266``destroy_inode``
267 this method is called by destroy_inode() to release resources
268 allocated for struct inode. It is only required if
269 ->alloc_inode was defined and simply undoes anything done by
270 ->alloc_inode.
271
272``dirty_inode``
273 this method is called by the VFS to mark an inode dirty.
274
275``write_inode``
276 this method is called when the VFS needs to write an inode to
277 disc. The second parameter indicates whether the write should
278 be synchronous or not, not all filesystems check this flag.
279
280``drop_inode``
281 called when the last access to the inode is dropped, with the
282 inode->i_lock spinlock held.
283
284 This method should be either NULL (normal UNIX filesystem
285 semantics) or "generic_delete_inode" (for filesystems that do
286 not want to cache inodes - causing "delete_inode" to always be
287 called regardless of the value of i_nlink)
288
289 The "generic_delete_inode()" behavior is equivalent to the old
290 practice of using "force_delete" in the put_inode() case, but
291 does not have the races that the "force_delete()" approach had.
292
293``delete_inode``
294 called when the VFS wants to delete an inode
295
296``put_super``
297 called when the VFS wishes to free the superblock
298 (i.e. unmount). This is called with the superblock lock held
299
300``sync_fs``
301 called when VFS is writing out all dirty data associated with a
302 superblock. The second parameter indicates whether the method
303 should wait until the write out has been completed. Optional.
304
305``freeze_fs``
306 called when VFS is locking a filesystem and forcing it into a
307 consistent state. This method is currently used by the Logical
308 Volume Manager (LVM).
309
310``unfreeze_fs``
311 called when VFS is unlocking a filesystem and making it writable
312 again.
313
314``statfs``
315 called when the VFS needs to get filesystem statistics.
316
317``remount_fs``
318 called when the filesystem is remounted. This is called with
319 the kernel lock held
320
321``clear_inode``
322 called then the VFS clears the inode. Optional
323
324``umount_begin``
325 called when the VFS is unmounting a filesystem.
326
327``show_options``
328 called by the VFS to show mount options for /proc/<pid>/mounts.
329 (see "Mount Options" section)
330
331``quota_read``
332 called by the VFS to read from filesystem quota file.
333
334``quota_write``
335 called by the VFS to write to filesystem quota file.
336
337``nr_cached_objects``
338 called by the sb cache shrinking function for the filesystem to
339 return the number of freeable cached objects it contains.
340 Optional.
341
342``free_cache_objects``
343 called by the sb cache shrinking function for the filesystem to
344 scan the number of objects indicated to try to free them.
345 Optional, but any filesystem implementing this method needs to
346 also implement ->nr_cached_objects for it to be called
347 correctly.
348
349 We can't do anything with any errors that the filesystem might
350 encountered, hence the void return type. This will never be
351 called if the VM is trying to reclaim under GFP_NOFS conditions,
352 hence this method does not need to handle that situation itself.
353
354 Implementations must include conditional reschedule calls inside
355 any scanning loop that is done. This allows the VFS to
356 determine appropriate scan batch sizes without having to worry
357 about whether implementations will cause holdoff problems due to
358 large scan batch sizes.
359
360Whoever sets up the inode is responsible for filling in the "i_op"
361field. This is a pointer to a "struct inode_operations" which describes
362the methods that can be performed on individual inodes.
363
364
365struct xattr_handlers
366---------------------
367
368On filesystems that support extended attributes (xattrs), the s_xattr
369superblock field points to a NULL-terminated array of xattr handlers.
370Extended attributes are name:value pairs.
371
372``name``
373 Indicates that the handler matches attributes with the specified
374 name (such as "system.posix_acl_access"); the prefix field must
375 be NULL.
376
377``prefix``
378 Indicates that the handler matches all attributes with the
379 specified name prefix (such as "user."); the name field must be
380 NULL.
381
382``list``
383 Determine if attributes matching this xattr handler should be
384 listed for a particular dentry. Used by some listxattr
385 implementations like generic_listxattr.
386
387``get``
388 Called by the VFS to get the value of a particular extended
389 attribute. This method is called by the getxattr(2) system
390 call.
391
392``set``
393 Called by the VFS to set the value of a particular extended
394 attribute. When the new value is NULL, called to remove a
395 particular extended attribute. This method is called by the the
396 setxattr(2) and removexattr(2) system calls.
397
398When none of the xattr handlers of a filesystem match the specified
399attribute name or when a filesystem doesn't support extended attributes,
400the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
401
402
403The Inode Object
404================
405
406An inode object represents an object within the filesystem.
407
408
409struct inode_operations
410-----------------------
411
412This describes how the VFS can manipulate an inode in your filesystem.
413As of kernel 2.6.22, the following members are defined:
414
415.. code-block:: c
416
417 struct inode_operations {
418 int (*create) (struct inode *,struct dentry *, umode_t, bool);
419 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
420 int (*link) (struct dentry *,struct inode *,struct dentry *);
421 int (*unlink) (struct inode *,struct dentry *);
422 int (*symlink) (struct inode *,struct dentry *,const char *);
423 int (*mkdir) (struct inode *,struct dentry *,umode_t);
424 int (*rmdir) (struct inode *,struct dentry *);
425 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
426 int (*rename) (struct inode *, struct dentry *,
427 struct inode *, struct dentry *, unsigned int);
428 int (*readlink) (struct dentry *, char __user *,int);
429 const char *(*get_link) (struct dentry *, struct inode *,
430 struct delayed_call *);
431 int (*permission) (struct inode *, int);
432 int (*get_acl)(struct inode *, int);
433 int (*setattr) (struct dentry *, struct iattr *);
434 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
435 ssize_t (*listxattr) (struct dentry *, char *, size_t);
436 void (*update_time)(struct inode *, struct timespec *, int);
437 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
438 unsigned open_flag, umode_t create_mode);
439 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
440 };
441
442Again, all methods are called without any locks being held, unless
443otherwise noted.
444
445``create``
446 called by the open(2) and creat(2) system calls. Only required
447 if you want to support regular files. The dentry you get should
448 not have an inode (i.e. it should be a negative dentry). Here
449 you will probably call d_instantiate() with the dentry and the
450 newly created inode
451
452``lookup``
453 called when the VFS needs to look up an inode in a parent
454 directory. The name to look for is found in the dentry. This
455 method must call d_add() to insert the found inode into the
456 dentry. The "i_count" field in the inode structure should be
457 incremented. If the named inode does not exist a NULL inode
458 should be inserted into the dentry (this is called a negative
459 dentry). Returning an error code from this routine must only be
460 done on a real error, otherwise creating inodes with system
461 calls like create(2), mknod(2), mkdir(2) and so on will fail.
462 If you wish to overload the dentry methods then you should
463 initialise the "d_dop" field in the dentry; this is a pointer to
464 a struct "dentry_operations". This method is called with the
465 directory inode semaphore held
466
467``link``
468 called by the link(2) system call. Only required if you want to
469 support hard links. You will probably need to call
470 d_instantiate() just as you would in the create() method
471
472``unlink``
473 called by the unlink(2) system call. Only required if you want
474 to support deleting inodes
475
476``symlink``
477 called by the symlink(2) system call. Only required if you want
478 to support symlinks. You will probably need to call
479 d_instantiate() just as you would in the create() method
480
481``mkdir``
482 called by the mkdir(2) system call. Only required if you want
483 to support creating subdirectories. You will probably need to
484 call d_instantiate() just as you would in the create() method
485
486``rmdir``
487 called by the rmdir(2) system call. Only required if you want
488 to support deleting subdirectories
489
490``mknod``
491 called by the mknod(2) system call to create a device (char,
492 block) inode or a named pipe (FIFO) or socket. Only required if
493 you want to support creating these types of inodes. You will
494 probably need to call d_instantiate() just as you would in the
495 create() method
496
497``rename``
498 called by the rename(2) system call to rename the object to have
499 the parent and name given by the second inode and dentry.
500
501 The filesystem must return -EINVAL for any unsupported or
502 unknown flags. Currently the following flags are implemented:
503 (1) RENAME_NOREPLACE: this flag indicates that if the target of
504 the rename exists the rename should fail with -EEXIST instead of
505 replacing the target. The VFS already checks for existence, so
506 for local filesystems the RENAME_NOREPLACE implementation is
507 equivalent to plain rename.
508 (2) RENAME_EXCHANGE: exchange source and target. Both must
509 exist; this is checked by the VFS. Unlike plain rename, source
510 and target may be of different type.
511
512``get_link``
513 called by the VFS to follow a symbolic link to the inode it
514 points to. Only required if you want to support symbolic links.
515 This method returns the symlink body to traverse (and possibly
516 resets the current position with nd_jump_link()). If the body
517 won't go away until the inode is gone, nothing else is needed;
518 if it needs to be otherwise pinned, arrange for its release by
519 having get_link(..., ..., done) do set_delayed_call(done,
520 destructor, argument). In that case destructor(argument) will
521 be called once VFS is done with the body you've returned. May
522 be called in RCU mode; that is indicated by NULL dentry
523 argument. If request can't be handled without leaving RCU mode,
524 have it return ERR_PTR(-ECHILD).
525
526 If the filesystem stores the symlink target in ->i_link, the
527 VFS may use it directly without calling ->get_link(); however,
528 ->get_link() must still be provided. ->i_link must not be
529 freed until after an RCU grace period. Writing to ->i_link
530 post-iget() time requires a 'release' memory barrier.
531
532``readlink``
533 this is now just an override for use by readlink(2) for the
534 cases when ->get_link uses nd_jump_link() or object is not in
535 fact a symlink. Normally filesystems should only implement
536 ->get_link for symlinks and readlink(2) will automatically use
537 that.
538
539``permission``
540 called by the VFS to check for access rights on a POSIX-like
541 filesystem.
542
543 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
544 rcu-walk mode, the filesystem must check the permission without
545 blocking or storing to the inode.
546
547 If a situation is encountered that rcu-walk cannot handle,
548 return
549 -ECHILD and it will be called again in ref-walk mode.
550
551``setattr``
552 called by the VFS to set attributes for a file. This method is
553 called by chmod(2) and related system calls.
554
555``getattr``
556 called by the VFS to get attributes of a file. This method is
557 called by stat(2) and related system calls.
558
559``listxattr``
560 called by the VFS to list all extended attributes for a given
561 file. This method is called by the listxattr(2) system call.
562
563``update_time``
564 called by the VFS to update a specific time or the i_version of
565 an inode. If this is not defined the VFS will update the inode
566 itself and call mark_inode_dirty_sync.
567
568``atomic_open``
569 called on the last component of an open. Using this optional
570 method the filesystem can look up, possibly create and open the
571 file in one atomic operation. If it wants to leave actual
572 opening to the caller (e.g. if the file turned out to be a
573 symlink, device, or just something filesystem won't do atomic
574 open for), it may signal this by returning finish_no_open(file,
575 dentry). This method is only called if the last component is
576 negative or needs lookup. Cached positive dentries are still
577 handled by f_op->open(). If the file was created, FMODE_CREATED
578 flag should be set in file->f_mode. In case of O_EXCL the
579 method must only succeed if the file didn't exist and hence
580 FMODE_CREATED shall always be set on success.
581
582``tmpfile``
583 called in the end of O_TMPFILE open(). Optional, equivalent to
584 atomically creating, opening and unlinking a file in given
585 directory.
586
587
588The Address Space Object
589========================
590
591The address space object is used to group and manage pages in the page
592cache. It can be used to keep track of the pages in a file (or anything
593else) and also track the mapping of sections of the file into process
594address spaces.
595
596There are a number of distinct yet related services that an
597address-space can provide. These include communicating memory pressure,
598page lookup by address, and keeping track of pages tagged as Dirty or
599Writeback.
600
601The first can be used independently to the others. The VM can try to
602either write dirty pages in order to clean them, or release clean pages
603in order to reuse them. To do this it can call the ->writepage method
604on dirty pages, and ->releasepage on clean pages with PagePrivate set.
605Clean pages without PagePrivate and with no external references will be
606released without notice being given to the address_space.
607
608To achieve this functionality, pages need to be placed on an LRU with
609lru_cache_add and mark_page_active needs to be called whenever the page
610is used.
