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1
2 Overview of the Linux Virtual File System
3
4 Original author: Richard Gooch <rgooch@atnf.csiro.au>
5
6 Last updated on June 24, 2007.
7
8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
10
11 This file is released under the GPLv2.
12
13
14Introduction
15============
16
17The Virtual File System (also known as the Virtual Filesystem Switch)
18is the software layer in the kernel that provides the filesystem
19interface to userspace programs. It also provides an abstraction
20within the kernel which allows different filesystem implementations to
21coexist.
22
23VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24on are called from a process context. Filesystem locking is described
25in the document Documentation/filesystems/Locking.
26
27
28Directory Entry Cache (dcache)
29------------------------------
30
31The VFS implements the open(2), stat(2), chmod(2), and similar system
32calls. The pathname argument that is passed to them is used by the VFS
33to search through the directory entry cache (also known as the dentry
34cache or dcache). This provides a very fast look-up mechanism to
35translate a pathname (filename) into a specific dentry. Dentries live
36in RAM and are never saved to disc: they exist only for performance.
37
38The dentry cache is meant to be a view into your entire filespace. As
39most computers cannot fit all dentries in the RAM at the same time,
40some bits of the cache are missing. In order to resolve your pathname
41into a dentry, the VFS may have to resort to creating dentries along
42the way, and then loading the inode. This is done by looking up the
43inode.
44
45
46The Inode Object
47----------------
48
49An individual dentry usually has a pointer to an inode. Inodes are
50filesystem objects such as regular files, directories, FIFOs and other
51beasts. They live either on the disc (for block device filesystems)
52or in the memory (for pseudo filesystems). Inodes that live on the
53disc are copied into the memory when required and changes to the inode
54are written back to disc. A single inode can be pointed to by multiple
55dentries (hard links, for example, do this).
56
57To look up an inode requires that the VFS calls the lookup() method of
58the parent directory inode. This method is installed by the specific
59filesystem implementation that the inode lives in. Once the VFS has
60the required dentry (and hence the inode), we can do all those boring
61things like open(2) the file, or stat(2) it to peek at the inode
62data. The stat(2) operation is fairly simple: once the VFS has the
63dentry, it peeks at the inode data and passes some of it back to
64userspace.
65
66
67The File Object
68---------------
69
70Opening a file requires another operation: allocation of a file
71structure (this is the kernel-side implementation of file
72descriptors). The freshly allocated file structure is initialized with
73a pointer to the dentry and a set of file operation member functions.
74These are taken from the inode data. The open() file method is then
75called so the specific filesystem implementation can do its work. You
76can see that this is another switch performed by the VFS. The file
77structure is placed into the file descriptor table for the process.
78
79Reading, writing and closing files (and other assorted VFS operations)
80is done by using the userspace file descriptor to grab the appropriate
81file structure, and then calling the required file structure method to
82do whatever is required. For as long as the file is open, it keeps the
83dentry in use, which in turn means that the VFS inode is still in use.
84
85
86Registering and Mounting a Filesystem
87=====================================
88
89To register and unregister a filesystem, use the following API
90functions:
91
92 #include <linux/fs.h>
93
94 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
96
97The passed struct file_system_type describes your filesystem. When a
98request is made to mount a filesystem onto a directory in your namespace,
99the VFS will call the appropriate mount() method for the specific
100filesystem. New vfsmount referring to the tree returned by ->mount()
101will be attached to the mountpoint, so that when pathname resolution
102reaches the mountpoint it will jump into the root of that vfsmount.
103
104You can see all filesystems that are registered to the kernel in the
105file /proc/filesystems.
106
107
108struct file_system_type
109-----------------------
110
111This describes the filesystem. As of kernel 2.6.39, the following
112members are defined:
113
114struct file_system_type {
115 const char *name;
116 int fs_flags;
117 struct dentry *(*mount) (struct file_system_type *, int,
118 const char *, void *);
119 void (*kill_sb) (struct super_block *);
120 struct module *owner;
121 struct file_system_type * next;
122 struct list_head fs_supers;
123 struct lock_class_key s_lock_key;
124 struct lock_class_key s_umount_key;
125};
126
127 name: the name of the filesystem type, such as "ext2", "iso9660",
128 "msdos" and so on
129
130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131
132 mount: the method to call when a new instance of this
133 filesystem should be mounted
134
135 kill_sb: the method to call when an instance of this filesystem
136 should be shut down
137
138 owner: for internal VFS use: you should initialize this to THIS_MODULE in
139 most cases.
