Loading...
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_type {
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 when an inode is marked dirty.
274 This is specifically for the inode itself being marked dirty,
275 not its data. If the update needs to be persisted by fdatasync(),
276 then I_DIRTY_DATASYNC will be set in the flags argument.
277 I_DIRTY_TIME will be set in the flags in case lazytime is enabled
278 and struct inode has times updated since the last ->dirty_inode
279 call.
280
281``write_inode``
282 this method is called when the VFS needs to write an inode to
283 disc. The second parameter indicates whether the write should
284 be synchronous or not, not all filesystems check this flag.
285
286``drop_inode``
287 called when the last access to the inode is dropped, with the
288 inode->i_lock spinlock held.
289
290 This method should be either NULL (normal UNIX filesystem
291 semantics) or "generic_delete_inode" (for filesystems that do
292 not want to cache inodes - causing "delete_inode" to always be
293 called regardless of the value of i_nlink)
294
295 The "generic_delete_inode()" behavior is equivalent to the old
296 practice of using "force_delete" in the put_inode() case, but
297 does not have the races that the "force_delete()" approach had.
298
299``delete_inode``
300 called when the VFS wants to delete an inode
301
302``put_super``
303 called when the VFS wishes to free the superblock
304 (i.e. unmount). This is called with the superblock lock held
305
306``sync_fs``
307 called when VFS is writing out all dirty data associated with a
308 superblock. The second parameter indicates whether the method
309 should wait until the write out has been completed. Optional.
310
311``freeze_fs``
312 called when VFS is locking a filesystem and forcing it into a
313 consistent state. This method is currently used by the Logical
314 Volume Manager (LVM).
315
316``unfreeze_fs``
317 called when VFS is unlocking a filesystem and making it writable
318 again.
319
320``statfs``
321 called when the VFS needs to get filesystem statistics.
322
323``remount_fs``
324 called when the filesystem is remounted. This is called with
325 the kernel lock held
326
327``clear_inode``
328 called then the VFS clears the inode. Optional
329
330``umount_begin``
331 called when the VFS is unmounting a filesystem.
332
333``show_options``
334 called by the VFS to show mount options for /proc/<pid>/mounts.
335 (see "Mount Options" section)
336
337``quota_read``
338 called by the VFS to read from filesystem quota file.
339
340``quota_write``
341 called by the VFS to write to filesystem quota file.
342
343``nr_cached_objects``
344 called by the sb cache shrinking function for the filesystem to
345 return the number of freeable cached objects it contains.
346 Optional.
347
348``free_cache_objects``
349 called by the sb cache shrinking function for the filesystem to
350 scan the number of objects indicated to try to free them.
351 Optional, but any filesystem implementing this method needs to
352 also implement ->nr_cached_objects for it to be called
353 correctly.
354
355 We can't do anything with any errors that the filesystem might
356 encountered, hence the void return type. This will never be
357 called if the VM is trying to reclaim under GFP_NOFS conditions,
358 hence this method does not need to handle that situation itself.
359
360 Implementations must include conditional reschedule calls inside
361 any scanning loop that is done. This allows the VFS to
362 determine appropriate scan batch sizes without having to worry
363 about whether implementations will cause holdoff problems due to
364 large scan batch sizes.
365
366Whoever sets up the inode is responsible for filling in the "i_op"
367field. This is a pointer to a "struct inode_operations" which describes
368the methods that can be performed on individual inodes.
369
370
371struct xattr_handlers
372---------------------
373
374On filesystems that support extended attributes (xattrs), the s_xattr
375superblock field points to a NULL-terminated array of xattr handlers.
376Extended attributes are name:value pairs.
377
378``name``
379 Indicates that the handler matches attributes with the specified
380 name (such as "system.posix_acl_access"); the prefix field must
381 be NULL.
382
383``prefix``
384 Indicates that the handler matches all attributes with the
385 specified name prefix (such as "user."); the name field must be
386 NULL.
387
388``list``
389 Determine if attributes matching this xattr handler should be
390 listed for a particular dentry. Used by some listxattr
391 implementations like generic_listxattr.
392
393``get``
394 Called by the VFS to get the value of a particular extended
395 attribute. This method is called by the getxattr(2) system
396 call.
397
398``set``
399 Called by the VFS to set the value of a particular extended
400 attribute. When the new value is NULL, called to remove a
401 particular extended attribute. This method is called by the
402 setxattr(2) and removexattr(2) system calls.
403
404When none of the xattr handlers of a filesystem match the specified
405attribute name or when a filesystem doesn't support extended attributes,
406the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
407
408
409The Inode Object
410================
411
412An inode object represents an object within the filesystem.
413
414
415struct inode_operations
416-----------------------
417
418This describes how the VFS can manipulate an inode in your filesystem.
419As of kernel 2.6.22, the following members are defined:
420
421.. code-block:: c
422
423 struct inode_operations {
424 int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
425 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
426 int (*link) (struct dentry *,struct inode *,struct dentry *);
427 int (*unlink) (struct inode *,struct dentry *);
428 int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
429 int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
430 int (*rmdir) (struct inode *,struct dentry *);
431 int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
432 int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
433 struct inode *, struct dentry *, unsigned int);
434 int (*readlink) (struct dentry *, char __user *,int);
435 const char *(*get_link) (struct dentry *, struct inode *,
436 struct delayed_call *);
437 int (*permission) (struct user_namespace *, struct inode *, int);
438 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
439 int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
440 int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
441 ssize_t (*listxattr) (struct dentry *, char *, size_t);
442 void (*update_time)(struct inode *, struct timespec *, int);
443 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
444 unsigned open_flag, umode_t create_mode);
445 int (*tmpfile) (struct user_namespace *, struct inode *, struct file *, umode_t);
446 struct posix_acl * (*get_acl)(struct user_namespace *, struct dentry *, int);
447 int (*set_acl)(struct user_namespace *, struct dentry *, struct posix_acl *, int);
448 int (*fileattr_set)(struct user_namespace *mnt_userns,
449 struct dentry *dentry, struct fileattr *fa);
450 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
451 };
452
453Again, all methods are called without any locks being held, unless
454otherwise noted.
455
456``create``
457 called by the open(2) and creat(2) system calls. Only required
458 if you want to support regular files. The dentry you get should
459 not have an inode (i.e. it should be a negative dentry). Here
460 you will probably call d_instantiate() with the dentry and the
461 newly created inode
462
463``lookup``
464 called when the VFS needs to look up an inode in a parent
465 directory. The name to look for is found in the dentry. This
466 method must call d_add() to insert the found inode into the
467 dentry. The "i_count" field in the inode structure should be
468 incremented. If the named inode does not exist a NULL inode
469 should be inserted into the dentry (this is called a negative
470 dentry). Returning an error code from this routine must only be
471 done on a real error, otherwise creating inodes with system
472 calls like create(2), mknod(2), mkdir(2) and so on will fail.
473 If you wish to overload the dentry methods then you should
474 initialise the "d_dop" field in the dentry; this is a pointer to
475 a struct "dentry_operations". This method is called with the
476 directory inode semaphore held
477
478``link``
479 called by the link(2) system call. Only required if you want to
480 support hard links. You will probably need to call
481 d_instantiate() just as you would in the create() method
482
483``unlink``
484 called by the unlink(2) system call. Only required if you want
485 to support deleting inodes
486
487``symlink``
488 called by the symlink(2) system call. Only required if you want
489 to support symlinks. You will probably need to call
490 d_instantiate() just as you would in the create() method
491
492``mkdir``
493 called by the mkdir(2) system call. Only required if you want
494 to support creating subdirectories. You will probably need to
495 call d_instantiate() just as you would in the create() method
496
497``rmdir``
498 called by the rmdir(2) system call. Only required if you want
499 to support deleting subdirectories
500
501``mknod``
502 called by the mknod(2) system call to create a device (char,
503 block) inode or a named pipe (FIFO) or socket. Only required if
504 you want to support creating these types of inodes. You will
505 probably need to call d_instantiate() just as you would in the
506 create() method
507
508``rename``
509 called by the rename(2) system call to rename the object to have
510 the parent and name given by the second inode and dentry.
511
512 The filesystem must return -EINVAL for any unsupported or
513 unknown flags. Currently the following flags are implemented:
514 (1) RENAME_NOREPLACE: this flag indicates that if the target of
515 the rename exists the rename should fail with -EEXIST instead of
516 replacing the target. The VFS already checks for existence, so
517 for local filesystems the RENAME_NOREPLACE implementation is
518 equivalent to plain rename.
519 (2) RENAME_EXCHANGE: exchange source and target. Both must
520 exist; this is checked by the VFS. Unlike plain rename, source
521 and target may be of different type.
522
523``get_link``
524 called by the VFS to follow a symbolic link to the inode it
525 points to. Only required if you want to support symbolic links.
526 This method returns the symlink body to traverse (and possibly
527 resets the current position with nd_jump_link()). If the body
528 won't go away until the inode is gone, nothing else is needed;
529 if it needs to be otherwise pinned, arrange for its release by
530 having get_link(..., ..., done) do set_delayed_call(done,
531 destructor, argument). In that case destructor(argument) will
532 be called once VFS is done with the body you've returned. May
533 be called in RCU mode; that is indicated by NULL dentry
534 argument. If request can't be handled without leaving RCU mode,
535 have it return ERR_PTR(-ECHILD).
536
537 If the filesystem stores the symlink target in ->i_link, the
538 VFS may use it directly without calling ->get_link(); however,
539 ->get_link() must still be provided. ->i_link must not be
540 freed until after an RCU grace period. Writing to ->i_link
541 post-iget() time requires a 'release' memory barrier.
542
543``readlink``
544 this is now just an override for use by readlink(2) for the
545 cases when ->get_link uses nd_jump_link() or object is not in
546 fact a symlink. Normally filesystems should only implement
547 ->get_link for symlinks and readlink(2) will automatically use
548 that.
549
550``permission``
551 called by the VFS to check for access rights on a POSIX-like
552 filesystem.
553
554 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
555 rcu-walk mode, the filesystem must check the permission without
556 blocking or storing to the inode.
557
558 If a situation is encountered that rcu-walk cannot handle,
559 return
560 -ECHILD and it will be called again in ref-walk mode.
561
562``setattr``
563 called by the VFS to set attributes for a file. This method is
564 called by chmod(2) and related system calls.
565
566``getattr``
567 called by the VFS to get attributes of a file. This method is
568 called by stat(2) and related system calls.
569
570``listxattr``
571 called by the VFS to list all extended attributes for a given
572 file. This method is called by the listxattr(2) system call.
573
574``update_time``
575 called by the VFS to update a specific time or the i_version of
576 an inode. If this is not defined the VFS will update the inode
577 itself and call mark_inode_dirty_sync.