611
612Pages are normally kept in a radix tree index by ->index. This tree
613maintains information about the PG_Dirty and PG_Writeback status of each
614page, so that pages with either of these flags can be found quickly.
615
616The Dirty tag is primarily used by mpage_writepages - the default
617->writepages method. It uses the tag to find dirty pages to call
618->writepage on. If mpage_writepages is not used (i.e. the address
619provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
620unused. write_inode_now and sync_inode do use it (through
621__sync_single_inode) to check if ->writepages has been successful in
622writing out the whole address_space.
623
624The Writeback tag is used by filemap*wait* and sync_page* functions, via
625filemap_fdatawait_range, to wait for all writeback to complete.
626
627An address_space handler may attach extra information to a page,
628typically using the 'private' field in the 'struct page'. If such
629information is attached, the PG_Private flag should be set. This will
630cause various VM routines to make extra calls into the address_space
631handler to deal with that data.
632
633An address space acts as an intermediate between storage and
634application. Data is read into the address space a whole page at a
635time, and provided to the application either by copying of the page, or
636by memory-mapping the page. Data is written into the address space by
637the application, and then written-back to storage typically in whole
638pages, however the address_space has finer control of write sizes.
639
640The read process essentially only requires 'readpage'. The write
641process is more complicated and uses write_begin/write_end or
642set_page_dirty to write data into the address_space, and writepage and
643writepages to writeback data to storage.
644
645Adding and removing pages to/from an address_space is protected by the
646inode's i_mutex.
647
648When data is written to a page, the PG_Dirty flag should be set. It
649typically remains set until writepage asks for it to be written. This
650should clear PG_Dirty and set PG_Writeback. It can be actually written
651at any point after PG_Dirty is clear. Once it is known to be safe,
652PG_Writeback is cleared.
653
654Writeback makes use of a writeback_control structure to direct the
655operations. This gives the the writepage and writepages operations some
656information about the nature of and reason for the writeback request,
657and the constraints under which it is being done. It is also used to
658return information back to the caller about the result of a writepage or
659writepages request.
660
661
662Handling errors during writeback
663--------------------------------
664
665Most applications that do buffered I/O will periodically call a file
666synchronization call (fsync, fdatasync, msync or sync_file_range) to
667ensure that data written has made it to the backing store. When there
668is an error during writeback, they expect that error to be reported when
669a file sync request is made. After an error has been reported on one
670request, subsequent requests on the same file descriptor should return
6710, unless further writeback errors have occurred since the previous file
672syncronization.
673
674Ideally, the kernel would report errors only on file descriptions on
675which writes were done that subsequently failed to be written back. The
676generic pagecache infrastructure does not track the file descriptions
677that have dirtied each individual page however, so determining which
678file descriptors should get back an error is not possible.
679
680Instead, the generic writeback error tracking infrastructure in the
681kernel settles for reporting errors to fsync on all file descriptions
682that were open at the time that the error occurred. In a situation with
683multiple writers, all of them will get back an error on a subsequent
684fsync, even if all of the writes done through that particular file
685descriptor succeeded (or even if there were no writes on that file
686descriptor at all).
687
688Filesystems that wish to use this infrastructure should call
689mapping_set_error to record the error in the address_space when it
690occurs. Then, after writing back data from the pagecache in their
691file->fsync operation, they should call file_check_and_advance_wb_err to
692ensure that the struct file's error cursor has advanced to the correct
693point in the stream of errors emitted by the backing device(s).
694
695
696struct address_space_operations
697-------------------------------
698
699This describes how the VFS can manipulate mapping of a file to page
700cache in your filesystem. The following members are defined:
701
702.. code-block:: c
703
704 struct address_space_operations {
705 int (*writepage)(struct page *page, struct writeback_control *wbc);
706 int (*readpage)(struct file *, struct page *);
707 int (*writepages)(struct address_space *, struct writeback_control *);
708 int (*set_page_dirty)(struct page *page);
709 int (*readpages)(struct file *filp, struct address_space *mapping,
710 struct list_head *pages, unsigned nr_pages);
711 int (*write_begin)(struct file *, struct address_space *mapping,
712 loff_t pos, unsigned len, unsigned flags,
713 struct page **pagep, void **fsdata);
714 int (*write_end)(struct file *, struct address_space *mapping,
715 loff_t pos, unsigned len, unsigned copied,
716 struct page *page, void *fsdata);
717 sector_t (*bmap)(struct address_space *, sector_t);
718 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
719 int (*releasepage) (struct page *, int);
720 void (*freepage)(struct page *);
721 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
722 /* isolate a page for migration */
723 bool (*isolate_page) (struct page *, isolate_mode_t);
724 /* migrate the contents of a page to the specified target */
725 int (*migratepage) (struct page *, struct page *);
726 /* put migration-failed page back to right list */
727 void (*putback_page) (struct page *);
728 int (*launder_page) (struct page *);
729
730 int (*is_partially_uptodate) (struct page *, unsigned long,
731 unsigned long);
732 void (*is_dirty_writeback) (struct page *, bool *, bool *);
733 int (*error_remove_page) (struct mapping *mapping, struct page *page);
734 int (*swap_activate)(struct file *);
735 int (*swap_deactivate)(struct file *);
736 };
737
738``writepage``
739 called by the VM to write a dirty page to backing store. This
740 may happen for data integrity reasons (i.e. 'sync'), or to free
741 up memory (flush). The difference can be seen in
742 wbc->sync_mode. The PG_Dirty flag has been cleared and
743 PageLocked is true. writepage should start writeout, should set
744 PG_Writeback, and should make sure the page is unlocked, either
745 synchronously or asynchronously when the write operation
746 completes.
747
748 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
749 try too hard if there are problems, and may choose to write out
750 other pages from the mapping if that is easier (e.g. due to
751 internal dependencies). If it chooses not to start writeout, it
752 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
753 keep calling ->writepage on that page.
754
755 See the file "Locking" for more details.
756
757``readpage``
758 called by the VM to read a page from backing store. The page
759 will be Locked when readpage is called, and should be unlocked
760 and marked uptodate once the read completes. If ->readpage
761 discovers that it needs to unlock the page for some reason, it
762 can do so, and then return AOP_TRUNCATED_PAGE. In this case,
763 the page will be relocated, relocked and if that all succeeds,
764 ->readpage will be called again.
765
766``writepages``
767 called by the VM to write out pages associated with the
768 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
769 the writeback_control will specify a range of pages that must be
770 written out. If it is WBC_SYNC_NONE, then a nr_to_write is
771 given and that many pages should be written if possible. If no
772 ->writepages is given, then mpage_writepages is used instead.
773 This will choose pages from the address space that are tagged as
774 DIRTY and will pass them to ->writepage.
775
776``set_page_dirty``
777 called by the VM to set a page dirty. This is particularly
778 needed if an address space attaches private data to a page, and
779 that data needs to be updated when a page is dirtied. This is
780 called, for example, when a memory mapped page gets modified.
781 If defined, it should set the PageDirty flag, and the
782 PAGECACHE_TAG_DIRTY tag in the radix tree.
783
784``readpages``
785 called by the VM to read pages associated with the address_space
786 object. This is essentially just a vector version of readpage.
787 Instead of just one page, several pages are requested.
788 readpages is only used for read-ahead, so read errors are
789 ignored. If anything goes wrong, feel free to give up.
790
791``write_begin``
792 Called by the generic buffered write code to ask the filesystem
793 to prepare to write len bytes at the given offset in the file.
794 The address_space should check that the write will be able to
795 complete, by allocating space if necessary and doing any other
796 internal housekeeping. If the write will update parts of any
797 basic-blocks on storage, then those blocks should be pre-read
798 (if they haven't been read already) so that the updated blocks
799 can be written out properly.
800
801 The filesystem must return the locked pagecache page for the
802 specified offset, in ``*pagep``, for the caller to write into.
803
804 It must be able to cope with short writes (where the length
805 passed to write_begin is greater than the number of bytes copied
806 into the page).
807
808 flags is a field for AOP_FLAG_xxx flags, described in
809 include/linux/fs.h.
810
811 A void * may be returned in fsdata, which then gets passed into
812 write_end.
813
814 Returns 0 on success; < 0 on failure (which is the error code),
815 in which case write_end is not called.
816
817``write_end``
818 After a successful write_begin, and data copy, write_end must be
819 called. len is the original len passed to write_begin, and
820 copied is the amount that was able to be copied.
821
822 The filesystem must take care of unlocking the page and
823 releasing it refcount, and updating i_size.
824
825 Returns < 0 on failure, otherwise the number of bytes (<=
826 'copied') that were able to be copied into pagecache.
827
828``bmap``
829 called by the VFS to map a logical block offset within object to
830 physical block number. This method is used by the FIBMAP ioctl
831 and for working with swap-files. To be able to swap to a file,
832 the file must have a stable mapping to a block device. The swap
833 system does not go through the filesystem but instead uses bmap
834 to find out where the blocks in the file are and uses those
835 addresses directly.
836
837``invalidatepage``
838 If a page has PagePrivate set, then invalidatepage will be
839 called when part or all of the page is to be removed from the
840 address space. This generally corresponds to either a
841 truncation, punch hole or a complete invalidation of the address
842 space (in the latter case 'offset' will always be 0 and 'length'
843 will be PAGE_SIZE). Any private data associated with the page
844 should be updated to reflect this truncation. If offset is 0
845 and length is PAGE_SIZE, then the private data should be
846 released, because the page must be able to be completely
847 discarded. This may be done by calling the ->releasepage
848 function, but in this case the release MUST succeed.
849
850``releasepage``
851 releasepage is called on PagePrivate pages to indicate that the
852 page should be freed if possible. ->releasepage should remove
853 any private data from the page and clear the PagePrivate flag.
854 If releasepage() fails for some reason, it must indicate failure
855 with a 0 return value. releasepage() is used in two distinct
856 though related cases. The first is when the VM finds a clean
857 page with no active users and wants to make it a free page. If
858 ->releasepage succeeds, the page will be removed from the
859 address_space and become free.
860
861 The second case is when a request has been made to invalidate
862 some or all pages in an address_space. This can happen through
863 the fadvise(POSIX_FADV_DONTNEED) system call or by the
864 filesystem explicitly requesting it as nfs and 9fs do (when they
865 believe the cache may be out of date with storage) by calling
866 invalidate_inode_pages2(). If the filesystem makes such a call,
867 and needs to be certain that all pages are invalidated, then its
868 releasepage will need to ensure this. Possibly it can clear the
869 PageUptodate bit if it cannot free private data yet.
870
871``freepage``
872 freepage is called once the page is no longer visible in the
873 page cache in order to allow the cleanup of any private data.
874 Since it may be called by the memory reclaimer, it should not
875 assume that the original address_space mapping still exists, and
876 it should not block.
877
878``direct_IO``
879 called by the generic read/write routines to perform direct_IO -
880 that is IO requests which bypass the page cache and transfer
881 data directly between the storage and the application's address
882 space.
883
884``isolate_page``
885 Called by the VM when isolating a movable non-lru page. If page
886 is successfully isolated, VM marks the page as PG_isolated via
887 __SetPageIsolated.
888
889``migrate_page``
890 This is used to compact the physical memory usage. If the VM
891 wants to relocate a page (maybe off a memory card that is
892 signalling imminent failure) it will pass a new page and an old
893 page to this function. migrate_page should transfer any private
894 data across and update any references that it has to the page.
895
896``putback_page``
897 Called by the VM when isolated page's migration fails.
898
899``launder_page``
900 Called before freeing a page - it writes back the dirty page.
901 To prevent redirtying the page, it is kept locked during the
902 whole operation.
903
904``is_partially_uptodate``
905 Called by the VM when reading a file through the pagecache when
906 the underlying blocksize != pagesize. If the required block is
907 up to date then the read can complete without needing the IO to
908 bring the whole page up to date.
909
910``is_dirty_writeback``
911 Called by the VM when attempting to reclaim a page. The VM uses
912 dirty and writeback information to determine if it needs to
913 stall to allow flushers a chance to complete some IO.