140
141 next: for internal VFS use: you should initialize this to NULL
142
143 s_lock_key, s_umount_key: lockdep-specific
144
145The mount() method has the following arguments:
146
147 struct file_system_type *fs_type: describes the filesystem, partly initialized
148 by the specific filesystem code
149
150 int flags: mount flags
151
152 const char *dev_name: the device name we are mounting.
153
154 void *data: arbitrary mount options, usually comes as an ASCII
155 string (see "Mount Options" section)
156
157The mount() method must return the root dentry of the tree requested by
158caller. An active reference to its superblock must be grabbed and the
159superblock must be locked. On failure it should return ERR_PTR(error).
160
161The arguments match those of mount(2) and their interpretation
162depends on filesystem type. E.g. for block filesystems, dev_name is
163interpreted as block device name, that device is opened and if it
164contains a suitable filesystem image the method creates and initializes
165struct super_block accordingly, returning its root dentry to caller.
166
167->mount() may choose to return a subtree of existing filesystem - it
168doesn't have to create a new one. The main result from the caller's
169point of view is a reference to dentry at the root of (sub)tree to
170be attached; creation of new superblock is a common side effect.
171
172The most interesting member of the superblock structure that the
173mount() method fills in is the "s_op" field. This is a pointer to
174a "struct super_operations" which describes the next level of the
175filesystem implementation.
176
177Usually, a filesystem uses one of the generic mount() implementations
178and provides a fill_super() callback instead. The generic variants are:
179
180 mount_bdev: mount a filesystem residing on a block device
181
182 mount_nodev: mount a filesystem that is not backed by a device
183
184 mount_single: mount a filesystem which shares the instance between
185 all mounts
186
187A fill_super() callback implementation has the following arguments:
188
189 struct super_block *sb: the superblock structure. The callback
190 must initialize this properly.
191
192 void *data: arbitrary mount options, usually comes as an ASCII
193 string (see "Mount Options" section)
194
195 int silent: whether or not to be silent on error
196
197
198The Superblock Object
199=====================
200
201A superblock object represents a mounted filesystem.
202
203
204struct super_operations
205-----------------------
206
207This describes how the VFS can manipulate the superblock of your
208filesystem. As of kernel 2.6.22, the following members are defined:
209
210struct super_operations {
211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
213
214 void (*dirty_inode) (struct inode *, int flags);
215 int (*write_inode) (struct inode *, int);
216 void (*drop_inode) (struct inode *);
217 void (*delete_inode) (struct inode *);
218 void (*put_super) (struct super_block *);
219 int (*sync_fs)(struct super_block *sb, int wait);
220 int (*freeze_fs) (struct super_block *);
221 int (*unfreeze_fs) (struct super_block *);
222 int (*statfs) (struct dentry *, struct kstatfs *);
223 int (*remount_fs) (struct super_block *, int *, char *);
224 void (*clear_inode) (struct inode *);
225 void (*umount_begin) (struct super_block *);
226
227 int (*show_options)(struct seq_file *, struct dentry *);
228
229 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
231 int (*nr_cached_objects)(struct super_block *);
232 void (*free_cached_objects)(struct super_block *, int);
233};
234
235All methods are called without any locks being held, unless otherwise
236noted. This means that most methods can block safely. All methods are
237only called from a process context (i.e. not from an interrupt handler
238or bottom half).
239
240 alloc_inode: this method is called by alloc_inode() to allocate memory
241 for struct inode and initialize it. If this function is not
242 defined, a simple 'struct inode' is allocated. Normally
243 alloc_inode will be used to allocate a larger structure which
244 contains a 'struct inode' embedded within it.
245
246 destroy_inode: this method is called by destroy_inode() to release
247 resources allocated for struct inode. It is only required if
248 ->alloc_inode was defined and simply undoes anything done by
249 ->alloc_inode.
250
251 dirty_inode: this method is called by the VFS to mark an inode dirty.
252
253 write_inode: this method is called when the VFS needs to write an
254 inode to disc. The second parameter indicates whether the write
255 should be synchronous or not, not all filesystems check this flag.
256
257 drop_inode: called when the last access to the inode is dropped,
258 with the inode->i_lock spinlock held.