578
579``atomic_open``
580 called on the last component of an open. Using this optional
581 method the filesystem can look up, possibly create and open the
582 file in one atomic operation. If it wants to leave actual
583 opening to the caller (e.g. if the file turned out to be a
584 symlink, device, or just something filesystem won't do atomic
585 open for), it may signal this by returning finish_no_open(file,
586 dentry). This method is only called if the last component is
587 negative or needs lookup. Cached positive dentries are still
588 handled by f_op->open(). If the file was created, FMODE_CREATED
589 flag should be set in file->f_mode. In case of O_EXCL the
590 method must only succeed if the file didn't exist and hence
591 FMODE_CREATED shall always be set on success.
592
593``tmpfile``
594 called in the end of O_TMPFILE open(). Optional, equivalent to
595 atomically creating, opening and unlinking a file in given
596 directory. On success needs to return with the file already
597 open; this can be done by calling finish_open_simple() right at
598 the end.
599
600``fileattr_get``
601 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
602 retrieve miscellaneous file flags and attributes. Also called
603 before the relevant SET operation to check what is being changed
604 (in this case with i_rwsem locked exclusive). If unset, then
605 fall back to f_op->ioctl().
606
607``fileattr_set``
608 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
609 change miscellaneous file flags and attributes. Callers hold
610 i_rwsem exclusive. If unset, then fall back to f_op->ioctl().
611
612
613The Address Space Object
614========================
615
616The address space object is used to group and manage pages in the page
617cache. It can be used to keep track of the pages in a file (or anything
618else) and also track the mapping of sections of the file into process
619address spaces.
620
621There are a number of distinct yet related services that an
622address-space can provide. These include communicating memory pressure,
623page lookup by address, and keeping track of pages tagged as Dirty or
624Writeback.
625
626The first can be used independently to the others. The VM can try to
627either write dirty pages in order to clean them, or release clean pages
628in order to reuse them. To do this it can call the ->writepage method
629on dirty pages, and ->release_folio on clean folios with the private
630flag set. Clean pages without PagePrivate and with no external references
631will be released without notice being given to the address_space.
632
633To achieve this functionality, pages need to be placed on an LRU with
634lru_cache_add and mark_page_active needs to be called whenever the page
635is used.
636
637Pages are normally kept in a radix tree index by ->index. This tree
638maintains information about the PG_Dirty and PG_Writeback status of each
639page, so that pages with either of these flags can be found quickly.
640
641The Dirty tag is primarily used by mpage_writepages - the default
642->writepages method. It uses the tag to find dirty pages to call
643->writepage on. If mpage_writepages is not used (i.e. the address
644provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
645unused. write_inode_now and sync_inode do use it (through
646__sync_single_inode) to check if ->writepages has been successful in
647writing out the whole address_space.
648
649The Writeback tag is used by filemap*wait* and sync_page* functions, via
650filemap_fdatawait_range, to wait for all writeback to complete.
651
652An address_space handler may attach extra information to a page,
653typically using the 'private' field in the 'struct page'. If such
654information is attached, the PG_Private flag should be set. This will
655cause various VM routines to make extra calls into the address_space
656handler to deal with that data.
657
658An address space acts as an intermediate between storage and
659application. Data is read into the address space a whole page at a
660time, and provided to the application either by copying of the page, or
661by memory-mapping the page. Data is written into the address space by
662the application, and then written-back to storage typically in whole
663pages, however the address_space has finer control of write sizes.
664
665The read process essentially only requires 'read_folio'. The write
666process is more complicated and uses write_begin/write_end or
667dirty_folio to write data into the address_space, and writepage and
668writepages to writeback data to storage.
669
670Adding and removing pages to/from an address_space is protected by the
671inode's i_mutex.
672
673When data is written to a page, the PG_Dirty flag should be set. It
674typically remains set until writepage asks for it to be written. This
675should clear PG_Dirty and set PG_Writeback. It can be actually written
676at any point after PG_Dirty is clear. Once it is known to be safe,
677PG_Writeback is cleared.
678
679Writeback makes use of a writeback_control structure to direct the
680operations. This gives the writepage and writepages operations some
681information about the nature of and reason for the writeback request,
682and the constraints under which it is being done. It is also used to
683return information back to the caller about the result of a writepage or
684writepages request.
685
686
687Handling errors during writeback
688--------------------------------
689
690Most applications that do buffered I/O will periodically call a file
691synchronization call (fsync, fdatasync, msync or sync_file_range) to
692ensure that data written has made it to the backing store. When there
693is an error during writeback, they expect that error to be reported when
694a file sync request is made. After an error has been reported on one
695request, subsequent requests on the same file descriptor should return
6960, unless further writeback errors have occurred since the previous file
697syncronization.
698
699Ideally, the kernel would report errors only on file descriptions on
700which writes were done that subsequently failed to be written back. The
701generic pagecache infrastructure does not track the file descriptions
702that have dirtied each individual page however, so determining which
703file descriptors should get back an error is not possible.
704
705Instead, the generic writeback error tracking infrastructure in the
706kernel settles for reporting errors to fsync on all file descriptions
707that were open at the time that the error occurred. In a situation with
708multiple writers, all of them will get back an error on a subsequent
709fsync, even if all of the writes done through that particular file
710descriptor succeeded (or even if there were no writes on that file
711descriptor at all).
712
713Filesystems that wish to use this infrastructure should call
714mapping_set_error to record the error in the address_space when it
715occurs. Then, after writing back data from the pagecache in their
716file->fsync operation, they should call file_check_and_advance_wb_err to
717ensure that the struct file's error cursor has advanced to the correct
718point in the stream of errors emitted by the backing device(s).
719
720
721struct address_space_operations
722-------------------------------
723
724This describes how the VFS can manipulate mapping of a file to page
725cache in your filesystem. The following members are defined:
726
727.. code-block:: c
728
729 struct address_space_operations {
730 int (*writepage)(struct page *page, struct writeback_control *wbc);
731 int (*read_folio)(struct file *, struct folio *);
732 int (*writepages)(struct address_space *, struct writeback_control *);
733 bool (*dirty_folio)(struct address_space *, struct folio *);
734 void (*readahead)(struct readahead_control *);
735 int (*write_begin)(struct file *, struct address_space *mapping,
736 loff_t pos, unsigned len,
737 struct page **pagep, void **fsdata);
738 int (*write_end)(struct file *, struct address_space *mapping,
739 loff_t pos, unsigned len, unsigned copied,
740 struct page *page, void *fsdata);
741 sector_t (*bmap)(struct address_space *, sector_t);
742 void (*invalidate_folio) (struct folio *, size_t start, size_t len);
743 bool (*release_folio)(struct folio *, gfp_t);
744 void (*free_folio)(struct folio *);
745 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
746 int (*migrate_folio)(struct mapping *, struct folio *dst,
747 struct folio *src, enum migrate_mode);
748 int (*launder_folio) (struct folio *);
749
750 bool (*is_partially_uptodate) (struct folio *, size_t from,
751 size_t count);
752 void (*is_dirty_writeback)(struct folio *, bool *, bool *);
753 int (*error_remove_page) (struct mapping *mapping, struct page *page);
754 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
755 int (*swap_deactivate)(struct file *);
756 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
757 };
758
759``writepage``
760 called by the VM to write a dirty page to backing store. This
761 may happen for data integrity reasons (i.e. 'sync'), or to free
762 up memory (flush). The difference can be seen in
763 wbc->sync_mode. The PG_Dirty flag has been cleared and
764 PageLocked is true. writepage should start writeout, should set
765 PG_Writeback, and should make sure the page is unlocked, either
766 synchronously or asynchronously when the write operation
767 completes.
768
769 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
770 try too hard if there are problems, and may choose to write out
771 other pages from the mapping if that is easier (e.g. due to
772 internal dependencies). If it chooses not to start writeout, it
773 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
774 keep calling ->writepage on that page.
775
776 See the file "Locking" for more details.
777
778``read_folio``
779 Called by the page cache to read a folio from the backing store.
780 The 'file' argument supplies authentication information to network
781 filesystems, and is generally not used by block based filesystems.
782 It may be NULL if the caller does not have an open file (eg if
783 the kernel is performing a read for itself rather than on behalf
784 of a userspace process with an open file).
785
786 If the mapping does not support large folios, the folio will
787 contain a single page. The folio will be locked when read_folio
788 is called. If the read completes successfully, the folio should
789 be marked uptodate. The filesystem should unlock the folio
790 once the read has completed, whether it was successful or not.
791 The filesystem does not need to modify the refcount on the folio;
792 the page cache holds a reference count and that will not be
793 released until the folio is unlocked.
794
795 Filesystems may implement ->read_folio() synchronously.
796 In normal operation, folios are read through the ->readahead()
797 method. Only if this fails, or if the caller needs to wait for
798 the read to complete will the page cache call ->read_folio().
799 Filesystems should not attempt to perform their own readahead
800 in the ->read_folio() operation.
801
802 If the filesystem cannot perform the read at this time, it can
803 unlock the folio, do whatever action it needs to ensure that the
804 read will succeed in the future and return AOP_TRUNCATED_PAGE.
805 In this case, the caller should look up the folio, lock it,
806 and call ->read_folio again.
807
808 Callers may invoke the ->read_folio() method directly, but using
809 read_mapping_folio() will take care of locking, waiting for the
810 read to complete and handle cases such as AOP_TRUNCATED_PAGE.
811
812``writepages``
813 called by the VM to write out pages associated with the
814 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
815 the writeback_control will specify a range of pages that must be
816 written out. If it is WB_SYNC_NONE, then a nr_to_write is
817 given and that many pages should be written if possible. If no
818 ->writepages is given, then mpage_writepages is used instead.
819 This will choose pages from the address space that are tagged as
820 DIRTY and will pass them to ->writepage.
821
822``dirty_folio``
823 called by the VM to mark a folio as dirty. This is particularly
824 needed if an address space attaches private data to a folio, and
825 that data needs to be updated when a folio is dirtied. This is
826 called, for example, when a memory mapped page gets modified.
827 If defined, it should set the folio dirty flag, and the
828 PAGECACHE_TAG_DIRTY search mark in i_pages.
829
830``readahead``
831 Called by the VM to read pages associated with the address_space
832 object. The pages are consecutive in the page cache and are
833 locked. The implementation should decrement the page refcount
834 after starting I/O on each page. Usually the page will be
835 unlocked by the I/O completion handler. The set of pages are
836 divided into some sync pages followed by some async pages,
837 rac->ra->async_size gives the number of async pages. The
838 filesystem should attempt to read all sync pages but may decide
839 to stop once it reaches the async pages. If it does decide to
840 stop attempting I/O, it can simply return. The caller will
841 remove the remaining pages from the address space, unlock them
842 and decrement the page refcount. Set PageUptodate if the I/O
843 completes successfully. Setting PageError on any page will be
844 ignored; simply unlock the page if an I/O error occurs.