914 Ordinarily it can use PageDirty and PageWriteback but some
915 filesystems have more complex state (unstable pages in NFS
916 prevent reclaim) or do not set those flags due to locking
917 problems. This callback allows a filesystem to indicate to the
918 VM if a page should be treated as dirty or writeback for the
919 purposes of stalling.
920
921``error_remove_page``
922 normally set to generic_error_remove_page if truncation is ok
923 for this address space. Used for memory failure handling.
924 Setting this implies you deal with pages going away under you,
925 unless you have them locked or reference counts increased.
926
927``swap_activate``
928 Called when swapon is used on a file to allocate space if
929 necessary and pin the block lookup information in memory. A
930 return value of zero indicates success, in which case this file
931 can be used to back swapspace.
932
933``swap_deactivate``
934 Called during swapoff on files where swap_activate was
935 successful.
936
937
938The File Object
939===============
940
941A file object represents a file opened by a process. This is also known
942as an "open file description" in POSIX parlance.
943
944
945struct file_operations
946----------------------
947
948This describes how the VFS can manipulate an open file. As of kernel
9494.18, the following members are defined:
950
951.. code-block:: c
952
953 struct file_operations {
954 struct module *owner;
955 loff_t (*llseek) (struct file *, loff_t, int);
956 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
957 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
958 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
959 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
960 int (*iopoll)(struct kiocb *kiocb, bool spin);
961 int (*iterate) (struct file *, struct dir_context *);
962 int (*iterate_shared) (struct file *, struct dir_context *);
963 __poll_t (*poll) (struct file *, struct poll_table_struct *);
964 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
965 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
966 int (*mmap) (struct file *, struct vm_area_struct *);
967 int (*open) (struct inode *, struct file *);
968 int (*flush) (struct file *, fl_owner_t id);
969 int (*release) (struct inode *, struct file *);
970 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
971 int (*fasync) (int, struct file *, int);
972 int (*lock) (struct file *, int, struct file_lock *);
973 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
974 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
975 int (*check_flags)(int);
976 int (*flock) (struct file *, int, struct file_lock *);
977 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
978 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
979 int (*setlease)(struct file *, long, struct file_lock **, void **);
980 long (*fallocate)(struct file *file, int mode, loff_t offset,
981 loff_t len);
982 void (*show_fdinfo)(struct seq_file *m, struct file *f);
983 #ifndef CONFIG_MMU
984 unsigned (*mmap_capabilities)(struct file *);
985 #endif
986 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
987 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
988 struct file *file_out, loff_t pos_out,
989 loff_t len, unsigned int remap_flags);
990 int (*fadvise)(struct file *, loff_t, loff_t, int);
991 };
992
993Again, all methods are called without any locks being held, unless
994otherwise noted.
995
996``llseek``
997 called when the VFS needs to move the file position index
998
999``read``
1000 called by read(2) and related system calls
1001
1002``read_iter``
1003 possibly asynchronous read with iov_iter as destination
1004
1005``write``
1006 called by write(2) and related system calls
1007
1008``write_iter``
1009 possibly asynchronous write with iov_iter as source
1010
1011``iopoll``
1012 called when aio wants to poll for completions on HIPRI iocbs
1013
1014``iterate``
1015 called when the VFS needs to read the directory contents
1016
1017``iterate_shared``
1018 called when the VFS needs to read the directory contents when
1019 filesystem supports concurrent dir iterators
1020
1021``poll``
1022 called by the VFS when a process wants to check if there is
1023 activity on this file and (optionally) go to sleep until there
1024 is activity. Called by the select(2) and poll(2) system calls
1025
1026``unlocked_ioctl``
1027 called by the ioctl(2) system call.
1028
1029``compat_ioctl``
1030 called by the ioctl(2) system call when 32 bit system calls are
1031 used on 64 bit kernels.
1032
1033``mmap``
1034 called by the mmap(2) system call
1035
1036``open``
1037 called by the VFS when an inode should be opened. When the VFS
1038 opens a file, it creates a new "struct file". It then calls the
1039 open method for the newly allocated file structure. You might
1040 think that the open method really belongs in "struct
1041 inode_operations", and you may be right. I think it's done the
1042 way it is because it makes filesystems simpler to implement.
1043 The open() method is a good place to initialize the
1044 "private_data" member in the file structure if you want to point
1045 to a device structure
1046
1047``flush``
1048 called by the close(2) system call to flush a file
1049
1050``release``
1051 called when the last reference to an open file is closed
1052
1053``fsync``
1054 called by the fsync(2) system call. Also see the section above
1055 entitled "Handling errors during writeback".
1056
1057``fasync``
1058 called by the fcntl(2) system call when asynchronous
1059 (non-blocking) mode is enabled for a file
1060
1061``lock``
1062 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1063 F_SETLKW commands
1064
1065``get_unmapped_area``
1066 called by the mmap(2) system call
1067
1068``check_flags``
1069 called by the fcntl(2) system call for F_SETFL command
1070
1071``flock``
1072 called by the flock(2) system call
1073
1074``splice_write``
1075 called by the VFS to splice data from a pipe to a file. This
1076 method is used by the splice(2) system call
1077
1078``splice_read``
1079 called by the VFS to splice data from file to a pipe. This
1080 method is used by the splice(2) system call
1081
1082``setlease``
1083 called by the VFS to set or release a file lock lease. setlease
1084 implementations should call generic_setlease to record or remove
1085 the lease in the inode after setting it.
1086
1087``fallocate``
1088 called by the VFS to preallocate blocks or punch a hole.
1089
1090``copy_file_range``
1091 called by the copy_file_range(2) system call.
1092
1093``remap_file_range``
1094 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1095 and FIDEDUPERANGE commands to remap file ranges. An
1096 implementation should remap len bytes at pos_in of the source
1097 file into the dest file at pos_out. Implementations must handle
1098 callers passing in len == 0; this means "remap to the end of the
1099 source file". The return value should the number of bytes
1100 remapped, or the usual negative error code if errors occurred
1101 before any bytes were remapped. The remap_flags parameter
1102 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1103 implementation must only remap if the requested file ranges have
1104 identical contents. If REMAP_CAN_SHORTEN is set, the caller is
1105 ok with the implementation shortening the request length to
1106 satisfy alignment or EOF requirements (or any other reason).
1107
1108``fadvise``
1109 possibly called by the fadvise64() system call.
1110
1111Note that the file operations are implemented by the specific
1112filesystem in which the inode resides. When opening a device node
1113(character or block special) most filesystems will call special
1114support routines in the VFS which will locate the required device
1115driver information. These support routines replace the filesystem file
1116operations with those for the device driver, and then proceed to call
1117the new open() method for the file. This is how opening a device file
1118in the filesystem eventually ends up calling the device driver open()
1119method.
1120
1121
1122Directory Entry Cache (dcache)
1123==============================
1124
1125
1126struct dentry_operations
1127------------------------
1128
1129This describes how a filesystem can overload the standard dentry
1130operations. Dentries and the dcache are the domain of the VFS and the
1131individual filesystem implementations. Device drivers have no business
1132here. These methods may be set to NULL, as they are either optional or
1133the VFS uses a default. As of kernel 2.6.22, the following members are
1134defined:
1135
1136.. code-block:: c
1137
1138 struct dentry_operations {
1139 int (*d_revalidate)(struct dentry *, unsigned int);
1140 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1141 int (*d_hash)(const struct dentry *, struct qstr *);
1142 int (*d_compare)(const struct dentry *,
1143 unsigned int, const char *, const struct qstr *);
1144 int (*d_delete)(const struct dentry *);
1145 int (*d_init)(struct dentry *);
1146 void (*d_release)(struct dentry *);
1147 void (*d_iput)(struct dentry *, struct inode *);
1148 char *(*d_dname)(struct dentry *, char *, int);
1149 struct vfsmount *(*d_automount)(struct path *);
1150 int (*d_manage)(const struct path *, bool);
1151 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1152 };
1153
1154``d_revalidate``
1155 called when the VFS needs to revalidate a dentry. This is
1156 called whenever a name look-up finds a dentry in the dcache.
1157 Most local filesystems leave this as NULL, because all their
1158 dentries in the dcache are valid. Network filesystems are
1159 different since things can change on the server without the
1160 client necessarily being aware of it.
1161
1162 This function should return a positive value if the dentry is
1163 still valid, and zero or a negative error code if it isn't.
1164
1165 d_revalidate may be called in rcu-walk mode (flags &
1166 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1167 revalidate the dentry without blocking or storing to the dentry,
1168 d_parent and d_inode should not be used without care (because
1169 they can change and, in d_inode case, even become NULL under
1170 us).
1171
1172 If a situation is encountered that rcu-walk cannot handle,
1173 return
1174 -ECHILD and it will be called again in ref-walk mode.
1175
1176``_weak_revalidate``
1177 called when the VFS needs to revalidate a "jumped" dentry. This
1178 is called when a path-walk ends at dentry that was not acquired
1179 by doing a lookup in the parent directory. This includes "/",
1180 "." and "..", as well as procfs-style symlinks and mountpoint
1181 traversal.
1182
1183 In this case, we are less concerned with whether the dentry is
1184 still fully correct, but rather that the inode is still valid.
1185 As with d_revalidate, most local filesystems will set this to
1186 NULL since their dcache entries are always valid.
1187
1188 This function has the same return code semantics as
1189 d_revalidate.
1190
1191 d_weak_revalidate is only called after leaving rcu-walk mode.
1192
1193``d_hash``
1194 called when the VFS adds a dentry to the hash table. The first
1195 dentry passed to d_hash is the parent directory that the name is
1196 to be hashed into.
1197
1198 Same locking and synchronisation rules as d_compare regarding
1199 what is safe to dereference etc.
1200
1201``d_compare``
1202 called to compare a dentry name with a given name. The first
1203 dentry is the parent of the dentry to be compared, the second is
1204 the child dentry. len and name string are properties of the
1205 dentry to be compared. qstr is the name to compare it with.
1206
1207 Must be constant and idempotent, and should not take locks if
1208 possible, and should not or store into the dentry. Should not
1209 dereference pointers outside the dentry without lots of care
1210 (eg. d_parent, d_inode, d_name should not be used).
1211
1212 However, our vfsmount is pinned, and RCU held, so the dentries
1213 and inodes won't disappear, neither will our sb or filesystem
1214 module. ->d_sb may be used.
1215
1216 It is a tricky calling convention because it needs to be called
1217 under "rcu-walk", ie. without any locks or references on things.
1218
1219``d_delete``
1220 called when the last reference to a dentry is dropped and the
1221 dcache is deciding whether or not to cache it. Return 1 to
1222 delete immediately, or 0 to cache the dentry. Default is NULL
1223 which means to always cache a reachable dentry. d_delete must
1224 be constant and idempotent.
1225
1226``d_init``
1227 called when a dentry is allocated
1228
1229``d_release``
1230 called when a dentry is really deallocated
1231
1232``d_iput``
1233 called when a dentry loses its inode (just prior to its being
1234 deallocated). The default when this is NULL is that the VFS
1235 calls iput(). If you define this method, you must call iput()
1236 yourself
1237
1238``d_dname``
1239 called when the pathname of a dentry should be generated.
1240 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1241 delay pathname generation. (Instead of doing it when dentry is
1242 created, it's done only when the path is needed.). Real
1243 filesystems probably dont want to use it, because their dentries
1244 are present in global dcache hash, so their hash should be an
1245 invariant. As no lock is held, d_dname() should not try to
1246 modify the dentry itself, unless appropriate SMP safety is used.
1247 CAUTION : d_path() logic is quite tricky. The correct way to
1248 return for example "Hello" is to put it at the end of the
1249 buffer, and returns a pointer to the first char.
1250 dynamic_dname() helper function is provided to take care of
1251 this.
1252
1253 Example :
1254
1255.. code-block:: c
1256
1257 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1258 {
1259 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1260 dentry->d_inode->i_ino);
1261 }
1262
1263``d_automount``
1264 called when an automount dentry is to be traversed (optional).