259
260 This method should be either NULL (normal UNIX filesystem
261 semantics) or "generic_delete_inode" (for filesystems that do not
262 want to cache inodes - causing "delete_inode" to always be
263 called regardless of the value of i_nlink)
264
265 The "generic_delete_inode()" behavior is equivalent to the
266 old practice of using "force_delete" in the put_inode() case,
267 but does not have the races that the "force_delete()" approach
268 had.
269
270 delete_inode: called when the VFS wants to delete an inode
271
272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
274
275 sync_fs: called when VFS is writing out all dirty data associated with
276 a superblock. The second parameter indicates whether the method
277 should wait until the write out has been completed. Optional.
278
279 freeze_fs: called when VFS is locking a filesystem and
280 forcing it into a consistent state. This method is currently
281 used by the Logical Volume Manager (LVM).
282
283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284 again.
285
286 statfs: called when the VFS needs to get filesystem statistics.
287
288 remount_fs: called when the filesystem is remounted. This is called
289 with the kernel lock held
290
291 clear_inode: called then the VFS clears the inode. Optional
292
293 umount_begin: called when the VFS is unmounting a filesystem.
294
295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
297
298 quota_read: called by the VFS to read from filesystem quota file.
299
300 quota_write: called by the VFS to write to filesystem quota file.
301
302 nr_cached_objects: called by the sb cache shrinking function for the
303 filesystem to return the number of freeable cached objects it contains.
304 Optional.
305
306 free_cache_objects: called by the sb cache shrinking function for the
307 filesystem to scan the number of objects indicated to try to free them.
308 Optional, but any filesystem implementing this method needs to also
309 implement ->nr_cached_objects for it to be called correctly.
310
311 We can't do anything with any errors that the filesystem might
312 encountered, hence the void return type. This will never be called if
313 the VM is trying to reclaim under GFP_NOFS conditions, hence this
314 method does not need to handle that situation itself.
315
316 Implementations must include conditional reschedule calls inside any
317 scanning loop that is done. This allows the VFS to determine
318 appropriate scan batch sizes without having to worry about whether
319 implementations will cause holdoff problems due to large scan batch
320 sizes.
321
322Whoever sets up the inode is responsible for filling in the "i_op" field. This
323is a pointer to a "struct inode_operations" which describes the methods that
324can be performed on individual inodes.
325
326struct xattr_handlers
327---------------------
328
329On filesystems that support extended attributes (xattrs), the s_xattr
330superblock field points to a NULL-terminated array of xattr handlers. Extended
331attributes are name:value pairs.
332
333 name: Indicates that the handler matches attributes with the specified name
334 (such as "system.posix_acl_access"); the prefix field must be NULL.
335
336 prefix: Indicates that the handler matches all attributes with the specified
337 name prefix (such as "user."); the name field must be NULL.
338
339 list: Determine if attributes matching this xattr handler should be listed
340 for a particular dentry. Used by some listxattr implementations like
341 generic_listxattr.
342
343 get: Called by the VFS to get the value of a particular extended attribute.
344 This method is called by the getxattr(2) system call.
345
346 set: Called by the VFS to set the value of a particular extended attribute.
347 When the new value is NULL, called to remove a particular extended
348 attribute. This method is called by the the setxattr(2) and
349 removexattr(2) system calls.
350
351When none of the xattr handlers of a filesystem match the specified attribute
352name or when a filesystem doesn't support extended attributes, the various
353*xattr(2) system calls return -EOPNOTSUPP.
354
355
356The Inode Object
357================
358
359An inode object represents an object within the filesystem.
360
361
362struct inode_operations
363-----------------------
364
365This describes how the VFS can manipulate an inode in your
366filesystem. As of kernel 2.6.22, the following members are defined:
367
368struct inode_operations {
369 int (*create) (struct inode *,struct dentry *, umode_t, bool);
370 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
371 int (*link) (struct dentry *,struct inode *,struct dentry *);
372 int (*unlink) (struct inode *,struct dentry *);
373 int (*symlink) (struct inode *,struct dentry *,const char *);
374 int (*mkdir) (struct inode *,struct dentry *,umode_t);
375 int (*rmdir) (struct inode *,struct dentry *);
376 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
377 int (*rename) (struct inode *, struct dentry *,
378 struct inode *, struct dentry *, unsigned int);
379 int (*readlink) (struct dentry *, char __user *,int);
380 const char *(*get_link) (struct dentry *, struct inode *,
381 struct delayed_call *);
382 int (*permission) (struct inode *, int);
383 int (*get_acl)(struct inode *, int);
384 int (*setattr) (struct dentry *, struct iattr *);
385 int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
386 ssize_t (*listxattr) (struct dentry *, char *, size_t);
387 void (*update_time)(struct inode *, struct timespec *, int);
388 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
389 unsigned open_flag, umode_t create_mode, int *opened);
390 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
391};
392
393Again, all methods are called without any locks being held, unless
394otherwise noted.