845
846``write_begin``
847 Called by the generic buffered write code to ask the filesystem
848 to prepare to write len bytes at the given offset in the file.
849 The address_space should check that the write will be able to
850 complete, by allocating space if necessary and doing any other
851 internal housekeeping. If the write will update parts of any
852 basic-blocks on storage, then those blocks should be pre-read
853 (if they haven't been read already) so that the updated blocks
854 can be written out properly.
855
856 The filesystem must return the locked pagecache page for the
857 specified offset, in ``*pagep``, for the caller to write into.
858
859 It must be able to cope with short writes (where the length
860 passed to write_begin is greater than the number of bytes copied
861 into the page).
862
863 A void * may be returned in fsdata, which then gets passed into
864 write_end.
865
866 Returns 0 on success; < 0 on failure (which is the error code),
867 in which case write_end is not called.
868
869``write_end``
870 After a successful write_begin, and data copy, write_end must be
871 called. len is the original len passed to write_begin, and
872 copied is the amount that was able to be copied.
873
874 The filesystem must take care of unlocking the page and
875 releasing it refcount, and updating i_size.
876
877 Returns < 0 on failure, otherwise the number of bytes (<=
878 'copied') that were able to be copied into pagecache.
879
880``bmap``
881 called by the VFS to map a logical block offset within object to
882 physical block number. This method is used by the FIBMAP ioctl
883 and for working with swap-files. To be able to swap to a file,
884 the file must have a stable mapping to a block device. The swap
885 system does not go through the filesystem but instead uses bmap
886 to find out where the blocks in the file are and uses those
887 addresses directly.
888
889``invalidate_folio``
890 If a folio has private data, then invalidate_folio will be
891 called when part or all of the folio is to be removed from the
892 address space. This generally corresponds to either a
893 truncation, punch hole or a complete invalidation of the address
894 space (in the latter case 'offset' will always be 0 and 'length'
895 will be folio_size()). Any private data associated with the folio
896 should be updated to reflect this truncation. If offset is 0
897 and length is folio_size(), then the private data should be
898 released, because the folio must be able to be completely
899 discarded. This may be done by calling the ->release_folio
900 function, but in this case the release MUST succeed.
901
902``release_folio``
903 release_folio is called on folios with private data to tell the
904 filesystem that the folio is about to be freed. ->release_folio
905 should remove any private data from the folio and clear the
906 private flag. If release_folio() fails, it should return false.
907 release_folio() is used in two distinct though related cases.
908 The first is when the VM wants to free a clean folio with no
909 active users. If ->release_folio succeeds, the folio will be
910 removed from the address_space and be freed.
911
912 The second case is when a request has been made to invalidate
913 some or all folios in an address_space. This can happen
914 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
915 filesystem explicitly requesting it as nfs and 9p do (when they
916 believe the cache may be out of date with storage) by calling
917 invalidate_inode_pages2(). If the filesystem makes such a call,
918 and needs to be certain that all folios are invalidated, then
919 its release_folio will need to ensure this. Possibly it can
920 clear the uptodate flag if it cannot free private data yet.
921
922``free_folio``
923 free_folio is called once the folio is no longer visible in the
924 page cache in order to allow the cleanup of any private data.
925 Since it may be called by the memory reclaimer, it should not
926 assume that the original address_space mapping still exists, and
927 it should not block.
928
929``direct_IO``
930 called by the generic read/write routines to perform direct_IO -
931 that is IO requests which bypass the page cache and transfer
932 data directly between the storage and the application's address
933 space.
934
935``migrate_folio``
936 This is used to compact the physical memory usage. If the VM
937 wants to relocate a folio (maybe from a memory device that is
938 signalling imminent failure) it will pass a new folio and an old
939 folio to this function. migrate_folio should transfer any private
940 data across and update any references that it has to the folio.
941
942``launder_folio``
943 Called before freeing a folio - it writes back the dirty folio.
944 To prevent redirtying the folio, it is kept locked during the
945 whole operation.
946
947``is_partially_uptodate``
948 Called by the VM when reading a file through the pagecache when
949 the underlying blocksize is smaller than the size of the folio.
950 If the required block is up to date then the read can complete
951 without needing I/O to bring the whole page up to date.
952
953``is_dirty_writeback``
954 Called by the VM when attempting to reclaim a folio. The VM uses
955 dirty and writeback information to determine if it needs to
956 stall to allow flushers a chance to complete some IO.
957 Ordinarily it can use folio_test_dirty and folio_test_writeback but
958 some filesystems have more complex state (unstable folios in NFS
959 prevent reclaim) or do not set those flags due to locking
960 problems. This callback allows a filesystem to indicate to the
961 VM if a folio should be treated as dirty or writeback for the
962 purposes of stalling.
963
964``error_remove_page``
965 normally set to generic_error_remove_page if truncation is ok
966 for this address space. Used for memory failure handling.
967 Setting this implies you deal with pages going away under you,
968 unless you have them locked or reference counts increased.
969
970``swap_activate``
971
972 Called to prepare the given file for swap. It should perform
973 any validation and preparation necessary to ensure that writes
974 can be performed with minimal memory allocation. It should call
975 add_swap_extent(), or the helper iomap_swapfile_activate(), and
976 return the number of extents added. If IO should be submitted
977 through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
978 be submitted directly to the block device ``sis->bdev``.
979
980``swap_deactivate``
981 Called during swapoff on files where swap_activate was
982 successful.
983
984``swap_rw``
985 Called to read or write swap pages when SWP_FS_OPS is set.
986
987The File Object
988===============
989
990A file object represents a file opened by a process. This is also known
991as an "open file description" in POSIX parlance.
992
993
994struct file_operations
995----------------------
996
997This describes how the VFS can manipulate an open file. As of kernel
9984.18, the following members are defined:
999
1000.. code-block:: c
1001
1002 struct file_operations {
1003 struct module *owner;
1004 loff_t (*llseek) (struct file *, loff_t, int);
1005 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1006 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1007 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1008 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1009 int (*iopoll)(struct kiocb *kiocb, bool spin);
1010 int (*iterate) (struct file *, struct dir_context *);
1011 int (*iterate_shared) (struct file *, struct dir_context *);
1012 __poll_t (*poll) (struct file *, struct poll_table_struct *);
1013 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1014 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1015 int (*mmap) (struct file *, struct vm_area_struct *);
1016 int (*open) (struct inode *, struct file *);
1017 int (*flush) (struct file *, fl_owner_t id);
1018 int (*release) (struct inode *, struct file *);
1019 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1020 int (*fasync) (int, struct file *, int);
1021 int (*lock) (struct file *, int, struct file_lock *);
1022 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1023 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1024 int (*check_flags)(int);
1025 int (*flock) (struct file *, int, struct file_lock *);
1026 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1027 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1028 int (*setlease)(struct file *, long, struct file_lock **, void **);
1029 long (*fallocate)(struct file *file, int mode, loff_t offset,
1030 loff_t len);
1031 void (*show_fdinfo)(struct seq_file *m, struct file *f);
1032 #ifndef CONFIG_MMU
1033 unsigned (*mmap_capabilities)(struct file *);
1034 #endif
1035 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1036 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1037 struct file *file_out, loff_t pos_out,
1038 loff_t len, unsigned int remap_flags);
1039 int (*fadvise)(struct file *, loff_t, loff_t, int);
1040 };
1041
1042Again, all methods are called without any locks being held, unless
1043otherwise noted.
1044
1045``llseek``
1046 called when the VFS needs to move the file position index
1047
1048``read``
1049 called by read(2) and related system calls
1050
1051``read_iter``
1052 possibly asynchronous read with iov_iter as destination
1053
1054``write``
1055 called by write(2) and related system calls
1056
1057``write_iter``
1058 possibly asynchronous write with iov_iter as source
1059
1060``iopoll``
1061 called when aio wants to poll for completions on HIPRI iocbs
1062
1063``iterate``
1064 called when the VFS needs to read the directory contents
1065
1066``iterate_shared``
1067 called when the VFS needs to read the directory contents when
1068 filesystem supports concurrent dir iterators
1069
1070``poll``
1071 called by the VFS when a process wants to check if there is
1072 activity on this file and (optionally) go to sleep until there
1073 is activity. Called by the select(2) and poll(2) system calls
1074
1075``unlocked_ioctl``
1076 called by the ioctl(2) system call.
1077
1078``compat_ioctl``
1079 called by the ioctl(2) system call when 32 bit system calls are
1080 used on 64 bit kernels.
1081
1082``mmap``
1083 called by the mmap(2) system call
1084
1085``open``
1086 called by the VFS when an inode should be opened. When the VFS
1087 opens a file, it creates a new "struct file". It then calls the
1088 open method for the newly allocated file structure. You might
1089 think that the open method really belongs in "struct
1090 inode_operations", and you may be right. I think it's done the
1091 way it is because it makes filesystems simpler to implement.
1092 The open() method is a good place to initialize the
1093 "private_data" member in the file structure if you want to point
1094 to a device structure
1095
1096``flush``
1097 called by the close(2) system call to flush a file
1098
1099``release``
1100 called when the last reference to an open file is closed
1101
1102``fsync``
1103 called by the fsync(2) system call. Also see the section above
1104 entitled "Handling errors during writeback".
1105
1106``fasync``
1107 called by the fcntl(2) system call when asynchronous
1108 (non-blocking) mode is enabled for a file
1109
1110``lock``
1111 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1112 F_SETLKW commands
1113
1114``get_unmapped_area``
1115 called by the mmap(2) system call
1116
1117``check_flags``
1118 called by the fcntl(2) system call for F_SETFL command
1119
1120``flock``
1121 called by the flock(2) system call
1122
1123``splice_write``
1124 called by the VFS to splice data from a pipe to a file. This
1125 method is used by the splice(2) system call
1126
1127``splice_read``
1128 called by the VFS to splice data from file to a pipe. This
1129 method is used by the splice(2) system call
1130
1131``setlease``
1132 called by the VFS to set or release a file lock lease. setlease
1133 implementations should call generic_setlease to record or remove
1134 the lease in the inode after setting it.
1135
1136``fallocate``
1137 called by the VFS to preallocate blocks or punch a hole.
1138
1139``copy_file_range``
1140 called by the copy_file_range(2) system call.
1141
1142``remap_file_range``
1143 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1144 and FIDEDUPERANGE commands to remap file ranges. An
1145 implementation should remap len bytes at pos_in of the source
1146 file into the dest file at pos_out. Implementations must handle
1147 callers passing in len == 0; this means "remap to the end of the
1148 source file". The return value should the number of bytes
1149 remapped, or the usual negative error code if errors occurred
1150 before any bytes were remapped. The remap_flags parameter
1151 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1152 implementation must only remap if the requested file ranges have
1153 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
1154 ok with the implementation shortening the request length to
1155 satisfy alignment or EOF requirements (or any other reason).