1265 This should create a new VFS mount record and return the record
1266 to the caller. The caller is supplied with a path parameter
1267 giving the automount directory to describe the automount target
1268 and the parent VFS mount record to provide inheritable mount
1269 parameters. NULL should be returned if someone else managed to
1270 make the automount first. If the vfsmount creation failed, then
1271 an error code should be returned. If -EISDIR is returned, then
1272 the directory will be treated as an ordinary directory and
1273 returned to pathwalk to continue walking.
1274
1275 If a vfsmount is returned, the caller will attempt to mount it
1276 on the mountpoint and will remove the vfsmount from its
1277 expiration list in the case of failure. The vfsmount should be
1278 returned with 2 refs on it to prevent automatic expiration - the
1279 caller will clean up the additional ref.
1280
1281 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1282 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1283 set on the inode being added.
1284
1285``d_manage``
1286 called to allow the filesystem to manage the transition from a
1287 dentry (optional). This allows autofs, for example, to hold up
1288 clients waiting to explore behind a 'mountpoint' while letting
1289 the daemon go past and construct the subtree there. 0 should be
1290 returned to let the calling process continue. -EISDIR can be
1291 returned to tell pathwalk to use this directory as an ordinary
1292 directory and to ignore anything mounted on it and not to check
1293 the automount flag. Any other error code will abort pathwalk
1294 completely.
1295
1296 If the 'rcu_walk' parameter is true, then the caller is doing a
1297 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1298 mode, and the caller can be asked to leave it and call again by
1299 returning -ECHILD. -EISDIR may also be returned to tell
1300 pathwalk to ignore d_automount or any mounts.
1301
1302 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1303 the dentry being transited from.
1304
1305``d_real``
1306 overlay/union type filesystems implement this method to return
1307 one of the underlying dentries hidden by the overlay. It is
1308 used in two different modes:
1309
1310 Called from file_dentry() it returns the real dentry matching
1311 the inode argument. The real dentry may be from a lower layer
1312 already copied up, but still referenced from the file. This
1313 mode is selected with a non-NULL inode argument.
1314
1315 With NULL inode the topmost real underlying dentry is returned.
1316
1317Each dentry has a pointer to its parent dentry, as well as a hash list
1318of child dentries. Child dentries are basically like files in a
1319directory.
1320
1321
1322Directory Entry Cache API
1323--------------------------
1324
1325There are a number of functions defined which permit a filesystem to
1326manipulate dentries:
1327
1328``dget``
1329 open a new handle for an existing dentry (this just increments
1330 the usage count)
1331
1332``dput``
1333 close a handle for a dentry (decrements the usage count). If
1334 the usage count drops to 0, and the dentry is still in its
1335 parent's hash, the "d_delete" method is called to check whether
1336 it should be cached. If it should not be cached, or if the
1337 dentry is not hashed, it is deleted. Otherwise cached dentries
1338 are put into an LRU list to be reclaimed on memory shortage.
1339
1340``d_drop``
1341 this unhashes a dentry from its parents hash list. A subsequent
1342 call to dput() will deallocate the dentry if its usage count
1343 drops to 0
1344
1345``d_delete``
1346 delete a dentry. If there are no other open references to the
1347 dentry then the dentry is turned into a negative dentry (the
1348 d_iput() method is called). If there are other references, then
1349 d_drop() is called instead
1350
1351``d_add``
1352 add a dentry to its parents hash list and then calls
1353 d_instantiate()
1354
1355``d_instantiate``
1356 add a dentry to the alias hash list for the inode and updates
1357 the "d_inode" member. The "i_count" member in the inode
1358 structure should be set/incremented. If the inode pointer is
1359 NULL, the dentry is called a "negative dentry". This function
1360 is commonly called when an inode is created for an existing
1361 negative dentry
1362
1363``d_lookup``
1364 look up a dentry given its parent and path name component It
1365 looks up the child of that given name from the dcache hash
1366 table. If it is found, the reference count is incremented and
1367 the dentry is returned. The caller must use dput() to free the
1368 dentry when it finishes using it.
1369
1370
1371Mount Options
1372=============
1373
1374
1375Parsing options
1376---------------
1377
1378On mount and remount the filesystem is passed a string containing a
1379comma separated list of mount options. The options can have either of
1380these forms:
1381
1382 option
1383 option=value
1384
1385The <linux/parser.h> header defines an API that helps parse these
1386options. There are plenty of examples on how to use it in existing
1387filesystems.
1388
1389
1390Showing options
1391---------------
1392
1393If a filesystem accepts mount options, it must define show_options() to
1394show all the currently active options. The rules are:
1395
1396 - options MUST be shown which are not default or their values differ
1397 from the default
1398
1399 - options MAY be shown which are enabled by default or have their
1400 default value
1401
1402Options used only internally between a mount helper and the kernel (such
1403as file descriptors), or which only have an effect during the mounting
1404(such as ones controlling the creation of a journal) are exempt from the
1405above rules.
1406
1407The underlying reason for the above rules is to make sure, that a mount
1408can be accurately replicated (e.g. umounting and mounting again) based
1409on the information found in /proc/mounts.
1410
1411
1412Resources
1413=========
1414
1415(Note some of these resources are not up-to-date with the latest kernel
1416 version.)
1417
1418Creating Linux virtual filesystems. 2002
1419 <http://lwn.net/Articles/13325/>
1420
1421The Linux Virtual File-system Layer by Neil Brown. 1999
1422 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1423
1424A tour of the Linux VFS by Michael K. Johnson. 1996
1425 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1426
1427A small trail through the Linux kernel by Andries Brouwer. 2001
1428 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4Overview of the Linux Virtual File System
5=========================================
6
7Original author: Richard Gooch <rgooch@atnf.csiro.au>
8
9- Copyright (C) 1999 Richard Gooch
10- Copyright (C) 2005 Pekka Enberg
11
12
13Introduction
14============
15
16The Virtual File System (also known as the Virtual Filesystem Switch) is
17the software layer in the kernel that provides the filesystem interface
18to userspace programs. It also provides an abstraction within the
19kernel which allows different filesystem implementations to coexist.
20
21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22are called from a process context. Filesystem locking is described in
23the document Documentation/filesystems/locking.rst.
24
25
26Directory Entry Cache (dcache)
27------------------------------
28
29The VFS implements the open(2), stat(2), chmod(2), and similar system
30calls. The pathname argument that is passed to them is used by the VFS
31to search through the directory entry cache (also known as the dentry
32cache or dcache). This provides a very fast look-up mechanism to
33translate a pathname (filename) into a specific dentry. Dentries live
34in RAM and are never saved to disc: they exist only for performance.
35
36The dentry cache is meant to be a view into your entire filespace. As
37most computers cannot fit all dentries in the RAM at the same time, some
38bits of the cache are missing. In order to resolve your pathname into a
39dentry, the VFS may have to resort to creating dentries along the way,
40and then loading the inode. This is done by looking up the inode.
41
42
43The Inode Object
44----------------
45
46An individual dentry usually has a pointer to an inode. Inodes are
47filesystem objects such as regular files, directories, FIFOs and other
48beasts. They live either on the disc (for block device filesystems) or
49in the memory (for pseudo filesystems). Inodes that live on the disc
50are copied into the memory when required and changes to the inode are
51written back to disc. A single inode can be pointed to by multiple
52dentries (hard links, for example, do this).
53
54To look up an inode requires that the VFS calls the lookup() method of
55the parent directory inode. This method is installed by the specific
56filesystem implementation that the inode lives in. Once the VFS has the
57required dentry (and hence the inode), we can do all those boring things
58like open(2) the file, or stat(2) it to peek at the inode data. The
59stat(2) operation is fairly simple: once the VFS has the dentry, it
60peeks at the inode data and passes some of it back to userspace.
61
62
63The File Object
64---------------
65
66Opening a file requires another operation: allocation of a file
67structure (this is the kernel-side implementation of file descriptors).
68The freshly allocated file structure is initialized with a pointer to
69the dentry and a set of file operation member functions. These are
70taken from the inode data. The open() file method is then called so the
71specific filesystem implementation can do its work. You can see that
72this is another switch performed by the VFS. The file structure is
73placed into the file descriptor table for the process.
74
75Reading, writing and closing files (and other assorted VFS operations)
76is done by using the userspace file descriptor to grab the appropriate
77file structure, and then calling the required file structure method to
78do whatever is required. For as long as the file is open, it keeps the
79dentry in use, which in turn means that the VFS inode is still in use.
80
81
82Registering and Mounting a Filesystem
83=====================================
84
85To register and unregister a filesystem, use the following API
86functions:
87
88.. code-block:: c
89
90 #include <linux/fs.h>
91
92 extern int register_filesystem(struct file_system_type *);
93 extern int unregister_filesystem(struct file_system_type *);
94
95The passed struct file_system_type describes your filesystem. When a
96request is made to mount a filesystem onto a directory in your
97namespace, the VFS will call the appropriate mount() method for the
98specific filesystem. New vfsmount referring to the tree returned by
99->mount() will be attached to the mountpoint, so that when pathname
100resolution reaches the mountpoint it will jump into the root of that
101vfsmount.
102
103You can see all filesystems that are registered to the kernel in the
104file /proc/filesystems.
105
106
107struct file_system_type
108-----------------------
109
110This describes the filesystem. The following
111members are defined:
112
113.. code-block:: c
114
115 struct file_system_type {
116 const char *name;
117 int fs_flags;
118 int (*init_fs_context)(struct fs_context *);
119 const struct fs_parameter_spec *parameters;
120 struct dentry *(*mount) (struct file_system_type *, int,
121 const char *, void *);
122 void (*kill_sb) (struct super_block *);
123 struct module *owner;
124 struct file_system_type * next;
125 struct hlist_head fs_supers;
126
127 struct lock_class_key s_lock_key;
128 struct lock_class_key s_umount_key;
129 struct lock_class_key s_vfs_rename_key;
130 struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];
131
132 struct lock_class_key i_lock_key;
133 struct lock_class_key i_mutex_key;
134 struct lock_class_key invalidate_lock_key;
135 struct lock_class_key i_mutex_dir_key;
136 };
137
138``name``
139 the name of the filesystem type, such as "ext2", "iso9660",
140 "msdos" and so on
141
142``fs_flags``
143 various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
144
145``init_fs_context``
146 Initializes 'struct fs_context' ->ops and ->fs_private fields with
147 filesystem-specific data.
148
149``parameters``
150 Pointer to the array of filesystem parameters descriptors
151 'struct fs_parameter_spec'.
152 More info in Documentation/filesystems/mount_api.rst.
153
154``mount``
155 the method to call when a new instance of this filesystem should
156 be mounted
157
158``kill_sb``
159 the method to call when an instance of this filesystem should be
160 shut down
161
162
163``owner``
164 for internal VFS use: you should initialize this to THIS_MODULE
165 in most cases.
166
167``next``
168 for internal VFS use: you should initialize this to NULL
169
170``fs_supers``
171 for internal VFS use: hlist of filesystem instances (superblocks)
172
173 s_lock_key, s_umount_key, s_vfs_rename_key, s_writers_key,
174 i_lock_key, i_mutex_key, invalidate_lock_key, i_mutex_dir_key: lockdep-specific
175
176The mount() method has the following arguments:
177
178``struct file_system_type *fs_type``
179 describes the filesystem, partly initialized by the specific
180 filesystem code
181
182``int flags``
183 mount flags
184
185``const char *dev_name``
186 the device name we are mounting.
187
188``void *data``
189 arbitrary mount options, usually comes as an ASCII string (see
190 "Mount Options" section)
191
192The mount() method must return the root dentry of the tree requested by
193caller. An active reference to its superblock must be grabbed and the
194superblock must be locked. On failure it should return ERR_PTR(error).
195
196The arguments match those of mount(2) and their interpretation depends
197on filesystem type. E.g. for block filesystems, dev_name is interpreted
198as block device name, that device is opened and if it contains a
199suitable filesystem image the method creates and initializes struct
200super_block accordingly, returning its root dentry to caller.
201
202->mount() may choose to return a subtree of existing filesystem - it
203doesn't have to create a new one. The main result from the caller's
204point of view is a reference to dentry at the root of (sub)tree to be
205attached; creation of new superblock is a common side effect.