395
396 create: called by the open(2) and creat(2) system calls. Only
397 required if you want to support regular files. The dentry you
398 get should not have an inode (i.e. it should be a negative
399 dentry). Here you will probably call d_instantiate() with the
400 dentry and the newly created inode
401
402 lookup: called when the VFS needs to look up an inode in a parent
403 directory. The name to look for is found in the dentry. This
404 method must call d_add() to insert the found inode into the
405 dentry. The "i_count" field in the inode structure should be
406 incremented. If the named inode does not exist a NULL inode
407 should be inserted into the dentry (this is called a negative
408 dentry). Returning an error code from this routine must only
409 be done on a real error, otherwise creating inodes with system
410 calls like create(2), mknod(2), mkdir(2) and so on will fail.
411 If you wish to overload the dentry methods then you should
412 initialise the "d_dop" field in the dentry; this is a pointer
413 to a struct "dentry_operations".
414 This method is called with the directory inode semaphore held
415
416 link: called by the link(2) system call. Only required if you want
417 to support hard links. You will probably need to call
418 d_instantiate() just as you would in the create() method
419
420 unlink: called by the unlink(2) system call. Only required if you
421 want to support deleting inodes
422
423 symlink: called by the symlink(2) system call. Only required if you
424 want to support symlinks. You will probably need to call
425 d_instantiate() just as you would in the create() method
426
427 mkdir: called by the mkdir(2) system call. Only required if you want
428 to support creating subdirectories. You will probably need to
429 call d_instantiate() just as you would in the create() method
430
431 rmdir: called by the rmdir(2) system call. Only required if you want
432 to support deleting subdirectories
433
434 mknod: called by the mknod(2) system call to create a device (char,
435 block) inode or a named pipe (FIFO) or socket. Only required
436 if you want to support creating these types of inodes. You
437 will probably need to call d_instantiate() just as you would
438 in the create() method
439
440 rename: called by the rename(2) system call to rename the object to
441 have the parent and name given by the second inode and dentry.
442
443 The filesystem must return -EINVAL for any unsupported or
444 unknown flags. Currently the following flags are implemented:
445 (1) RENAME_NOREPLACE: this flag indicates that if the target
446 of the rename exists the rename should fail with -EEXIST
447 instead of replacing the target. The VFS already checks for
448 existence, so for local filesystems the RENAME_NOREPLACE
449 implementation is equivalent to plain rename.
450 (2) RENAME_EXCHANGE: exchange source and target. Both must
451 exist; this is checked by the VFS. Unlike plain rename,
452 source and target may be of different type.
453
454 get_link: called by the VFS to follow a symbolic link to the
455 inode it points to. Only required if you want to support
456 symbolic links. This method returns the symlink body
457 to traverse (and possibly resets the current position with
458 nd_jump_link()). If the body won't go away until the inode
459 is gone, nothing else is needed; if it needs to be otherwise
460 pinned, arrange for its release by having get_link(..., ..., done)
461 do set_delayed_call(done, destructor, argument).
462 In that case destructor(argument) will be called once VFS is
463 done with the body you've returned.
464 May be called in RCU mode; that is indicated by NULL dentry
465 argument. If request can't be handled without leaving RCU mode,
466 have it return ERR_PTR(-ECHILD).
467
468 readlink: this is now just an override for use by readlink(2) for the
469 cases when ->get_link uses nd_jump_link() or object is not in
470 fact a symlink. Normally filesystems should only implement
471 ->get_link for symlinks and readlink(2) will automatically use
472 that.
473
474 permission: called by the VFS to check for access rights on a POSIX-like
475 filesystem.
476
477 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
478 mode, the filesystem must check the permission without blocking or
479 storing to the inode.
480
481 If a situation is encountered that rcu-walk cannot handle, return
482 -ECHILD and it will be called again in ref-walk mode.
483
484 setattr: called by the VFS to set attributes for a file. This method
485 is called by chmod(2) and related system calls.
486
487 getattr: called by the VFS to get attributes of a file. This method
488 is called by stat(2) and related system calls.