1156
1157``fadvise``
1158 possibly called by the fadvise64() system call.
1159
1160Note that the file operations are implemented by the specific
1161filesystem in which the inode resides. When opening a device node
1162(character or block special) most filesystems will call special
1163support routines in the VFS which will locate the required device
1164driver information. These support routines replace the filesystem file
1165operations with those for the device driver, and then proceed to call
1166the new open() method for the file. This is how opening a device file
1167in the filesystem eventually ends up calling the device driver open()
1168method.
1169
1170
1171Directory Entry Cache (dcache)
1172==============================
1173
1174
1175struct dentry_operations
1176------------------------
1177
1178This describes how a filesystem can overload the standard dentry
1179operations. Dentries and the dcache are the domain of the VFS and the
1180individual filesystem implementations. Device drivers have no business
1181here. These methods may be set to NULL, as they are either optional or
1182the VFS uses a default. As of kernel 2.6.22, the following members are
1183defined:
1184
1185.. code-block:: c
1186
1187 struct dentry_operations {
1188 int (*d_revalidate)(struct dentry *, unsigned int);
1189 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1190 int (*d_hash)(const struct dentry *, struct qstr *);
1191 int (*d_compare)(const struct dentry *,
1192 unsigned int, const char *, const struct qstr *);
1193 int (*d_delete)(const struct dentry *);
1194 int (*d_init)(struct dentry *);
1195 void (*d_release)(struct dentry *);
1196 void (*d_iput)(struct dentry *, struct inode *);
1197 char *(*d_dname)(struct dentry *, char *, int);
1198 struct vfsmount *(*d_automount)(struct path *);
1199 int (*d_manage)(const struct path *, bool);
1200 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1201 };
1202
1203``d_revalidate``
1204 called when the VFS needs to revalidate a dentry. This is
1205 called whenever a name look-up finds a dentry in the dcache.
1206 Most local filesystems leave this as NULL, because all their
1207 dentries in the dcache are valid. Network filesystems are
1208 different since things can change on the server without the
1209 client necessarily being aware of it.
1210
1211 This function should return a positive value if the dentry is
1212 still valid, and zero or a negative error code if it isn't.
1213
1214 d_revalidate may be called in rcu-walk mode (flags &
1215 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1216 revalidate the dentry without blocking or storing to the dentry,
1217 d_parent and d_inode should not be used without care (because
1218 they can change and, in d_inode case, even become NULL under
1219 us).
1220
1221 If a situation is encountered that rcu-walk cannot handle,
1222 return
1223 -ECHILD and it will be called again in ref-walk mode.
1224
1225``_weak_revalidate``
1226 called when the VFS needs to revalidate a "jumped" dentry. This
1227 is called when a path-walk ends at dentry that was not acquired
1228 by doing a lookup in the parent directory. This includes "/",
1229 "." and "..", as well as procfs-style symlinks and mountpoint
1230 traversal.
1231
1232 In this case, we are less concerned with whether the dentry is
1233 still fully correct, but rather that the inode is still valid.
1234 As with d_revalidate, most local filesystems will set this to
1235 NULL since their dcache entries are always valid.
1236
1237 This function has the same return code semantics as
1238 d_revalidate.
1239
1240 d_weak_revalidate is only called after leaving rcu-walk mode.
1241
1242``d_hash``
1243 called when the VFS adds a dentry to the hash table. The first
1244 dentry passed to d_hash is the parent directory that the name is
1245 to be hashed into.
1246
1247 Same locking and synchronisation rules as d_compare regarding
1248 what is safe to dereference etc.
1249
1250``d_compare``
1251 called to compare a dentry name with a given name. The first
1252 dentry is the parent of the dentry to be compared, the second is
1253 the child dentry. len and name string are properties of the
1254 dentry to be compared. qstr is the name to compare it with.
1255
1256 Must be constant and idempotent, and should not take locks if
1257 possible, and should not or store into the dentry. Should not
1258 dereference pointers outside the dentry without lots of care
1259 (eg. d_parent, d_inode, d_name should not be used).
1260
1261 However, our vfsmount is pinned, and RCU held, so the dentries
1262 and inodes won't disappear, neither will our sb or filesystem
1263 module. ->d_sb may be used.
1264
1265 It is a tricky calling convention because it needs to be called
1266 under "rcu-walk", ie. without any locks or references on things.
1267
1268``d_delete``
1269 called when the last reference to a dentry is dropped and the
1270 dcache is deciding whether or not to cache it. Return 1 to
1271 delete immediately, or 0 to cache the dentry. Default is NULL
1272 which means to always cache a reachable dentry. d_delete must
1273 be constant and idempotent.
1274
1275``d_init``
1276 called when a dentry is allocated
1277
1278``d_release``
1279 called when a dentry is really deallocated
1280
1281``d_iput``
1282 called when a dentry loses its inode (just prior to its being
1283 deallocated). The default when this is NULL is that the VFS
1284 calls iput(). If you define this method, you must call iput()
1285 yourself
1286
1287``d_dname``
1288 called when the pathname of a dentry should be generated.
1289 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1290 delay pathname generation. (Instead of doing it when dentry is
1291 created, it's done only when the path is needed.). Real
1292 filesystems probably dont want to use it, because their dentries
1293 are present in global dcache hash, so their hash should be an
1294 invariant. As no lock is held, d_dname() should not try to
1295 modify the dentry itself, unless appropriate SMP safety is used.
1296 CAUTION : d_path() logic is quite tricky. The correct way to
1297 return for example "Hello" is to put it at the end of the
1298 buffer, and returns a pointer to the first char.
1299 dynamic_dname() helper function is provided to take care of
1300 this.
1301
1302 Example :
1303
1304.. code-block:: c
1305
1306 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1307 {
1308 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1309 dentry->d_inode->i_ino);
1310 }
1311
1312``d_automount``
1313 called when an automount dentry is to be traversed (optional).
1314 This should create a new VFS mount record and return the record
1315 to the caller. The caller is supplied with a path parameter
1316 giving the automount directory to describe the automount target
1317 and the parent VFS mount record to provide inheritable mount
1318 parameters. NULL should be returned if someone else managed to
1319 make the automount first. If the vfsmount creation failed, then
1320 an error code should be returned. If -EISDIR is returned, then
1321 the directory will be treated as an ordinary directory and
1322 returned to pathwalk to continue walking.
1323
1324 If a vfsmount is returned, the caller will attempt to mount it
1325 on the mountpoint and will remove the vfsmount from its
1326 expiration list in the case of failure. The vfsmount should be
1327 returned with 2 refs on it to prevent automatic expiration - the
1328 caller will clean up the additional ref.
1329
1330 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1331 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1332 set on the inode being added.
1333
1334``d_manage``
1335 called to allow the filesystem to manage the transition from a
1336 dentry (optional). This allows autofs, for example, to hold up
1337 clients waiting to explore behind a 'mountpoint' while letting
1338 the daemon go past and construct the subtree there. 0 should be
1339 returned to let the calling process continue. -EISDIR can be
1340 returned to tell pathwalk to use this directory as an ordinary
1341 directory and to ignore anything mounted on it and not to check
1342 the automount flag. Any other error code will abort pathwalk
1343 completely.
1344
1345 If the 'rcu_walk' parameter is true, then the caller is doing a
1346 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1347 mode, and the caller can be asked to leave it and call again by
1348 returning -ECHILD. -EISDIR may also be returned to tell
1349 pathwalk to ignore d_automount or any mounts.
1350
1351 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1352 the dentry being transited from.
1353
1354``d_real``
1355 overlay/union type filesystems implement this method to return
1356 one of the underlying dentries hidden by the overlay. It is
1357 used in two different modes:
1358
1359 Called from file_dentry() it returns the real dentry matching
1360 the inode argument. The real dentry may be from a lower layer
1361 already copied up, but still referenced from the file. This
1362 mode is selected with a non-NULL inode argument.
1363
1364 With NULL inode the topmost real underlying dentry is returned.
1365
1366Each dentry has a pointer to its parent dentry, as well as a hash list
1367of child dentries. Child dentries are basically like files in a
1368directory.
1369
1370
1371Directory Entry Cache API
1372--------------------------
1373
1374There are a number of functions defined which permit a filesystem to
1375manipulate dentries:
1376
1377``dget``
1378 open a new handle for an existing dentry (this just increments
1379 the usage count)
1380
1381``dput``
1382 close a handle for a dentry (decrements the usage count). If
1383 the usage count drops to 0, and the dentry is still in its
1384 parent's hash, the "d_delete" method is called to check whether
1385 it should be cached. If it should not be cached, or if the
1386 dentry is not hashed, it is deleted. Otherwise cached dentries
1387 are put into an LRU list to be reclaimed on memory shortage.
1388
1389``d_drop``
1390 this unhashes a dentry from its parents hash list. A subsequent
1391 call to dput() will deallocate the dentry if its usage count
1392 drops to 0
1393
1394``d_delete``
1395 delete a dentry. If there are no other open references to the
1396 dentry then the dentry is turned into a negative dentry (the
1397 d_iput() method is called). If there are other references, then
1398 d_drop() is called instead
1399
1400``d_add``
1401 add a dentry to its parents hash list and then calls
1402 d_instantiate()
1403
1404``d_instantiate``
1405 add a dentry to the alias hash list for the inode and updates
1406 the "d_inode" member. The "i_count" member in the inode
1407 structure should be set/incremented. If the inode pointer is
1408 NULL, the dentry is called a "negative dentry". This function
1409 is commonly called when an inode is created for an existing
1410 negative dentry
1411
1412``d_lookup``
1413 look up a dentry given its parent and path name component It
1414 looks up the child of that given name from the dcache hash
1415 table. If it is found, the reference count is incremented and
1416 the dentry is returned. The caller must use dput() to free the
1417 dentry when it finishes using it.
1418
1419
1420Mount Options
1421=============
1422
1423
1424Parsing options
1425---------------
1426
1427On mount and remount the filesystem is passed a string containing a
1428comma separated list of mount options. The options can have either of
1429these forms:
1430
1431 option
1432 option=value
1433
1434The <linux/parser.h> header defines an API that helps parse these
1435options. There are plenty of examples on how to use it in existing
1436filesystems.
1437
1438
1439Showing options
1440---------------
1441
1442If a filesystem accepts mount options, it must define show_options() to
1443show all the currently active options. The rules are:
1444
1445 - options MUST be shown which are not default or their values differ
1446 from the default
1447
1448 - options MAY be shown which are enabled by default or have their
1449 default value
1450
1451Options used only internally between a mount helper and the kernel (such
1452as file descriptors), or which only have an effect during the mounting
1453(such as ones controlling the creation of a journal) are exempt from the
1454above rules.
1455
1456The underlying reason for the above rules is to make sure, that a mount
1457can be accurately replicated (e.g. umounting and mounting again) based
1458on the information found in /proc/mounts.