206
207The most interesting member of the superblock structure that the mount()
208method fills in is the "s_op" field. This is a pointer to a "struct
209super_operations" which describes the next level of the filesystem
210implementation.
211
212Usually, a filesystem uses one of the generic mount() implementations
213and provides a fill_super() callback instead. The generic variants are:
214
215``mount_bdev``
216 mount a filesystem residing on a block device
217
218``mount_nodev``
219 mount a filesystem that is not backed by a device
220
221``mount_single``
222 mount a filesystem which shares the instance between all mounts
223
224A fill_super() callback implementation has the following arguments:
225
226``struct super_block *sb``
227 the superblock structure. The callback must initialize this
228 properly.
229
230``void *data``
231 arbitrary mount options, usually comes as an ASCII string (see
232 "Mount Options" section)
233
234``int silent``
235 whether or not to be silent on error
236
237
238The Superblock Object
239=====================
240
241A superblock object represents a mounted filesystem.
242
243
244struct super_operations
245-----------------------
246
247This describes how the VFS can manipulate the superblock of your
248filesystem. The following members are defined:
249
250.. code-block:: c
251
252 struct super_operations {
253 struct inode *(*alloc_inode)(struct super_block *sb);
254 void (*destroy_inode)(struct inode *);
255 void (*free_inode)(struct inode *);
256
257 void (*dirty_inode) (struct inode *, int flags);
258 int (*write_inode) (struct inode *, struct writeback_control *wbc);
259 int (*drop_inode) (struct inode *);
260 void (*evict_inode) (struct inode *);
261 void (*put_super) (struct super_block *);
262 int (*sync_fs)(struct super_block *sb, int wait);
263 int (*freeze_super) (struct super_block *sb,
264 enum freeze_holder who);
265 int (*freeze_fs) (struct super_block *);
266 int (*thaw_super) (struct super_block *sb,
267 enum freeze_wholder who);
268 int (*unfreeze_fs) (struct super_block *);
269 int (*statfs) (struct dentry *, struct kstatfs *);
270 int (*remount_fs) (struct super_block *, int *, char *);
271 void (*umount_begin) (struct super_block *);
272
273 int (*show_options)(struct seq_file *, struct dentry *);
274 int (*show_devname)(struct seq_file *, struct dentry *);
275 int (*show_path)(struct seq_file *, struct dentry *);
276 int (*show_stats)(struct seq_file *, struct dentry *);
277
278 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
279 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
280 struct dquot **(*get_dquots)(struct inode *);
281
282 long (*nr_cached_objects)(struct super_block *,
283 struct shrink_control *);
284 long (*free_cached_objects)(struct super_block *,
285 struct shrink_control *);
286 };
287
288All methods are called without any locks being held, unless otherwise
289noted. This means that most methods can block safely. All methods are
290only called from a process context (i.e. not from an interrupt handler
291or bottom half).
292
293``alloc_inode``
294 this method is called by alloc_inode() to allocate memory for
295 struct inode and initialize it. If this function is not
296 defined, a simple 'struct inode' is allocated. Normally
297 alloc_inode will be used to allocate a larger structure which
298 contains a 'struct inode' embedded within it.
299
300``destroy_inode``
301 this method is called by destroy_inode() to release resources
302 allocated for struct inode. It is only required if
303 ->alloc_inode was defined and simply undoes anything done by
304 ->alloc_inode.
305
306``free_inode``
307 this method is called from RCU callback. If you use call_rcu()
308 in ->destroy_inode to free 'struct inode' memory, then it's
309 better to release memory in this method.
310
311``dirty_inode``
312 this method is called by the VFS when an inode is marked dirty.
313 This is specifically for the inode itself being marked dirty,
314 not its data. If the update needs to be persisted by fdatasync(),
315 then I_DIRTY_DATASYNC will be set in the flags argument.
316 I_DIRTY_TIME will be set in the flags in case lazytime is enabled
317 and struct inode has times updated since the last ->dirty_inode
318 call.
319
320``write_inode``
321 this method is called when the VFS needs to write an inode to
322 disc. The second parameter indicates whether the write should
323 be synchronous or not, not all filesystems check this flag.
324
325``drop_inode``
326 called when the last access to the inode is dropped, with the
327 inode->i_lock spinlock held.
328
329 This method should be either NULL (normal UNIX filesystem
330 semantics) or "generic_delete_inode" (for filesystems that do
331 not want to cache inodes - causing "delete_inode" to always be
332 called regardless of the value of i_nlink)
333
334 The "generic_delete_inode()" behavior is equivalent to the old
335 practice of using "force_delete" in the put_inode() case, but
336 does not have the races that the "force_delete()" approach had.
337
338``evict_inode``
339 called when the VFS wants to evict an inode. Caller does
340 *not* evict the pagecache or inode-associated metadata buffers;
341 the method has to use truncate_inode_pages_final() to get rid
342 of those. Caller makes sure async writeback cannot be running for
343 the inode while (or after) ->evict_inode() is called. Optional.
344
345``put_super``
346 called when the VFS wishes to free the superblock
347 (i.e. unmount). This is called with the superblock lock held
348
349``sync_fs``
350 called when VFS is writing out all dirty data associated with a
351 superblock. The second parameter indicates whether the method
352 should wait until the write out has been completed. Optional.
353
354``freeze_super``
355 Called instead of ->freeze_fs callback if provided.
356 Main difference is that ->freeze_super is called without taking
357 down_write(&sb->s_umount). If filesystem implements it and wants
358 ->freeze_fs to be called too, then it has to call ->freeze_fs
359 explicitly from this callback. Optional.
360
361``freeze_fs``
362 called when VFS is locking a filesystem and forcing it into a
363 consistent state. This method is currently used by the Logical
364 Volume Manager (LVM) and ioctl(FIFREEZE). Optional.
365
366``thaw_super``
367 called when VFS is unlocking a filesystem and making it writable
368 again after ->freeze_super. Optional.
369
370``unfreeze_fs``
371 called when VFS is unlocking a filesystem and making it writable
372 again after ->freeze_fs. Optional.
373
374``statfs``
375 called when the VFS needs to get filesystem statistics.
376
377``remount_fs``
378 called when the filesystem is remounted. This is called with
379 the kernel lock held
380
381``umount_begin``
382 called when the VFS is unmounting a filesystem.
383
384``show_options``
385 called by the VFS to show mount options for /proc/<pid>/mounts
386 and /proc/<pid>/mountinfo.
387 (see "Mount Options" section)
388
389``show_devname``
390 Optional. Called by the VFS to show device name for
391 /proc/<pid>/{mounts,mountinfo,mountstats}. If not provided then
392 '(struct mount).mnt_devname' will be used.
393
394``show_path``
395 Optional. Called by the VFS (for /proc/<pid>/mountinfo) to show
396 the mount root dentry path relative to the filesystem root.
397
398``show_stats``
399 Optional. Called by the VFS (for /proc/<pid>/mountstats) to show
400 filesystem-specific mount statistics.
401
402``quota_read``
403 called by the VFS to read from filesystem quota file.
404
405``quota_write``
406 called by the VFS to write to filesystem quota file.
407
408``get_dquots``
409 called by quota to get 'struct dquot' array for a particular inode.
410 Optional.
411
412``nr_cached_objects``
413 called by the sb cache shrinking function for the filesystem to
414 return the number of freeable cached objects it contains.
415 Optional.
416
417``free_cache_objects``
418 called by the sb cache shrinking function for the filesystem to
419 scan the number of objects indicated to try to free them.
420 Optional, but any filesystem implementing this method needs to
421 also implement ->nr_cached_objects for it to be called
422 correctly.
423
424 We can't do anything with any errors that the filesystem might
425 encountered, hence the void return type. This will never be
426 called if the VM is trying to reclaim under GFP_NOFS conditions,
427 hence this method does not need to handle that situation itself.
428
429 Implementations must include conditional reschedule calls inside
430 any scanning loop that is done. This allows the VFS to
431 determine appropriate scan batch sizes without having to worry
432 about whether implementations will cause holdoff problems due to
433 large scan batch sizes.
434
435Whoever sets up the inode is responsible for filling in the "i_op"
436field. This is a pointer to a "struct inode_operations" which describes
437the methods that can be performed on individual inodes.
438
439
440struct xattr_handler
441---------------------
442
443On filesystems that support extended attributes (xattrs), the s_xattr
444superblock field points to a NULL-terminated array of xattr handlers.
445Extended attributes are name:value pairs.
446
447``name``
448 Indicates that the handler matches attributes with the specified
449 name (such as "system.posix_acl_access"); the prefix field must
450 be NULL.
451
452``prefix``
453 Indicates that the handler matches all attributes with the
454 specified name prefix (such as "user."); the name field must be
455 NULL.
456
457``list``
458 Determine if attributes matching this xattr handler should be
459 listed for a particular dentry. Used by some listxattr
460 implementations like generic_listxattr.
461
462``get``
463 Called by the VFS to get the value of a particular extended
464 attribute. This method is called by the getxattr(2) system
465 call.
466
467``set``
468 Called by the VFS to set the value of a particular extended
469 attribute. When the new value is NULL, called to remove a
470 particular extended attribute. This method is called by the
471 setxattr(2) and removexattr(2) system calls.
472
473When none of the xattr handlers of a filesystem match the specified
474attribute name or when a filesystem doesn't support extended attributes,
475the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
476
477
478The Inode Object
479================
480
481An inode object represents an object within the filesystem.
482
483
484struct inode_operations
485-----------------------
486
487This describes how the VFS can manipulate an inode in your filesystem.
488As of kernel 2.6.22, the following members are defined:
489
490.. code-block:: c
491
492 struct inode_operations {
493 int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t, bool);
494 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
495 int (*link) (struct dentry *,struct inode *,struct dentry *);
496 int (*unlink) (struct inode *,struct dentry *);
497 int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
498 int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
499 int (*rmdir) (struct inode *,struct dentry *);
500 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
501 int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
502 struct inode *, struct dentry *, unsigned int);
503 int (*readlink) (struct dentry *, char __user *,int);
504 const char *(*get_link) (struct dentry *, struct inode *,
505 struct delayed_call *);
506 int (*permission) (struct mnt_idmap *, struct inode *, int);
507 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
508 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
509 int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
510 ssize_t (*listxattr) (struct dentry *, char *, size_t);
511 void (*update_time)(struct inode *, struct timespec *, int);
512 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
513 unsigned open_flag, umode_t create_mode);
514 int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
515 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
516 int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int);
517 int (*fileattr_set)(struct mnt_idmap *idmap,
518 struct dentry *dentry, struct fileattr *fa);
519 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
520 struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
521 };
522
523Again, all methods are called without any locks being held, unless
524otherwise noted.
525
526``create``
527 called by the open(2) and creat(2) system calls. Only required
528 if you want to support regular files. The dentry you get should
529 not have an inode (i.e. it should be a negative dentry). Here
530 you will probably call d_instantiate() with the dentry and the
531 newly created inode
532
533``lookup``
534 called when the VFS needs to look up an inode in a parent
535 directory. The name to look for is found in the dentry. This
536 method must call d_add() to insert the found inode into the
537 dentry. The "i_count" field in the inode structure should be
538 incremented. If the named inode does not exist a NULL inode
539 should be inserted into the dentry (this is called a negative
540 dentry). Returning an error code from this routine must only be
541 done on a real error, otherwise creating inodes with system
542 calls like create(2), mknod(2), mkdir(2) and so on will fail.
543 If you wish to overload the dentry methods then you should
544 initialise the "d_dop" field in the dentry; this is a pointer to
545 a struct "dentry_operations". This method is called with the
546 directory inode semaphore held
547
548``link``
549 called by the link(2) system call. Only required if you want to
550 support hard links. You will probably need to call
551 d_instantiate() just as you would in the create() method
552
553``unlink``
554 called by the unlink(2) system call. Only required if you want
555 to support deleting inodes
556
557``symlink``
558 called by the symlink(2) system call. Only required if you want
559 to support symlinks. You will probably need to call
560 d_instantiate() just as you would in the create() method
561
562``mkdir``
563 called by the mkdir(2) system call. Only required if you want
564 to support creating subdirectories. You will probably need to
565 call d_instantiate() just as you would in the create() method
566
567``rmdir``
568 called by the rmdir(2) system call. Only required if you want
569 to support deleting subdirectories
570
571``mknod``
572 called by the mknod(2) system call to create a device (char,
573 block) inode or a named pipe (FIFO) or socket. Only required if
574 you want to support creating these types of inodes. You will
575 probably need to call d_instantiate() just as you would in the
576 create() method
577
578``rename``
579 called by the rename(2) system call to rename the object to have
580 the parent and name given by the second inode and dentry.