489
490 listxattr: called by the VFS to list all extended attributes for a
491 given file. This method is called by the listxattr(2) system call.
492
493 update_time: called by the VFS to update a specific time or the i_version of
494 an inode. If this is not defined the VFS will update the inode itself
495 and call mark_inode_dirty_sync.
496
497 atomic_open: called on the last component of an open. Using this optional
498 method the filesystem can look up, possibly create and open the file in
499 one atomic operation. If it cannot perform this (e.g. the file type
500 turned out to be wrong) it may signal this by returning 1 instead of
501 usual 0 or -ve . This method is only called if the last component is
502 negative or needs lookup. Cached positive dentries are still handled by
503 f_op->open(). If the file was created, the FILE_CREATED flag should be
504 set in "opened". In case of O_EXCL the method must only succeed if the
505 file didn't exist and hence FILE_CREATED shall always be set on success.
506
507 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
508 atomically creating, opening and unlinking a file in given directory.
509
510The Address Space Object
511========================
512
513The address space object is used to group and manage pages in the page
514cache. It can be used to keep track of the pages in a file (or
515anything else) and also track the mapping of sections of the file into
516process address spaces.
517
518There are a number of distinct yet related services that an
519address-space can provide. These include communicating memory
520pressure, page lookup by address, and keeping track of pages tagged as
521Dirty or Writeback.
522
523The first can be used independently to the others. The VM can try to
524either write dirty pages in order to clean them, or release clean
525pages in order to reuse them. To do this it can call the ->writepage
526method on dirty pages, and ->releasepage on clean pages with
527PagePrivate set. Clean pages without PagePrivate and with no external
528references will be released without notice being given to the
529address_space.
530
531To achieve this functionality, pages need to be placed on an LRU with
532lru_cache_add and mark_page_active needs to be called whenever the
533page is used.
534
535Pages are normally kept in a radix tree index by ->index. This tree
536maintains information about the PG_Dirty and PG_Writeback status of
537each page, so that pages with either of these flags can be found
538quickly.
539
540The Dirty tag is primarily used by mpage_writepages - the default
541->writepages method. It uses the tag to find dirty pages to call
542->writepage on. If mpage_writepages is not used (i.e. the address
543provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
544almost unused. write_inode_now and sync_inode do use it (through
545__sync_single_inode) to check if ->writepages has been successful in
546writing out the whole address_space.
547
548The Writeback tag is used by filemap*wait* and sync_page* functions,
549via filemap_fdatawait_range, to wait for all writeback to complete.
550
551An address_space handler may attach extra information to a page,
552typically using the 'private' field in the 'struct page'. If such
553information is attached, the PG_Private flag should be set. This will
554cause various VM routines to make extra calls into the address_space
555handler to deal with that data.
556
557An address space acts as an intermediate between storage and
558application. Data is read into the address space a whole page at a
559time, and provided to the application either by copying of the page,
560or by memory-mapping the page.
561Data is written into the address space by the application, and then
562written-back to storage typically in whole pages, however the
563address_space has finer control of write sizes.
564
565The read process essentially only requires 'readpage'. The write
566process is more complicated and uses write_begin/write_end or
567set_page_dirty to write data into the address_space, and writepage
568and writepages to writeback data to storage.
569
570Adding and removing pages to/from an address_space is protected by the
571inode's i_mutex.
572
573When data is written to a page, the PG_Dirty flag should be set. It
574typically remains set until writepage asks for it to be written. This
575should clear PG_Dirty and set PG_Writeback. It can be actually
576written at any point after PG_Dirty is clear. Once it is known to be
577safe, PG_Writeback is cleared.
578
579Writeback makes use of a writeback_control structure...