1459
1460
1461Resources
1462=========
1463
1464(Note some of these resources are not up-to-date with the latest kernel
1465 version.)
1466
1467Creating Linux virtual filesystems. 2002
1468 <https://lwn.net/Articles/13325/>
1469
1470The Linux Virtual File-system Layer by Neil Brown. 1999
1471 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1472
1473A tour of the Linux VFS by Michael K. Johnson. 1996
1474 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1475
1476A small trail through the Linux kernel by Andries Brouwer. 2001
1477 <https://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. As of kernel 2.6.39, 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 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 when an inode is marked dirty.
274 This is specifically for the inode itself being marked dirty,
275 not its data. If the update needs to be persisted by fdatasync(),
276 then I_DIRTY_DATASYNC will be set in the flags argument.
277
278``write_inode``
279 this method is called when the VFS needs to write an inode to
280 disc. The second parameter indicates whether the write should
281 be synchronous or not, not all filesystems check this flag.
282
283``drop_inode``
284 called when the last access to the inode is dropped, with the
285 inode->i_lock spinlock held.
286
287 This method should be either NULL (normal UNIX filesystem
288 semantics) or "generic_delete_inode" (for filesystems that do
289 not want to cache inodes - causing "delete_inode" to always be
290 called regardless of the value of i_nlink)
291
292 The "generic_delete_inode()" behavior is equivalent to the old
293 practice of using "force_delete" in the put_inode() case, but
294 does not have the races that the "force_delete()" approach had.
295
296``delete_inode``
297 called when the VFS wants to delete an inode
298
299``put_super``
300 called when the VFS wishes to free the superblock
301 (i.e. unmount). This is called with the superblock lock held
302
303``sync_fs``
304 called when VFS is writing out all dirty data associated with a
305 superblock. The second parameter indicates whether the method
306 should wait until the write out has been completed. Optional.
307
308``freeze_fs``
309 called when VFS is locking a filesystem and forcing it into a
310 consistent state. This method is currently used by the Logical
311 Volume Manager (LVM).
312
313``unfreeze_fs``
314 called when VFS is unlocking a filesystem and making it writable
315 again.
316
317``statfs``
318 called when the VFS needs to get filesystem statistics.
319
320``remount_fs``
321 called when the filesystem is remounted. This is called with
322 the kernel lock held
323
324``clear_inode``
325 called then the VFS clears the inode. Optional
326
327``umount_begin``
328 called when the VFS is unmounting a filesystem.
329
330``show_options``
331 called by the VFS to show mount options for /proc/<pid>/mounts.
332 (see "Mount Options" section)
333
334``quota_read``
335 called by the VFS to read from filesystem quota file.
336
337``quota_write``
338 called by the VFS to write to filesystem quota file.
339
340``nr_cached_objects``
341 called by the sb cache shrinking function for the filesystem to
342 return the number of freeable cached objects it contains.
343 Optional.
344
345``free_cache_objects``
346 called by the sb cache shrinking function for the filesystem to
347 scan the number of objects indicated to try to free them.
348 Optional, but any filesystem implementing this method needs to
349 also implement ->nr_cached_objects for it to be called
350 correctly.
351
352 We can't do anything with any errors that the filesystem might
353 encountered, hence the void return type. This will never be
354 called if the VM is trying to reclaim under GFP_NOFS conditions,
355 hence this method does not need to handle that situation itself.
356
357 Implementations must include conditional reschedule calls inside
358 any scanning loop that is done. This allows the VFS to
359 determine appropriate scan batch sizes without having to worry
360 about whether implementations will cause holdoff problems due to
361 large scan batch sizes.
362
363Whoever sets up the inode is responsible for filling in the "i_op"
364field. This is a pointer to a "struct inode_operations" which describes
365the methods that can be performed on individual inodes.
366
367
368struct xattr_handlers
369---------------------
370
371On filesystems that support extended attributes (xattrs), the s_xattr
372superblock field points to a NULL-terminated array of xattr handlers.
373Extended attributes are name:value pairs.
374
375``name``
376 Indicates that the handler matches attributes with the specified
377 name (such as "system.posix_acl_access"); the prefix field must
378 be NULL.
379
380``prefix``
381 Indicates that the handler matches all attributes with the
382 specified name prefix (such as "user."); the name field must be
383 NULL.
384
385``list``
386 Determine if attributes matching this xattr handler should be
387 listed for a particular dentry. Used by some listxattr
388 implementations like generic_listxattr.
389
390``get``
391 Called by the VFS to get the value of a particular extended
392 attribute. This method is called by the getxattr(2) system
393 call.
394
395``set``
396 Called by the VFS to set the value of a particular extended
397 attribute. When the new value is NULL, called to remove a
398 particular extended attribute. This method is called by the
399 setxattr(2) and removexattr(2) system calls.
400
401When none of the xattr handlers of a filesystem match the specified
402attribute name or when a filesystem doesn't support extended attributes,
403the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
404
405
406The Inode Object
407================
408
409An inode object represents an object within the filesystem.
410
411
412struct inode_operations
413-----------------------
414
415This describes how the VFS can manipulate an inode in your filesystem.
416As of kernel 2.6.22, the following members are defined:
417
418.. code-block:: c
419
420 struct inode_operations {
421 int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
422 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
423 int (*link) (struct dentry *,struct inode *,struct dentry *);
424 int (*unlink) (struct inode *,struct dentry *);
425 int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
426 int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
427 int (*rmdir) (struct inode *,struct dentry *);
428 int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
429 int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
430 struct inode *, struct dentry *, unsigned int);
431 int (*readlink) (struct dentry *, char __user *,int);
432 const char *(*get_link) (struct dentry *, struct inode *,
433 struct delayed_call *);
434 int (*permission) (struct user_namespace *, struct inode *, int);
435 int (*get_acl)(struct inode *, int);
436 int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
437 int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
438 ssize_t (*listxattr) (struct dentry *, char *, size_t);
439 void (*update_time)(struct inode *, struct timespec *, int);
440 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
441 unsigned open_flag, umode_t create_mode);
442 int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t);
443 int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
444 int (*fileattr_set)(struct user_namespace *mnt_userns,
445 struct dentry *dentry, struct fileattr *fa);
446 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
447 };
448
449Again, all methods are called without any locks being held, unless
450otherwise noted.
451
452``create``
453 called by the open(2) and creat(2) system calls. Only required
454 if you want to support regular files. The dentry you get should
455 not have an inode (i.e. it should be a negative dentry). Here
456 you will probably call d_instantiate() with the dentry and the
457 newly created inode
458
459``lookup``
460 called when the VFS needs to look up an inode in a parent
461 directory. The name to look for is found in the dentry. This
462 method must call d_add() to insert the found inode into the
463 dentry. The "i_count" field in the inode structure should be
464 incremented. If the named inode does not exist a NULL inode
465 should be inserted into the dentry (this is called a negative
466 dentry). Returning an error code from this routine must only be
467 done on a real error, otherwise creating inodes with system
468 calls like create(2), mknod(2), mkdir(2) and so on will fail.
469 If you wish to overload the dentry methods then you should
470 initialise the "d_dop" field in the dentry; this is a pointer to
471 a struct "dentry_operations". This method is called with the
472 directory inode semaphore held
473
474``link``
475 called by the link(2) system call. Only required if you want to
476 support hard links. You will probably need to call
477 d_instantiate() just as you would in the create() method
478
479``unlink``
480 called by the unlink(2) system call. Only required if you want
481 to support deleting inodes
482
483``symlink``
484 called by the symlink(2) system call. Only required if you want
485 to support symlinks. You will probably need to call
486 d_instantiate() just as you would in the create() method
487
488``mkdir``
489 called by the mkdir(2) system call. Only required if you want
490 to support creating subdirectories. You will probably need to
491 call d_instantiate() just as you would in the create() method
492
493``rmdir``
494 called by the rmdir(2) system call. Only required if you want
495 to support deleting subdirectories
496
497``mknod``
498 called by the mknod(2) system call to create a device (char,
499 block) inode or a named pipe (FIFO) or socket. Only required if
500 you want to support creating these types of inodes. You will
501 probably need to call d_instantiate() just as you would in the
502 create() method
503
504``rename``
505 called by the rename(2) system call to rename the object to have
506 the parent and name given by the second inode and dentry.
507
508 The filesystem must return -EINVAL for any unsupported or
509 unknown flags. Currently the following flags are implemented:
510 (1) RENAME_NOREPLACE: this flag indicates that if the target of
511 the rename exists the rename should fail with -EEXIST instead of
512 replacing the target. The VFS already checks for existence, so
513 for local filesystems the RENAME_NOREPLACE implementation is
514 equivalent to plain rename.
515 (2) RENAME_EXCHANGE: exchange source and target. Both must
516 exist; this is checked by the VFS. Unlike plain rename, source
517 and target may be of different type.
518
519``get_link``
520 called by the VFS to follow a symbolic link to the inode it
521 points to. Only required if you want to support symbolic links.
522 This method returns the symlink body to traverse (and possibly
523 resets the current position with nd_jump_link()). If the body
524 won't go away until the inode is gone, nothing else is needed;
525 if it needs to be otherwise pinned, arrange for its release by
526 having get_link(..., ..., done) do set_delayed_call(done,
527 destructor, argument). In that case destructor(argument) will
528 be called once VFS is done with the body you've returned. May
529 be called in RCU mode; that is indicated by NULL dentry
530 argument. If request can't be handled without leaving RCU mode,
531 have it return ERR_PTR(-ECHILD).
532
533 If the filesystem stores the symlink target in ->i_link, the
534 VFS may use it directly without calling ->get_link(); however,
535 ->get_link() must still be provided. ->i_link must not be
536 freed until after an RCU grace period. Writing to ->i_link
537 post-iget() time requires a 'release' memory barrier.
538
539``readlink``
540 this is now just an override for use by readlink(2) for the
541 cases when ->get_link uses nd_jump_link() or object is not in
542 fact a symlink. Normally filesystems should only implement
543 ->get_link for symlinks and readlink(2) will automatically use
544 that.
545
546``permission``
547 called by the VFS to check for access rights on a POSIX-like
548 filesystem.
549
550 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
551 rcu-walk mode, the filesystem must check the permission without
552 blocking or storing to the inode.
553
554 If a situation is encountered that rcu-walk cannot handle,
555 return
556 -ECHILD and it will be called again in ref-walk mode.
557
558``setattr``
559 called by the VFS to set attributes for a file. This method is
560 called by chmod(2) and related system calls.
561
562``getattr``
563 called by the VFS to get attributes of a file. This method is
564 called by stat(2) and related system calls.
565
566``listxattr``
567 called by the VFS to list all extended attributes for a given
568 file. This method is called by the listxattr(2) system call.
569
570``update_time``
571 called by the VFS to update a specific time or the i_version of
572 an inode. If this is not defined the VFS will update the inode
573 itself and call mark_inode_dirty_sync.