581
582 The filesystem must return -EINVAL for any unsupported or
583 unknown flags. Currently the following flags are implemented:
584 (1) RENAME_NOREPLACE: this flag indicates that if the target of
585 the rename exists the rename should fail with -EEXIST instead of
586 replacing the target. The VFS already checks for existence, so
587 for local filesystems the RENAME_NOREPLACE implementation is
588 equivalent to plain rename.
589 (2) RENAME_EXCHANGE: exchange source and target. Both must
590 exist; this is checked by the VFS. Unlike plain rename, source
591 and target may be of different type.
592
593``get_link``
594 called by the VFS to follow a symbolic link to the inode it
595 points to. Only required if you want to support symbolic links.
596 This method returns the symlink body to traverse (and possibly
597 resets the current position with nd_jump_link()). If the body
598 won't go away until the inode is gone, nothing else is needed;
599 if it needs to be otherwise pinned, arrange for its release by
600 having get_link(..., ..., done) do set_delayed_call(done,
601 destructor, argument). In that case destructor(argument) will
602 be called once VFS is done with the body you've returned. May
603 be called in RCU mode; that is indicated by NULL dentry
604 argument. If request can't be handled without leaving RCU mode,
605 have it return ERR_PTR(-ECHILD).
606
607 If the filesystem stores the symlink target in ->i_link, the
608 VFS may use it directly without calling ->get_link(); however,
609 ->get_link() must still be provided. ->i_link must not be
610 freed until after an RCU grace period. Writing to ->i_link
611 post-iget() time requires a 'release' memory barrier.
612
613``readlink``
614 this is now just an override for use by readlink(2) for the
615 cases when ->get_link uses nd_jump_link() or object is not in
616 fact a symlink. Normally filesystems should only implement
617 ->get_link for symlinks and readlink(2) will automatically use
618 that.
619
620``permission``
621 called by the VFS to check for access rights on a POSIX-like
622 filesystem.
623
624 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
625 rcu-walk mode, the filesystem must check the permission without
626 blocking or storing to the inode.
627
628 If a situation is encountered that rcu-walk cannot handle,
629 return
630 -ECHILD and it will be called again in ref-walk mode.
631
632``setattr``
633 called by the VFS to set attributes for a file. This method is
634 called by chmod(2) and related system calls.
635
636``getattr``
637 called by the VFS to get attributes of a file. This method is
638 called by stat(2) and related system calls.
639
640``listxattr``
641 called by the VFS to list all extended attributes for a given
642 file. This method is called by the listxattr(2) system call.
643
644``update_time``
645 called by the VFS to update a specific time or the i_version of
646 an inode. If this is not defined the VFS will update the inode
647 itself and call mark_inode_dirty_sync.
648
649``atomic_open``
650 called on the last component of an open. Using this optional
651 method the filesystem can look up, possibly create and open the
652 file in one atomic operation. If it wants to leave actual
653 opening to the caller (e.g. if the file turned out to be a
654 symlink, device, or just something filesystem won't do atomic
655 open for), it may signal this by returning finish_no_open(file,
656 dentry). This method is only called if the last component is
657 negative or needs lookup. Cached positive dentries are still
658 handled by f_op->open(). If the file was created, FMODE_CREATED
659 flag should be set in file->f_mode. In case of O_EXCL the
660 method must only succeed if the file didn't exist and hence
661 FMODE_CREATED shall always be set on success.
662
663``tmpfile``
664 called in the end of O_TMPFILE open(). Optional, equivalent to
665 atomically creating, opening and unlinking a file in given
666 directory. On success needs to return with the file already
667 open; this can be done by calling finish_open_simple() right at
668 the end.
669
670``fileattr_get``
671 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
672 retrieve miscellaneous file flags and attributes. Also called
673 before the relevant SET operation to check what is being changed
674 (in this case with i_rwsem locked exclusive). If unset, then
675 fall back to f_op->ioctl().
676
677``fileattr_set``
678 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
679 change miscellaneous file flags and attributes. Callers hold
680 i_rwsem exclusive. If unset, then fall back to f_op->ioctl().
681``get_offset_ctx``
682 called to get the offset context for a directory inode. A
683 filesystem must define this operation to use
684 simple_offset_dir_operations.
685
686The Address Space Object
687========================
688
689The address space object is used to group and manage pages in the page
690cache. It can be used to keep track of the pages in a file (or anything
691else) and also track the mapping of sections of the file into process
692address spaces.
693
694There are a number of distinct yet related services that an
695address-space can provide. These include communicating memory pressure,
696page lookup by address, and keeping track of pages tagged as Dirty or
697Writeback.
698
699The first can be used independently to the others. The VM can try to
700either write dirty pages in order to clean them, or release clean pages
701in order to reuse them. To do this it can call the ->writepage method
702on dirty pages, and ->release_folio on clean folios with the private
703flag set. Clean pages without PagePrivate and with no external references
704will be released without notice being given to the address_space.
705
706To achieve this functionality, pages need to be placed on an LRU with
707lru_cache_add and mark_page_active needs to be called whenever the page
708is used.
709
710Pages are normally kept in a radix tree index by ->index. This tree
711maintains information about the PG_Dirty and PG_Writeback status of each
712page, so that pages with either of these flags can be found quickly.
713
714The Dirty tag is primarily used by mpage_writepages - the default
715->writepages method. It uses the tag to find dirty pages to call
716->writepage on. If mpage_writepages is not used (i.e. the address
717provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
718unused. write_inode_now and sync_inode do use it (through
719__sync_single_inode) to check if ->writepages has been successful in
720writing out the whole address_space.
721
722The Writeback tag is used by filemap*wait* and sync_page* functions, via
723filemap_fdatawait_range, to wait for all writeback to complete.
724
725An address_space handler may attach extra information to a page,
726typically using the 'private' field in the 'struct page'. If such
727information is attached, the PG_Private flag should be set. This will
728cause various VM routines to make extra calls into the address_space
729handler to deal with that data.
730
731An address space acts as an intermediate between storage and
732application. Data is read into the address space a whole page at a
733time, and provided to the application either by copying of the page, or
734by memory-mapping the page. Data is written into the address space by
735the application, and then written-back to storage typically in whole
736pages, however the address_space has finer control of write sizes.
737
738The read process essentially only requires 'read_folio'. The write
739process is more complicated and uses write_begin/write_end or
740dirty_folio to write data into the address_space, and writepage and
741writepages to writeback data to storage.
742
743Adding and removing pages to/from an address_space is protected by the
744inode's i_mutex.
745
746When data is written to a page, the PG_Dirty flag should be set. It
747typically remains set until writepage asks for it to be written. This
748should clear PG_Dirty and set PG_Writeback. It can be actually written
749at any point after PG_Dirty is clear. Once it is known to be safe,
750PG_Writeback is cleared.
751
752Writeback makes use of a writeback_control structure to direct the
753operations. This gives the writepage and writepages operations some
754information about the nature of and reason for the writeback request,
755and the constraints under which it is being done. It is also used to
756return information back to the caller about the result of a writepage or
757writepages request.
758
759
760Handling errors during writeback
761--------------------------------
762
763Most applications that do buffered I/O will periodically call a file
764synchronization call (fsync, fdatasync, msync or sync_file_range) to
765ensure that data written has made it to the backing store. When there
766is an error during writeback, they expect that error to be reported when
767a file sync request is made. After an error has been reported on one
768request, subsequent requests on the same file descriptor should return
7690, unless further writeback errors have occurred since the previous file
770synchronization.
771
772Ideally, the kernel would report errors only on file descriptions on
773which writes were done that subsequently failed to be written back. The
774generic pagecache infrastructure does not track the file descriptions
775that have dirtied each individual page however, so determining which
776file descriptors should get back an error is not possible.
777
778Instead, the generic writeback error tracking infrastructure in the
779kernel settles for reporting errors to fsync on all file descriptions
780that were open at the time that the error occurred. In a situation with
781multiple writers, all of them will get back an error on a subsequent
782fsync, even if all of the writes done through that particular file
783descriptor succeeded (or even if there were no writes on that file
784descriptor at all).
785
786Filesystems that wish to use this infrastructure should call
787mapping_set_error to record the error in the address_space when it
788occurs. Then, after writing back data from the pagecache in their
789file->fsync operation, they should call file_check_and_advance_wb_err to
790ensure that the struct file's error cursor has advanced to the correct
791point in the stream of errors emitted by the backing device(s).
792
793
794struct address_space_operations
795-------------------------------
796
797This describes how the VFS can manipulate mapping of a file to page
798cache in your filesystem. The following members are defined:
799
800.. code-block:: c
801
802 struct address_space_operations {
803 int (*writepage)(struct page *page, struct writeback_control *wbc);
804 int (*read_folio)(struct file *, struct folio *);
805 int (*writepages)(struct address_space *, struct writeback_control *);
806 bool (*dirty_folio)(struct address_space *, struct folio *);
807 void (*readahead)(struct readahead_control *);
808 int (*write_begin)(struct file *, struct address_space *mapping,
809 loff_t pos, unsigned len,
810 struct page **pagep, void **fsdata);
811 int (*write_end)(struct file *, struct address_space *mapping,
812 loff_t pos, unsigned len, unsigned copied,
813 struct page *page, void *fsdata);
814 sector_t (*bmap)(struct address_space *, sector_t);
815 void (*invalidate_folio) (struct folio *, size_t start, size_t len);
816 bool (*release_folio)(struct folio *, gfp_t);
817 void (*free_folio)(struct folio *);
818 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
819 int (*migrate_folio)(struct mapping *, struct folio *dst,
820 struct folio *src, enum migrate_mode);
821 int (*launder_folio) (struct folio *);
822
823 bool (*is_partially_uptodate) (struct folio *, size_t from,
824 size_t count);
825 void (*is_dirty_writeback)(struct folio *, bool *, bool *);
826 int (*error_remove_folio)(struct mapping *mapping, struct folio *);
827 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
828 int (*swap_deactivate)(struct file *);
829 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
830 };
831
832``writepage``
833 called by the VM to write a dirty page to backing store. This
834 may happen for data integrity reasons (i.e. 'sync'), or to free
835 up memory (flush). The difference can be seen in
836 wbc->sync_mode. The PG_Dirty flag has been cleared and
837 PageLocked is true. writepage should start writeout, should set
838 PG_Writeback, and should make sure the page is unlocked, either
839 synchronously or asynchronously when the write operation
840 completes.
841
842 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
843 try too hard if there are problems, and may choose to write out
844 other pages from the mapping if that is easier (e.g. due to
845 internal dependencies). If it chooses not to start writeout, it
846 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
847 keep calling ->writepage on that page.
848
849 See the file "Locking" for more details.
850
851``read_folio``
852 Called by the page cache to read a folio from the backing store.
853 The 'file' argument supplies authentication information to network
854 filesystems, and is generally not used by block based filesystems.
855 It may be NULL if the caller does not have an open file (eg if
856 the kernel is performing a read for itself rather than on behalf
857 of a userspace process with an open file).
858
859 If the mapping does not support large folios, the folio will
860 contain a single page. The folio will be locked when read_folio
861 is called. If the read completes successfully, the folio should
862 be marked uptodate. The filesystem should unlock the folio
863 once the read has completed, whether it was successful or not.
864 The filesystem does not need to modify the refcount on the folio;
865 the page cache holds a reference count and that will not be
866 released until the folio is unlocked.
867
868 Filesystems may implement ->read_folio() synchronously.
869 In normal operation, folios are read through the ->readahead()
870 method. Only if this fails, or if the caller needs to wait for
871 the read to complete will the page cache call ->read_folio().
872 Filesystems should not attempt to perform their own readahead
873 in the ->read_folio() operation.
874
875 If the filesystem cannot perform the read at this time, it can
876 unlock the folio, do whatever action it needs to ensure that the
877 read will succeed in the future and return AOP_TRUNCATED_PAGE.
878 In this case, the caller should look up the folio, lock it,
879 and call ->read_folio again.
880
881 Callers may invoke the ->read_folio() method directly, but using
882 read_mapping_folio() will take care of locking, waiting for the
883 read to complete and handle cases such as AOP_TRUNCATED_PAGE.
884
885``writepages``
886 called by the VM to write out pages associated with the
887 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
888 the writeback_control will specify a range of pages that must be
889 written out. If it is WB_SYNC_NONE, then a nr_to_write is
890 given and that many pages should be written if possible. If no
891 ->writepages is given, then mpage_writepages is used instead.
892 This will choose pages from the address space that are tagged as
893 DIRTY and will pass them to ->writepage.
894
895``dirty_folio``
896 called by the VM to mark a folio as dirty. This is particularly
897 needed if an address space attaches private data to a folio, and
898 that data needs to be updated when a folio is dirtied. This is
899 called, for example, when a memory mapped page gets modified.
900 If defined, it should set the folio dirty flag, and the
901 PAGECACHE_TAG_DIRTY search mark in i_pages.
902
903``readahead``
904 Called by the VM to read pages associated with the address_space
905 object. The pages are consecutive in the page cache and are
906 locked. The implementation should decrement the page refcount
907 after starting I/O on each page. Usually the page will be
908 unlocked by the I/O completion handler. The set of pages are
909 divided into some sync pages followed by some async pages,
910 rac->ra->async_size gives the number of async pages. The
911 filesystem should attempt to read all sync pages but may decide
912 to stop once it reaches the async pages. If it does decide to
913 stop attempting I/O, it can simply return. The caller will
914 remove the remaining pages from the address space, unlock them
915 and decrement the page refcount. Set PageUptodate if the I/O
916 completes successfully. Setting PageError on any page will be
917 ignored; simply unlock the page if an I/O error occurs.
918
919``write_begin``
920 Called by the generic buffered write code to ask the filesystem
921 to prepare to write len bytes at the given offset in the file.
922 The address_space should check that the write will be able to
923 complete, by allocating space if necessary and doing any other
924 internal housekeeping. If the write will update parts of any
925 basic-blocks on storage, then those blocks should be pre-read
926 (if they haven't been read already) so that the updated blocks
927 can be written out properly.
928
929 The filesystem must return the locked pagecache page for the
930 specified offset, in ``*pagep``, for the caller to write into.
931
932 It must be able to cope with short writes (where the length
933 passed to write_begin is greater than the number of bytes copied
934 into the page).
935
936 A void * may be returned in fsdata, which then gets passed into
937 write_end.
938
939 Returns 0 on success; < 0 on failure (which is the error code),
940 in which case write_end is not called.
941
942``write_end``
943 After a successful write_begin, and data copy, write_end must be
944 called. len is the original len passed to write_begin, and
945 copied is the amount that was able to be copied.
946
947 The filesystem must take care of unlocking the page and
948 releasing it refcount, and updating i_size.
949
950 Returns < 0 on failure, otherwise the number of bytes (<=
951 'copied') that were able to be copied into pagecache.
952
953``bmap``
954 called by the VFS to map a logical block offset within object to
955 physical block number. This method is used by the FIBMAP ioctl
956 and for working with swap-files. To be able to swap to a file,
957 the file must have a stable mapping to a block device. The swap
958 system does not go through the filesystem but instead uses bmap
959 to find out where the blocks in the file are and uses those
960 addresses directly.
961
962``invalidate_folio``
963 If a folio has private data, then invalidate_folio will be
964 called when part or all of the folio is to be removed from the
965 address space. This generally corresponds to either a
966 truncation, punch hole or a complete invalidation of the address
967 space (in the latter case 'offset' will always be 0 and 'length'
968 will be folio_size()). Any private data associated with the folio
969 should be updated to reflect this truncation. If offset is 0
970 and length is folio_size(), then the private data should be
971 released, because the folio must be able to be completely
972 discarded. This may be done by calling the ->release_folio
973 function, but in this case the release MUST succeed.
974
975``release_folio``
976 release_folio is called on folios with private data to tell the
977 filesystem that the folio is about to be freed. ->release_folio
978 should remove any private data from the folio and clear the
979 private flag. If release_folio() fails, it should return false.
980 release_folio() is used in two distinct though related cases.
981 The first is when the VM wants to free a clean folio with no
982 active users. If ->release_folio succeeds, the folio will be
983 removed from the address_space and be freed.
984
985 The second case is when a request has been made to invalidate
986 some or all folios in an address_space. This can happen
987 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
988 filesystem explicitly requesting it as nfs and 9p do (when they
989 believe the cache may be out of date with storage) by calling
990 invalidate_inode_pages2(). If the filesystem makes such a call,
991 and needs to be certain that all folios are invalidated, then
992 its release_folio will need to ensure this. Possibly it can
993 clear the uptodate flag if it cannot free private data yet.
994
995``free_folio``
996 free_folio is called once the folio is no longer visible in the
997 page cache in order to allow the cleanup of any private data.
998 Since it may be called by the memory reclaimer, it should not
999 assume that the original address_space mapping still exists, and
1000 it should not block.
1001
1002``direct_IO``
1003 called by the generic read/write routines to perform direct_IO -
1004 that is IO requests which bypass the page cache and transfer
1005 data directly between the storage and the application's address
1006 space.
1007
1008``migrate_folio``
1009 This is used to compact the physical memory usage. If the VM
1010 wants to relocate a folio (maybe from a memory device that is
1011 signalling imminent failure) it will pass a new folio and an old
1012 folio to this function. migrate_folio should transfer any private
1013 data across and update any references that it has to the folio.
1014
1015``launder_folio``
1016 Called before freeing a folio - it writes back the dirty folio.
1017 To prevent redirtying the folio, it is kept locked during the
1018 whole operation.
1019
1020``is_partially_uptodate``
1021 Called by the VM when reading a file through the pagecache when
1022 the underlying blocksize is smaller than the size of the folio.
1023 If the required block is up to date then the read can complete
1024 without needing I/O to bring the whole page up to date.
1025
1026``is_dirty_writeback``
1027 Called by the VM when attempting to reclaim a folio. The VM uses
1028 dirty and writeback information to determine if it needs to
1029 stall to allow flushers a chance to complete some IO.
1030 Ordinarily it can use folio_test_dirty and folio_test_writeback but
1031 some filesystems have more complex state (unstable folios in NFS
1032 prevent reclaim) or do not set those flags due to locking
1033 problems. This callback allows a filesystem to indicate to the
1034 VM if a folio should be treated as dirty or writeback for the
1035 purposes of stalling.
1036
1037``error_remove_folio``
1038 normally set to generic_error_remove_folio if truncation is ok
1039 for this address space. Used for memory failure handling.
1040 Setting this implies you deal with pages going away under you,
1041 unless you have them locked or reference counts increased.
1042
1043``swap_activate``
1044
1045 Called to prepare the given file for swap. It should perform
1046 any validation and preparation necessary to ensure that writes
1047 can be performed with minimal memory allocation. It should call
1048 add_swap_extent(), or the helper iomap_swapfile_activate(), and
1049 return the number of extents added. If IO should be submitted
1050 through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
1051 be submitted directly to the block device ``sis->bdev``.
1052
1053``swap_deactivate``
1054 Called during swapoff on files where swap_activate was
1055 successful.
1056
1057``swap_rw``
1058 Called to read or write swap pages when SWP_FS_OPS is set.
1059
1060The File Object
1061===============
1062
1063A file object represents a file opened by a process. This is also known
1064as an "open file description" in POSIX parlance.
1065
1066
1067struct file_operations
1068----------------------
1069
1070This describes how the VFS can manipulate an open file. As of kernel
10714.18, the following members are defined:
1072
1073.. code-block:: c
1074
1075 struct file_operations {
1076 struct module *owner;
1077 loff_t (*llseek) (struct file *, loff_t, int);
1078 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1079 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1080 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1081 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1082 int (*iopoll)(struct kiocb *kiocb, bool spin);
1083 int (*iterate_shared) (struct file *, struct dir_context *);
1084 __poll_t (*poll) (struct file *, struct poll_table_struct *);
1085 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1086 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1087 int (*mmap) (struct file *, struct vm_area_struct *);
1088 int (*open) (struct inode *, struct file *);
1089 int (*flush) (struct file *, fl_owner_t id);
1090 int (*release) (struct inode *, struct file *);
1091 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1092 int (*fasync) (int, struct file *, int);
1093 int (*lock) (struct file *, int, struct file_lock *);
1094 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1095 int (*check_flags)(int);
1096 int (*flock) (struct file *, int, struct file_lock *);
1097 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1098 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1099 int (*setlease)(struct file *, long, struct file_lock **, void **);
1100 long (*fallocate)(struct file *file, int mode, loff_t offset,
1101 loff_t len);
1102 void (*show_fdinfo)(struct seq_file *m, struct file *f);
1103 #ifndef CONFIG_MMU
1104 unsigned (*mmap_capabilities)(struct file *);
1105 #endif
1106 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1107 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1108 struct file *file_out, loff_t pos_out,
1109 loff_t len, unsigned int remap_flags);
1110 int (*fadvise)(struct file *, loff_t, loff_t, int);
1111 };
1112
1113Again, all methods are called without any locks being held, unless
1114otherwise noted.
1115
1116``llseek``
1117 called when the VFS needs to move the file position index
1118
1119``read``
1120 called by read(2) and related system calls
1121
1122``read_iter``
1123 possibly asynchronous read with iov_iter as destination
1124
1125``write``
1126 called by write(2) and related system calls
1127
1128``write_iter``
1129 possibly asynchronous write with iov_iter as source
1130
1131``iopoll``
1132 called when aio wants to poll for completions on HIPRI iocbs
1133
1134``iterate_shared``
1135 called when the VFS needs to read the directory contents
1136
1137``poll``
1138 called by the VFS when a process wants to check if there is
1139 activity on this file and (optionally) go to sleep until there
1140 is activity. Called by the select(2) and poll(2) system calls
1141
1142``unlocked_ioctl``
1143 called by the ioctl(2) system call.
1144
1145``compat_ioctl``
1146 called by the ioctl(2) system call when 32 bit system calls are
1147 used on 64 bit kernels.
1148
1149``mmap``
1150 called by the mmap(2) system call
1151
1152``open``
1153 called by the VFS when an inode should be opened. When the VFS
1154 opens a file, it creates a new "struct file". It then calls the
1155 open method for the newly allocated file structure. You might
1156 think that the open method really belongs in "struct
1157 inode_operations", and you may be right. I think it's done the
1158 way it is because it makes filesystems simpler to implement.
1159 The open() method is a good place to initialize the
1160 "private_data" member in the file structure if you want to point
1161 to a device structure
1162
1163``flush``
1164 called by the close(2) system call to flush a file
1165
1166``release``
1167 called when the last reference to an open file is closed
1168
1169``fsync``
1170 called by the fsync(2) system call. Also see the section above
1171 entitled "Handling errors during writeback".
1172
1173``fasync``
1174 called by the fcntl(2) system call when asynchronous
1175 (non-blocking) mode is enabled for a file
1176
1177``lock``
1178 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1179 F_SETLKW commands
1180
1181``get_unmapped_area``
1182 called by the mmap(2) system call
1183
1184``check_flags``
1185 called by the fcntl(2) system call for F_SETFL command
1186
1187``flock``
1188 called by the flock(2) system call
1189
1190``splice_write``
1191 called by the VFS to splice data from a pipe to a file. This
1192 method is used by the splice(2) system call
1193
1194``splice_read``
1195 called by the VFS to splice data from file to a pipe. This
1196 method is used by the splice(2) system call
1197
1198``setlease``
1199 called by the VFS to set or release a file lock lease. setlease
1200 implementations should call generic_setlease to record or remove
1201 the lease in the inode after setting it.