574
575``atomic_open``
576 called on the last component of an open. Using this optional
577 method the filesystem can look up, possibly create and open the
578 file in one atomic operation. If it wants to leave actual
579 opening to the caller (e.g. if the file turned out to be a
580 symlink, device, or just something filesystem won't do atomic
581 open for), it may signal this by returning finish_no_open(file,
582 dentry). This method is only called if the last component is
583 negative or needs lookup. Cached positive dentries are still
584 handled by f_op->open(). If the file was created, FMODE_CREATED
585 flag should be set in file->f_mode. In case of O_EXCL the
586 method must only succeed if the file didn't exist and hence
587 FMODE_CREATED shall always be set on success.
588
589``tmpfile``
590 called in the end of O_TMPFILE open(). Optional, equivalent to
591 atomically creating, opening and unlinking a file in given
592 directory.
593
594``fileattr_get``
595 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
596 retrieve miscellaneous file flags and attributes. Also called
597 before the relevant SET operation to check what is being changed
598 (in this case with i_rwsem locked exclusive). If unset, then
599 fall back to f_op->ioctl().
600
601``fileattr_set``
602 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
603 change miscellaneous file flags and attributes. Callers hold
604 i_rwsem exclusive. If unset, then fall back to f_op->ioctl().
605
606
607The Address Space Object
608========================
609
610The address space object is used to group and manage pages in the page
611cache. It can be used to keep track of the pages in a file (or anything
612else) and also track the mapping of sections of the file into process
613address spaces.
614
615There are a number of distinct yet related services that an
616address-space can provide. These include communicating memory pressure,
617page lookup by address, and keeping track of pages tagged as Dirty or
618Writeback.
619
620The first can be used independently to the others. The VM can try to
621either write dirty pages in order to clean them, or release clean pages
622in order to reuse them. To do this it can call the ->writepage method
623on dirty pages, and ->releasepage on clean pages with PagePrivate set.
624Clean pages without PagePrivate and with no external references will be
625released without notice being given to the address_space.
626
627To achieve this functionality, pages need to be placed on an LRU with
628lru_cache_add and mark_page_active needs to be called whenever the page
629is used.
630
631Pages are normally kept in a radix tree index by ->index. This tree
632maintains information about the PG_Dirty and PG_Writeback status of each
633page, so that pages with either of these flags can be found quickly.
634
635The Dirty tag is primarily used by mpage_writepages - the default
636->writepages method. It uses the tag to find dirty pages to call
637->writepage on. If mpage_writepages is not used (i.e. the address
638provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
639unused. write_inode_now and sync_inode do use it (through
640__sync_single_inode) to check if ->writepages has been successful in
641writing out the whole address_space.
642
643The Writeback tag is used by filemap*wait* and sync_page* functions, via
644filemap_fdatawait_range, to wait for all writeback to complete.
645
646An address_space handler may attach extra information to a page,
647typically using the 'private' field in the 'struct page'. If such
648information is attached, the PG_Private flag should be set. This will
649cause various VM routines to make extra calls into the address_space
650handler to deal with that data.
651
652An address space acts as an intermediate between storage and
653application. Data is read into the address space a whole page at a
654time, and provided to the application either by copying of the page, or
655by memory-mapping the page. Data is written into the address space by
656the application, and then written-back to storage typically in whole
657pages, however the address_space has finer control of write sizes.
658
659The read process essentially only requires 'readpage'. The write
660process is more complicated and uses write_begin/write_end or
661set_page_dirty to write data into the address_space, and writepage and
662writepages to writeback data to storage.
663
664Adding and removing pages to/from an address_space is protected by the
665inode's i_mutex.
666
667When data is written to a page, the PG_Dirty flag should be set. It
668typically remains set until writepage asks for it to be written. This
669should clear PG_Dirty and set PG_Writeback. It can be actually written
670at any point after PG_Dirty is clear. Once it is known to be safe,
671PG_Writeback is cleared.
672
673Writeback makes use of a writeback_control structure to direct the
674operations. This gives the writepage and writepages operations some
675information about the nature of and reason for the writeback request,
676and the constraints under which it is being done. It is also used to
677return information back to the caller about the result of a writepage or
678writepages request.
679
680
681Handling errors during writeback
682--------------------------------
683
684Most applications that do buffered I/O will periodically call a file
685synchronization call (fsync, fdatasync, msync or sync_file_range) to
686ensure that data written has made it to the backing store. When there
687is an error during writeback, they expect that error to be reported when
688a file sync request is made. After an error has been reported on one
689request, subsequent requests on the same file descriptor should return
6900, unless further writeback errors have occurred since the previous file
691syncronization.
692
693Ideally, the kernel would report errors only on file descriptions on
694which writes were done that subsequently failed to be written back. The
695generic pagecache infrastructure does not track the file descriptions
696that have dirtied each individual page however, so determining which
697file descriptors should get back an error is not possible.
698
699Instead, the generic writeback error tracking infrastructure in the
700kernel settles for reporting errors to fsync on all file descriptions
701that were open at the time that the error occurred. In a situation with
702multiple writers, all of them will get back an error on a subsequent
703fsync, even if all of the writes done through that particular file
704descriptor succeeded (or even if there were no writes on that file
705descriptor at all).
706
707Filesystems that wish to use this infrastructure should call
708mapping_set_error to record the error in the address_space when it
709occurs. Then, after writing back data from the pagecache in their
710file->fsync operation, they should call file_check_and_advance_wb_err to
711ensure that the struct file's error cursor has advanced to the correct
712point in the stream of errors emitted by the backing device(s).
713
714
715struct address_space_operations
716-------------------------------
717
718This describes how the VFS can manipulate mapping of a file to page
719cache in your filesystem. The following members are defined:
720
721.. code-block:: c
722
723 struct address_space_operations {
724 int (*writepage)(struct page *page, struct writeback_control *wbc);
725 int (*readpage)(struct file *, struct page *);
726 int (*writepages)(struct address_space *, struct writeback_control *);
727 int (*set_page_dirty)(struct page *page);
728 void (*readahead)(struct readahead_control *);
729 int (*readpages)(struct file *filp, struct address_space *mapping,
730 struct list_head *pages, unsigned nr_pages);
731 int (*write_begin)(struct file *, struct address_space *mapping,
732 loff_t pos, unsigned len, unsigned flags,
733 struct page **pagep, void **fsdata);
734 int (*write_end)(struct file *, struct address_space *mapping,
735 loff_t pos, unsigned len, unsigned copied,
736 struct page *page, void *fsdata);
737 sector_t (*bmap)(struct address_space *, sector_t);
738 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
739 int (*releasepage) (struct page *, int);
740 void (*freepage)(struct page *);
741 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
742 /* isolate a page for migration */
743 bool (*isolate_page) (struct page *, isolate_mode_t);
744 /* migrate the contents of a page to the specified target */
745 int (*migratepage) (struct page *, struct page *);
746 /* put migration-failed page back to right list */
747 void (*putback_page) (struct page *);
748 int (*launder_page) (struct page *);
749
750 int (*is_partially_uptodate) (struct page *, unsigned long,
751 unsigned long);
752 void (*is_dirty_writeback) (struct page *, bool *, bool *);
753 int (*error_remove_page) (struct mapping *mapping, struct page *page);
754 int (*swap_activate)(struct file *);
755 int (*swap_deactivate)(struct file *);
756 };
757
758``writepage``
759 called by the VM to write a dirty page to backing store. This
760 may happen for data integrity reasons (i.e. 'sync'), or to free
761 up memory (flush). The difference can be seen in
762 wbc->sync_mode. The PG_Dirty flag has been cleared and
763 PageLocked is true. writepage should start writeout, should set
764 PG_Writeback, and should make sure the page is unlocked, either
765 synchronously or asynchronously when the write operation
766 completes.
767
768 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
769 try too hard if there are problems, and may choose to write out
770 other pages from the mapping if that is easier (e.g. due to
771 internal dependencies). If it chooses not to start writeout, it
772 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
773 keep calling ->writepage on that page.
774
775 See the file "Locking" for more details.
776
777``readpage``
778 called by the VM to read a page from backing store. The page
779 will be Locked when readpage is called, and should be unlocked
780 and marked uptodate once the read completes. If ->readpage
781 discovers that it needs to unlock the page for some reason, it
782 can do so, and then return AOP_TRUNCATED_PAGE. In this case,
783 the page will be relocated, relocked and if that all succeeds,
784 ->readpage will be called again.
785
786``writepages``
787 called by the VM to write out pages associated with the
788 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
789 the writeback_control will specify a range of pages that must be
790 written out. If it is WB_SYNC_NONE, then a nr_to_write is
791 given and that many pages should be written if possible. If no
792 ->writepages is given, then mpage_writepages is used instead.
793 This will choose pages from the address space that are tagged as
794 DIRTY and will pass them to ->writepage.
795
796``set_page_dirty``
797 called by the VM to set a page dirty. This is particularly
798 needed if an address space attaches private data to a page, and
799 that data needs to be updated when a page is dirtied. This is
800 called, for example, when a memory mapped page gets modified.
801 If defined, it should set the PageDirty flag, and the
802 PAGECACHE_TAG_DIRTY tag in the radix tree.
803
804``readahead``
805 Called by the VM to read pages associated with the address_space
806 object. The pages are consecutive in the page cache and are
807 locked. The implementation should decrement the page refcount
808 after starting I/O on each page. Usually the page will be
809 unlocked by the I/O completion handler. If the filesystem decides
810 to stop attempting I/O before reaching the end of the readahead
811 window, it can simply return. The caller will decrement the page
812 refcount and unlock the remaining pages for you. Set PageUptodate
813 if the I/O completes successfully. Setting PageError on any page
814 will be ignored; simply unlock the page if an I/O error occurs.
815
816``readpages``
817 called by the VM to read pages associated with the address_space
818 object. This is essentially just a vector version of readpage.
819 Instead of just one page, several pages are requested.
820 readpages is only used for read-ahead, so read errors are
821 ignored. If anything goes wrong, feel free to give up.
822 This interface is deprecated and will be removed by the end of
823 2020; implement readahead instead.
824
825``write_begin``
826 Called by the generic buffered write code to ask the filesystem
827 to prepare to write len bytes at the given offset in the file.
828 The address_space should check that the write will be able to
829 complete, by allocating space if necessary and doing any other
830 internal housekeeping. If the write will update parts of any
831 basic-blocks on storage, then those blocks should be pre-read
832 (if they haven't been read already) so that the updated blocks
833 can be written out properly.
834
835 The filesystem must return the locked pagecache page for the
836 specified offset, in ``*pagep``, for the caller to write into.
837
838 It must be able to cope with short writes (where the length
839 passed to write_begin is greater than the number of bytes copied
840 into the page).
841
842 flags is a field for AOP_FLAG_xxx flags, described in
843 include/linux/fs.h.
844
845 A void * may be returned in fsdata, which then gets passed into
846 write_end.
847
848 Returns 0 on success; < 0 on failure (which is the error code),
849 in which case write_end is not called.
850
851``write_end``
852 After a successful write_begin, and data copy, write_end must be
853 called. len is the original len passed to write_begin, and
854 copied is the amount that was able to be copied.
855
856 The filesystem must take care of unlocking the page and
857 releasing it refcount, and updating i_size.
858
859 Returns < 0 on failure, otherwise the number of bytes (<=
860 'copied') that were able to be copied into pagecache.
861
862``bmap``
863 called by the VFS to map a logical block offset within object to
864 physical block number. This method is used by the FIBMAP ioctl
865 and for working with swap-files. To be able to swap to a file,
866 the file must have a stable mapping to a block device. The swap
867 system does not go through the filesystem but instead uses bmap
868 to find out where the blocks in the file are and uses those
869 addresses directly.
870
871``invalidatepage``
872 If a page has PagePrivate set, then invalidatepage will be
873 called when part or all of the page is to be removed from the
874 address space. This generally corresponds to either a
875 truncation, punch hole or a complete invalidation of the address
876 space (in the latter case 'offset' will always be 0 and 'length'
877 will be PAGE_SIZE). Any private data associated with the page
878 should be updated to reflect this truncation. If offset is 0
879 and length is PAGE_SIZE, then the private data should be
880 released, because the page must be able to be completely
881 discarded. This may be done by calling the ->releasepage
882 function, but in this case the release MUST succeed.
883
884``releasepage``
885 releasepage is called on PagePrivate pages to indicate that the
886 page should be freed if possible. ->releasepage should remove
887 any private data from the page and clear the PagePrivate flag.
888 If releasepage() fails for some reason, it must indicate failure
889 with a 0 return value. releasepage() is used in two distinct
890 though related cases. The first is when the VM finds a clean
891 page with no active users and wants to make it a free page. If
892 ->releasepage succeeds, the page will be removed from the
893 address_space and become free.
894
895 The second case is when a request has been made to invalidate
896 some or all pages in an address_space. This can happen through
897 the fadvise(POSIX_FADV_DONTNEED) system call or by the
898 filesystem explicitly requesting it as nfs and 9fs do (when they
899 believe the cache may be out of date with storage) by calling
900 invalidate_inode_pages2(). If the filesystem makes such a call,
901 and needs to be certain that all pages are invalidated, then its
902 releasepage will need to ensure this. Possibly it can clear the
903 PageUptodate bit if it cannot free private data yet.
904
905``freepage``
906 freepage is called once the page is no longer visible in the
907 page cache in order to allow the cleanup of any private data.
908 Since it may be called by the memory reclaimer, it should not
909 assume that the original address_space mapping still exists, and
910 it should not block.
911
912``direct_IO``
913 called by the generic read/write routines to perform direct_IO -
914 that is IO requests which bypass the page cache and transfer
915 data directly between the storage and the application's address
916 space.
917
918``isolate_page``
919 Called by the VM when isolating a movable non-lru page. If page
920 is successfully isolated, VM marks the page as PG_isolated via
921 __SetPageIsolated.
922
923``migrate_page``
924 This is used to compact the physical memory usage. If the VM
925 wants to relocate a page (maybe off a memory card that is
926 signalling imminent failure) it will pass a new page and an old
927 page to this function. migrate_page should transfer any private
928 data across and update any references that it has to the page.
929
930``putback_page``
931 Called by the VM when isolated page's migration fails.
932
933``launder_page``
934 Called before freeing a page - it writes back the dirty page.
935 To prevent redirtying the page, it is kept locked during the
936 whole operation.
937
938``is_partially_uptodate``
939 Called by the VM when reading a file through the pagecache when
940 the underlying blocksize != pagesize. If the required block is
941 up to date then the read can complete without needing the IO to
942 bring the whole page up to date.
943
944``is_dirty_writeback``
945 Called by the VM when attempting to reclaim a page. The VM uses
946 dirty and writeback information to determine if it needs to
947 stall to allow flushers a chance to complete some IO.
948 Ordinarily it can use PageDirty and PageWriteback but some
949 filesystems have more complex state (unstable pages in NFS
950 prevent reclaim) or do not set those flags due to locking
951 problems. This callback allows a filesystem to indicate to the
952 VM if a page should be treated as dirty or writeback for the
953 purposes of stalling.
954
955``error_remove_page``
956 normally set to generic_error_remove_page if truncation is ok
957 for this address space. Used for memory failure handling.
958 Setting this implies you deal with pages going away under you,
959 unless you have them locked or reference counts increased.
960
961``swap_activate``
962 Called when swapon is used on a file to allocate space if
963 necessary and pin the block lookup information in memory. A
964 return value of zero indicates success, in which case this file
965 can be used to back swapspace.
966
967``swap_deactivate``
968 Called during swapoff on files where swap_activate was
969 successful.
970
971
972The File Object
973===============
974
975A file object represents a file opened by a process. This is also known
976as an "open file description" in POSIX parlance.
977
978
979struct file_operations
980----------------------
981
982This describes how the VFS can manipulate an open file. As of kernel
9834.18, the following members are defined:
984
985.. code-block:: c
986
987 struct file_operations {
988 struct module *owner;
989 loff_t (*llseek) (struct file *, loff_t, int);
990 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
991 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
992 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
993 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
994 int (*iopoll)(struct kiocb *kiocb, bool spin);
995 int (*iterate) (struct file *, struct dir_context *);
996 int (*iterate_shared) (struct file *, struct dir_context *);
997 __poll_t (*poll) (struct file *, struct poll_table_struct *);
998 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
999 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1000 int (*mmap) (struct file *, struct vm_area_struct *);
1001 int (*open) (struct inode *, struct file *);
1002 int (*flush) (struct file *, fl_owner_t id);
1003 int (*release) (struct inode *, struct file *);
1004 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1005 int (*fasync) (int, struct file *, int);
1006 int (*lock) (struct file *, int, struct file_lock *);
1007 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1008 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1009 int (*check_flags)(int);
1010 int (*flock) (struct file *, int, struct file_lock *);
1011 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1012 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1013 int (*setlease)(struct file *, long, struct file_lock **, void **);
1014 long (*fallocate)(struct file *file, int mode, loff_t offset,
1015 loff_t len);
1016 void (*show_fdinfo)(struct seq_file *m, struct file *f);
1017 #ifndef CONFIG_MMU
1018 unsigned (*mmap_capabilities)(struct file *);
1019 #endif
1020 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1021 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1022 struct file *file_out, loff_t pos_out,
1023 loff_t len, unsigned int remap_flags);
1024 int (*fadvise)(struct file *, loff_t, loff_t, int);
1025 };
1026
1027Again, all methods are called without any locks being held, unless
1028otherwise noted.
1029
1030``llseek``
1031 called when the VFS needs to move the file position index
1032
1033``read``
1034 called by read(2) and related system calls
1035
1036``read_iter``
1037 possibly asynchronous read with iov_iter as destination
1038
1039``write``
1040 called by write(2) and related system calls
1041
1042``write_iter``
1043 possibly asynchronous write with iov_iter as source
1044
1045``iopoll``
1046 called when aio wants to poll for completions on HIPRI iocbs
1047
1048``iterate``
1049 called when the VFS needs to read the directory contents
1050
1051``iterate_shared``
1052 called when the VFS needs to read the directory contents when
1053 filesystem supports concurrent dir iterators
1054
1055``poll``
1056 called by the VFS when a process wants to check if there is
1057 activity on this file and (optionally) go to sleep until there
1058 is activity. Called by the select(2) and poll(2) system calls
1059
1060``unlocked_ioctl``
1061 called by the ioctl(2) system call.
1062
1063``compat_ioctl``
1064 called by the ioctl(2) system call when 32 bit system calls are
1065 used on 64 bit kernels.
1066
1067``mmap``
1068 called by the mmap(2) system call
1069
1070``open``
1071 called by the VFS when an inode should be opened. When the VFS
1072 opens a file, it creates a new "struct file". It then calls the
1073 open method for the newly allocated file structure. You might
1074 think that the open method really belongs in "struct
1075 inode_operations", and you may be right. I think it's done the
1076 way it is because it makes filesystems simpler to implement.
1077 The open() method is a good place to initialize the
1078 "private_data" member in the file structure if you want to point
1079 to a device structure
1080
1081``flush``
1082 called by the close(2) system call to flush a file
1083
1084``release``
1085 called when the last reference to an open file is closed
1086
1087``fsync``
1088 called by the fsync(2) system call. Also see the section above
1089 entitled "Handling errors during writeback".
1090
1091``fasync``
1092 called by the fcntl(2) system call when asynchronous
1093 (non-blocking) mode is enabled for a file
1094
1095``lock``
1096 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1097 F_SETLKW commands
1098
1099``get_unmapped_area``
1100 called by the mmap(2) system call
1101
1102``check_flags``
1103 called by the fcntl(2) system call for F_SETFL command
1104
1105``flock``
1106 called by the flock(2) system call
1107
1108``splice_write``
1109 called by the VFS to splice data from a pipe to a file. This
1110 method is used by the splice(2) system call
1111
1112``splice_read``
1113 called by the VFS to splice data from file to a pipe. This
1114 method is used by the splice(2) system call
1115
1116``setlease``
1117 called by the VFS to set or release a file lock lease. setlease
1118 implementations should call generic_setlease to record or remove
1119 the lease in the inode after setting it.
1120
1121``fallocate``
1122 called by the VFS to preallocate blocks or punch a hole.
1123
1124``copy_file_range``
1125 called by the copy_file_range(2) system call.
1126
1127``remap_file_range``
1128 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1129 and FIDEDUPERANGE commands to remap file ranges. An
1130 implementation should remap len bytes at pos_in of the source
1131 file into the dest file at pos_out. Implementations must handle
1132 callers passing in len == 0; this means "remap to the end of the
1133 source file". The return value should the number of bytes
1134 remapped, or the usual negative error code if errors occurred
1135 before any bytes were remapped. The remap_flags parameter
1136 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1137 implementation must only remap if the requested file ranges have
1138 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
1139 ok with the implementation shortening the request length to
1140 satisfy alignment or EOF requirements (or any other reason).
1141
1142``fadvise``
1143 possibly called by the fadvise64() system call.
1144
1145Note that the file operations are implemented by the specific
1146filesystem in which the inode resides. When opening a device node
1147(character or block special) most filesystems will call special
1148support routines in the VFS which will locate the required device
1149driver information. These support routines replace the filesystem file
1150operations with those for the device driver, and then proceed to call
1151the new open() method for the file. This is how opening a device file
1152in the filesystem eventually ends up calling the device driver open()
1153method.