1202
1203``fallocate``
1204 called by the VFS to preallocate blocks or punch a hole.
1205
1206``copy_file_range``
1207 called by the copy_file_range(2) system call.
1208
1209``remap_file_range``
1210 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1211 and FIDEDUPERANGE commands to remap file ranges. An
1212 implementation should remap len bytes at pos_in of the source
1213 file into the dest file at pos_out. Implementations must handle
1214 callers passing in len == 0; this means "remap to the end of the
1215 source file". The return value should the number of bytes
1216 remapped, or the usual negative error code if errors occurred
1217 before any bytes were remapped. The remap_flags parameter
1218 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1219 implementation must only remap if the requested file ranges have
1220 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
1221 ok with the implementation shortening the request length to
1222 satisfy alignment or EOF requirements (or any other reason).
1223
1224``fadvise``
1225 possibly called by the fadvise64() system call.
1226
1227Note that the file operations are implemented by the specific
1228filesystem in which the inode resides. When opening a device node
1229(character or block special) most filesystems will call special
1230support routines in the VFS which will locate the required device
1231driver information. These support routines replace the filesystem file
1232operations with those for the device driver, and then proceed to call
1233the new open() method for the file. This is how opening a device file
1234in the filesystem eventually ends up calling the device driver open()
1235method.
1236
1237
1238Directory Entry Cache (dcache)
1239==============================
1240
1241
1242struct dentry_operations
1243------------------------
1244
1245This describes how a filesystem can overload the standard dentry
1246operations. Dentries and the dcache are the domain of the VFS and the
1247individual filesystem implementations. Device drivers have no business
1248here. These methods may be set to NULL, as they are either optional or
1249the VFS uses a default. As of kernel 2.6.22, the following members are
1250defined:
1251
1252.. code-block:: c
1253
1254 struct dentry_operations {
1255 int (*d_revalidate)(struct dentry *, unsigned int);
1256 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1257 int (*d_hash)(const struct dentry *, struct qstr *);
1258 int (*d_compare)(const struct dentry *,
1259 unsigned int, const char *, const struct qstr *);
1260 int (*d_delete)(const struct dentry *);
1261 int (*d_init)(struct dentry *);
1262 void (*d_release)(struct dentry *);
1263 void (*d_iput)(struct dentry *, struct inode *);
1264 char *(*d_dname)(struct dentry *, char *, int);
1265 struct vfsmount *(*d_automount)(struct path *);
1266 int (*d_manage)(const struct path *, bool);
1267 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1268 };
1269
1270``d_revalidate``
1271 called when the VFS needs to revalidate a dentry. This is
1272 called whenever a name look-up finds a dentry in the dcache.
1273 Most local filesystems leave this as NULL, because all their
1274 dentries in the dcache are valid. Network filesystems are
1275 different since things can change on the server without the
1276 client necessarily being aware of it.
1277
1278 This function should return a positive value if the dentry is
1279 still valid, and zero or a negative error code if it isn't.
1280
1281 d_revalidate may be called in rcu-walk mode (flags &
1282 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1283 revalidate the dentry without blocking or storing to the dentry,
1284 d_parent and d_inode should not be used without care (because
1285 they can change and, in d_inode case, even become NULL under
1286 us).
1287
1288 If a situation is encountered that rcu-walk cannot handle,
1289 return
1290 -ECHILD and it will be called again in ref-walk mode.
1291
1292``d_weak_revalidate``
1293 called when the VFS needs to revalidate a "jumped" dentry. This
1294 is called when a path-walk ends at dentry that was not acquired
1295 by doing a lookup in the parent directory. This includes "/",
1296 "." and "..", as well as procfs-style symlinks and mountpoint
1297 traversal.
1298
1299 In this case, we are less concerned with whether the dentry is
1300 still fully correct, but rather that the inode is still valid.
1301 As with d_revalidate, most local filesystems will set this to
1302 NULL since their dcache entries are always valid.
1303
1304 This function has the same return code semantics as
1305 d_revalidate.
1306
1307 d_weak_revalidate is only called after leaving rcu-walk mode.
1308
1309``d_hash``
1310 called when the VFS adds a dentry to the hash table. The first
1311 dentry passed to d_hash is the parent directory that the name is
1312 to be hashed into.
1313
1314 Same locking and synchronisation rules as d_compare regarding
1315 what is safe to dereference etc.
1316
1317``d_compare``
1318 called to compare a dentry name with a given name. The first
1319 dentry is the parent of the dentry to be compared, the second is
1320 the child dentry. len and name string are properties of the
1321 dentry to be compared. qstr is the name to compare it with.
1322
1323 Must be constant and idempotent, and should not take locks if
1324 possible, and should not or store into the dentry. Should not
1325 dereference pointers outside the dentry without lots of care
1326 (eg. d_parent, d_inode, d_name should not be used).
1327
1328 However, our vfsmount is pinned, and RCU held, so the dentries
1329 and inodes won't disappear, neither will our sb or filesystem
1330 module. ->d_sb may be used.
1331
1332 It is a tricky calling convention because it needs to be called
1333 under "rcu-walk", ie. without any locks or references on things.
1334
1335``d_delete``
1336 called when the last reference to a dentry is dropped and the
1337 dcache is deciding whether or not to cache it. Return 1 to
1338 delete immediately, or 0 to cache the dentry. Default is NULL
1339 which means to always cache a reachable dentry. d_delete must
1340 be constant and idempotent.
1341
1342``d_init``
1343 called when a dentry is allocated
1344
1345``d_release``
1346 called when a dentry is really deallocated
1347
1348``d_iput``
1349 called when a dentry loses its inode (just prior to its being
1350 deallocated). The default when this is NULL is that the VFS
1351 calls iput(). If you define this method, you must call iput()
1352 yourself
1353
1354``d_dname``
1355 called when the pathname of a dentry should be generated.
1356 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1357 delay pathname generation. (Instead of doing it when dentry is
1358 created, it's done only when the path is needed.). Real
1359 filesystems probably dont want to use it, because their dentries
1360 are present in global dcache hash, so their hash should be an
1361 invariant. As no lock is held, d_dname() should not try to
1362 modify the dentry itself, unless appropriate SMP safety is used.
1363 CAUTION : d_path() logic is quite tricky. The correct way to
1364 return for example "Hello" is to put it at the end of the
1365 buffer, and returns a pointer to the first char.
1366 dynamic_dname() helper function is provided to take care of
1367 this.
1368
1369 Example :
1370
1371.. code-block:: c
1372
1373 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1374 {
1375 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1376 dentry->d_inode->i_ino);
1377 }
1378
1379``d_automount``
1380 called when an automount dentry is to be traversed (optional).
1381 This should create a new VFS mount record and return the record
1382 to the caller. The caller is supplied with a path parameter
1383 giving the automount directory to describe the automount target
1384 and the parent VFS mount record to provide inheritable mount
1385 parameters. NULL should be returned if someone else managed to
1386 make the automount first. If the vfsmount creation failed, then
1387 an error code should be returned. If -EISDIR is returned, then
1388 the directory will be treated as an ordinary directory and
1389 returned to pathwalk to continue walking.
1390
1391 If a vfsmount is returned, the caller will attempt to mount it
1392 on the mountpoint and will remove the vfsmount from its
1393 expiration list in the case of failure. The vfsmount should be
1394 returned with 2 refs on it to prevent automatic expiration - the
1395 caller will clean up the additional ref.
1396
1397 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1398 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1399 set on the inode being added.
1400
1401``d_manage``
1402 called to allow the filesystem to manage the transition from a
1403 dentry (optional). This allows autofs, for example, to hold up
1404 clients waiting to explore behind a 'mountpoint' while letting
1405 the daemon go past and construct the subtree there. 0 should be
1406 returned to let the calling process continue. -EISDIR can be
1407 returned to tell pathwalk to use this directory as an ordinary
1408 directory and to ignore anything mounted on it and not to check
1409 the automount flag. Any other error code will abort pathwalk
1410 completely.
1411
1412 If the 'rcu_walk' parameter is true, then the caller is doing a
1413 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1414 mode, and the caller can be asked to leave it and call again by
1415 returning -ECHILD. -EISDIR may also be returned to tell
1416 pathwalk to ignore d_automount or any mounts.
1417
1418 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1419 the dentry being transited from.
1420
1421``d_real``
1422 overlay/union type filesystems implement this method to return
1423 one of the underlying dentries hidden by the overlay. It is
1424 used in two different modes:
1425
1426 Called from file_dentry() it returns the real dentry matching
1427 the inode argument. The real dentry may be from a lower layer
1428 already copied up, but still referenced from the file. This
1429 mode is selected with a non-NULL inode argument.
1430
1431 With NULL inode the topmost real underlying dentry is returned.
1432
1433Each dentry has a pointer to its parent dentry, as well as a hash list
1434of child dentries. Child dentries are basically like files in a
1435directory.
1436
1437
1438Directory Entry Cache API
1439--------------------------
1440
1441There are a number of functions defined which permit a filesystem to
1442manipulate dentries:
1443
1444``dget``
1445 open a new handle for an existing dentry (this just increments
1446 the usage count)
1447
1448``dput``
1449 close a handle for a dentry (decrements the usage count). If
1450 the usage count drops to 0, and the dentry is still in its
1451 parent's hash, the "d_delete" method is called to check whether
1452 it should be cached. If it should not be cached, or if the
1453 dentry is not hashed, it is deleted. Otherwise cached dentries
1454 are put into an LRU list to be reclaimed on memory shortage.
1455
1456``d_drop``
1457 this unhashes a dentry from its parents hash list. A subsequent
1458 call to dput() will deallocate the dentry if its usage count
1459 drops to 0
1460
1461``d_delete``
1462 delete a dentry. If there are no other open references to the
1463 dentry then the dentry is turned into a negative dentry (the
1464 d_iput() method is called). If there are other references, then
1465 d_drop() is called instead
1466
1467``d_add``
1468 add a dentry to its parents hash list and then calls
1469 d_instantiate()
1470
1471``d_instantiate``
1472 add a dentry to the alias hash list for the inode and updates
1473 the "d_inode" member. The "i_count" member in the inode
1474 structure should be set/incremented. If the inode pointer is
1475 NULL, the dentry is called a "negative dentry". This function
1476 is commonly called when an inode is created for an existing
1477 negative dentry
1478
1479``d_lookup``
1480 look up a dentry given its parent and path name component It
1481 looks up the child of that given name from the dcache hash
1482 table. If it is found, the reference count is incremented and
1483 the dentry is returned. The caller must use dput() to free the
1484 dentry when it finishes using it.
1485
1486
1487Mount Options
1488=============
1489
1490
1491Parsing options
1492---------------
1493
1494On mount and remount the filesystem is passed a string containing a
1495comma separated list of mount options. The options can have either of
1496these forms:
1497
1498 option
1499 option=value
1500
1501The <linux/parser.h> header defines an API that helps parse these
1502options. There are plenty of examples on how to use it in existing
1503filesystems.
1504
1505
1506Showing options
1507---------------
1508
1509If a filesystem accepts mount options, it must define show_options() to
1510show all the currently active options. The rules are:
1511
1512 - options MUST be shown which are not default or their values differ
1513 from the default
1514
1515 - options MAY be shown which are enabled by default or have their
1516 default value
1517
1518Options used only internally between a mount helper and the kernel (such
1519as file descriptors), or which only have an effect during the mounting
1520(such as ones controlling the creation of a journal) are exempt from the
1521above rules.
1522
1523The underlying reason for the above rules is to make sure, that a mount
1524can be accurately replicated (e.g. umounting and mounting again) based
1525on the information found in /proc/mounts.
1526
1527
1528Resources
1529=========
1530
1531(Note some of these resources are not up-to-date with the latest kernel
1532 version.)
1533
1534Creating Linux virtual filesystems. 2002
1535 <https://lwn.net/Articles/13325/>
1536
1537The Linux Virtual File-system Layer by Neil Brown. 1999
1538 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1539
1540A tour of the Linux VFS by Michael K. Johnson. 1996
1541 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1542
1543A small trail through the Linux kernel by Andries Brouwer. 2001
1544 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>