1154
1155
1156Directory Entry Cache (dcache)
1157==============================
1158
1159
1160struct dentry_operations
1161------------------------
1162
1163This describes how a filesystem can overload the standard dentry
1164operations. Dentries and the dcache are the domain of the VFS and the
1165individual filesystem implementations. Device drivers have no business
1166here. These methods may be set to NULL, as they are either optional or
1167the VFS uses a default. As of kernel 2.6.22, the following members are
1168defined:
1169
1170.. code-block:: c
1171
1172 struct dentry_operations {
1173 int (*d_revalidate)(struct dentry *, unsigned int);
1174 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1175 int (*d_hash)(const struct dentry *, struct qstr *);
1176 int (*d_compare)(const struct dentry *,
1177 unsigned int, const char *, const struct qstr *);
1178 int (*d_delete)(const struct dentry *);
1179 int (*d_init)(struct dentry *);
1180 void (*d_release)(struct dentry *);
1181 void (*d_iput)(struct dentry *, struct inode *);
1182 char *(*d_dname)(struct dentry *, char *, int);
1183 struct vfsmount *(*d_automount)(struct path *);
1184 int (*d_manage)(const struct path *, bool);
1185 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1186 };
1187
1188``d_revalidate``
1189 called when the VFS needs to revalidate a dentry. This is
1190 called whenever a name look-up finds a dentry in the dcache.
1191 Most local filesystems leave this as NULL, because all their
1192 dentries in the dcache are valid. Network filesystems are
1193 different since things can change on the server without the
1194 client necessarily being aware of it.
1195
1196 This function should return a positive value if the dentry is
1197 still valid, and zero or a negative error code if it isn't.
1198
1199 d_revalidate may be called in rcu-walk mode (flags &
1200 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1201 revalidate the dentry without blocking or storing to the dentry,
1202 d_parent and d_inode should not be used without care (because
1203 they can change and, in d_inode case, even become NULL under
1204 us).
1205
1206 If a situation is encountered that rcu-walk cannot handle,
1207 return
1208 -ECHILD and it will be called again in ref-walk mode.
1209
1210``_weak_revalidate``
1211 called when the VFS needs to revalidate a "jumped" dentry. This
1212 is called when a path-walk ends at dentry that was not acquired
1213 by doing a lookup in the parent directory. This includes "/",
1214 "." and "..", as well as procfs-style symlinks and mountpoint
1215 traversal.
1216
1217 In this case, we are less concerned with whether the dentry is
1218 still fully correct, but rather that the inode is still valid.
1219 As with d_revalidate, most local filesystems will set this to
1220 NULL since their dcache entries are always valid.
1221
1222 This function has the same return code semantics as
1223 d_revalidate.
1224
1225 d_weak_revalidate is only called after leaving rcu-walk mode.
1226
1227``d_hash``
1228 called when the VFS adds a dentry to the hash table. The first
1229 dentry passed to d_hash is the parent directory that the name is
1230 to be hashed into.
1231
1232 Same locking and synchronisation rules as d_compare regarding
1233 what is safe to dereference etc.
1234
1235``d_compare``
1236 called to compare a dentry name with a given name. The first
1237 dentry is the parent of the dentry to be compared, the second is
1238 the child dentry. len and name string are properties of the
1239 dentry to be compared. qstr is the name to compare it with.
1240
1241 Must be constant and idempotent, and should not take locks if
1242 possible, and should not or store into the dentry. Should not
1243 dereference pointers outside the dentry without lots of care
1244 (eg. d_parent, d_inode, d_name should not be used).
1245
1246 However, our vfsmount is pinned, and RCU held, so the dentries
1247 and inodes won't disappear, neither will our sb or filesystem
1248 module. ->d_sb may be used.
1249
1250 It is a tricky calling convention because it needs to be called
1251 under "rcu-walk", ie. without any locks or references on things.
1252
1253``d_delete``
1254 called when the last reference to a dentry is dropped and the
1255 dcache is deciding whether or not to cache it. Return 1 to
1256 delete immediately, or 0 to cache the dentry. Default is NULL
1257 which means to always cache a reachable dentry. d_delete must
1258 be constant and idempotent.
1259
1260``d_init``
1261 called when a dentry is allocated
1262
1263``d_release``
1264 called when a dentry is really deallocated
1265
1266``d_iput``
1267 called when a dentry loses its inode (just prior to its being
1268 deallocated). The default when this is NULL is that the VFS
1269 calls iput(). If you define this method, you must call iput()
1270 yourself
1271
1272``d_dname``
1273 called when the pathname of a dentry should be generated.
1274 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1275 delay pathname generation. (Instead of doing it when dentry is
1276 created, it's done only when the path is needed.). Real
1277 filesystems probably dont want to use it, because their dentries
1278 are present in global dcache hash, so their hash should be an
1279 invariant. As no lock is held, d_dname() should not try to
1280 modify the dentry itself, unless appropriate SMP safety is used.
1281 CAUTION : d_path() logic is quite tricky. The correct way to
1282 return for example "Hello" is to put it at the end of the
1283 buffer, and returns a pointer to the first char.
1284 dynamic_dname() helper function is provided to take care of
1285 this.
1286
1287 Example :
1288
1289.. code-block:: c
1290
1291 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1292 {
1293 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1294 dentry->d_inode->i_ino);
1295 }
1296
1297``d_automount``
1298 called when an automount dentry is to be traversed (optional).
1299 This should create a new VFS mount record and return the record
1300 to the caller. The caller is supplied with a path parameter
1301 giving the automount directory to describe the automount target
1302 and the parent VFS mount record to provide inheritable mount
1303 parameters. NULL should be returned if someone else managed to
1304 make the automount first. If the vfsmount creation failed, then
1305 an error code should be returned. If -EISDIR is returned, then
1306 the directory will be treated as an ordinary directory and
1307 returned to pathwalk to continue walking.
1308
1309 If a vfsmount is returned, the caller will attempt to mount it
1310 on the mountpoint and will remove the vfsmount from its
1311 expiration list in the case of failure. The vfsmount should be
1312 returned with 2 refs on it to prevent automatic expiration - the
1313 caller will clean up the additional ref.
1314
1315 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1316 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1317 set on the inode being added.
1318
1319``d_manage``
1320 called to allow the filesystem to manage the transition from a
1321 dentry (optional). This allows autofs, for example, to hold up
1322 clients waiting to explore behind a 'mountpoint' while letting
1323 the daemon go past and construct the subtree there. 0 should be
1324 returned to let the calling process continue. -EISDIR can be
1325 returned to tell pathwalk to use this directory as an ordinary
1326 directory and to ignore anything mounted on it and not to check
1327 the automount flag. Any other error code will abort pathwalk
1328 completely.
1329
1330 If the 'rcu_walk' parameter is true, then the caller is doing a
1331 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1332 mode, and the caller can be asked to leave it and call again by
1333 returning -ECHILD. -EISDIR may also be returned to tell
1334 pathwalk to ignore d_automount or any mounts.
1335
1336 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1337 the dentry being transited from.
1338
1339``d_real``
1340 overlay/union type filesystems implement this method to return
1341 one of the underlying dentries hidden by the overlay. It is
1342 used in two different modes:
1343
1344 Called from file_dentry() it returns the real dentry matching
1345 the inode argument. The real dentry may be from a lower layer
1346 already copied up, but still referenced from the file. This
1347 mode is selected with a non-NULL inode argument.
1348
1349 With NULL inode the topmost real underlying dentry is returned.
1350
1351Each dentry has a pointer to its parent dentry, as well as a hash list
1352of child dentries. Child dentries are basically like files in a
1353directory.
1354
1355
1356Directory Entry Cache API
1357--------------------------
1358
1359There are a number of functions defined which permit a filesystem to
1360manipulate dentries:
1361
1362``dget``
1363 open a new handle for an existing dentry (this just increments
1364 the usage count)
1365
1366``dput``
1367 close a handle for a dentry (decrements the usage count). If
1368 the usage count drops to 0, and the dentry is still in its
1369 parent's hash, the "d_delete" method is called to check whether
1370 it should be cached. If it should not be cached, or if the
1371 dentry is not hashed, it is deleted. Otherwise cached dentries
1372 are put into an LRU list to be reclaimed on memory shortage.
1373
1374``d_drop``
1375 this unhashes a dentry from its parents hash list. A subsequent
1376 call to dput() will deallocate the dentry if its usage count
1377 drops to 0
1378
1379``d_delete``
1380 delete a dentry. If there are no other open references to the
1381 dentry then the dentry is turned into a negative dentry (the
1382 d_iput() method is called). If there are other references, then
1383 d_drop() is called instead
1384
1385``d_add``
1386 add a dentry to its parents hash list and then calls
1387 d_instantiate()
1388
1389``d_instantiate``
1390 add a dentry to the alias hash list for the inode and updates
1391 the "d_inode" member. The "i_count" member in the inode
1392 structure should be set/incremented. If the inode pointer is
1393 NULL, the dentry is called a "negative dentry". This function
1394 is commonly called when an inode is created for an existing
1395 negative dentry
1396
1397``d_lookup``
1398 look up a dentry given its parent and path name component It
1399 looks up the child of that given name from the dcache hash
1400 table. If it is found, the reference count is incremented and
1401 the dentry is returned. The caller must use dput() to free the
1402 dentry when it finishes using it.
1403
1404
1405Mount Options
1406=============
1407
1408
1409Parsing options
1410---------------
1411
1412On mount and remount the filesystem is passed a string containing a
1413comma separated list of mount options. The options can have either of
1414these forms:
1415
1416 option
1417 option=value
1418
1419The <linux/parser.h> header defines an API that helps parse these
1420options. There are plenty of examples on how to use it in existing
1421filesystems.
1422
1423
1424Showing options
1425---------------
1426
1427If a filesystem accepts mount options, it must define show_options() to
1428show all the currently active options. The rules are:
1429
1430 - options MUST be shown which are not default or their values differ
1431 from the default
1432
1433 - options MAY be shown which are enabled by default or have their
1434 default value
1435
1436Options used only internally between a mount helper and the kernel (such
1437as file descriptors), or which only have an effect during the mounting
1438(such as ones controlling the creation of a journal) are exempt from the
1439above rules.
1440
1441The underlying reason for the above rules is to make sure, that a mount
1442can be accurately replicated (e.g. umounting and mounting again) based
1443on the information found in /proc/mounts.
1444
1445
1446Resources
1447=========
1448
1449(Note some of these resources are not up-to-date with the latest kernel
1450 version.)
1451
1452Creating Linux virtual filesystems. 2002
1453 <https://lwn.net/Articles/13325/>
1454
1455The Linux Virtual File-system Layer by Neil Brown. 1999
1456 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1457
1458A tour of the Linux VFS by Michael K. Johnson. 1996
1459 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1460
1461A small trail through the Linux kernel by Andries Brouwer. 2001
1462 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>