580
581struct address_space_operations
582-------------------------------
583
584This describes how the VFS can manipulate mapping of a file to page cache in
585your filesystem. The following members are defined:
586
587struct address_space_operations {
588 int (*writepage)(struct page *page, struct writeback_control *wbc);
589 int (*readpage)(struct file *, struct page *);
590 int (*writepages)(struct address_space *, struct writeback_control *);
591 int (*set_page_dirty)(struct page *page);
592 int (*readpages)(struct file *filp, struct address_space *mapping,
593 struct list_head *pages, unsigned nr_pages);
594 int (*write_begin)(struct file *, struct address_space *mapping,
595 loff_t pos, unsigned len, unsigned flags,
596 struct page **pagep, void **fsdata);
597 int (*write_end)(struct file *, struct address_space *mapping,
598 loff_t pos, unsigned len, unsigned copied,
599 struct page *page, void *fsdata);
600 sector_t (*bmap)(struct address_space *, sector_t);
601 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
602 int (*releasepage) (struct page *, int);
603 void (*freepage)(struct page *);
604 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
605 /* isolate a page for migration */
606 bool (*isolate_page) (struct page *, isolate_mode_t);
607 /* migrate the contents of a page to the specified target */
608 int (*migratepage) (struct page *, struct page *);
609 /* put migration-failed page back to right list */
610 void (*putback_page) (struct page *);
611 int (*launder_page) (struct page *);
612
613 int (*is_partially_uptodate) (struct page *, unsigned long,
614 unsigned long);
615 void (*is_dirty_writeback) (struct page *, bool *, bool *);
616 int (*error_remove_page) (struct mapping *mapping, struct page *page);
617 int (*swap_activate)(struct file *);
618 int (*swap_deactivate)(struct file *);
619};
620
621 writepage: called by the VM to write a dirty page to backing store.
622 This may happen for data integrity reasons (i.e. 'sync'), or
623 to free up memory (flush). The difference can be seen in
624 wbc->sync_mode.
625 The PG_Dirty flag has been cleared and PageLocked is true.
626 writepage should start writeout, should set PG_Writeback,
627 and should make sure the page is unlocked, either synchronously
628 or asynchronously when the write operation completes.
629
630 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
631 try too hard if there are problems, and may choose to write out
632 other pages from the mapping if that is easier (e.g. due to
633 internal dependencies). If it chooses not to start writeout, it
634 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
635 calling ->writepage on that page.
636
637 See the file "Locking" for more details.
638
639 readpage: called by the VM to read a page from backing store.
640 The page will be Locked when readpage is called, and should be
641 unlocked and marked uptodate once the read completes.
642 If ->readpage discovers that it needs to unlock the page for
643 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
644 In this case, the page will be relocated, relocked and if
645 that all succeeds, ->readpage will be called again.
646
647 writepages: called by the VM to write out pages associated with the
648 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
649 the writeback_control will specify a range of pages that must be
650 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
651 and that many pages should be written if possible.
652 If no ->writepages is given, then mpage_writepages is used
653 instead. This will choose pages from the address space that are
654 tagged as DIRTY and will pass them to ->writepage.
655
656 set_page_dirty: called by the VM to set a page dirty.
657 This is particularly needed if an address space attaches
658 private data to a page, and that data needs to be updated when
659 a page is dirtied. This is called, for example, when a memory
660 mapped page gets modified.
661 If defined, it should set the PageDirty flag, and the
662 PAGECACHE_TAG_DIRTY tag in the radix tree.
663
664 readpages: called by the VM to read pages associated with the address_space
665 object. This is essentially just a vector version of
666 readpage. Instead of just one page, several pages are
667 requested.
668 readpages is only used for read-ahead, so read errors are
669 ignored. If anything goes wrong, feel free to give up.
670
671 write_begin:
672 Called by the generic buffered write code to ask the filesystem to
673 prepare to write len bytes at the given offset in the file. The
674 address_space should check that the write will be able to complete,
675 by allocating space if necessary and doing any other internal
676 housekeeping. If the write will update parts of any basic-blocks on
677 storage, then those blocks should be pre-read (if they haven't been
678 read already) so that the updated blocks can be written out properly.
679
680 The filesystem must return the locked pagecache page for the specified
681 offset, in *pagep, for the caller to write into.
682
683 It must be able to cope with short writes (where the length passed to
684 write_begin is greater than the number of bytes copied into the page).
685
686 flags is a field for AOP_FLAG_xxx flags, described in
687 include/linux/fs.h.
688
689 A void * may be returned in fsdata, which then gets passed into
690 write_end.
691
692 Returns 0 on success; < 0 on failure (which is the error code), in
693 which case write_end is not called.
694
695 write_end: After a successful write_begin, and data copy, write_end must
696 be called. len is the original len passed to write_begin, and copied
697 is the amount that was able to be copied (copied == len is always true
698 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
699
700 The filesystem must take care of unlocking the page and releasing it
701 refcount, and updating i_size.
702
703 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
704 that were able to be copied into pagecache.
705
706 bmap: called by the VFS to map a logical block offset within object to
707 physical block number. This method is used by the FIBMAP
708 ioctl and for working with swap-files. To be able to swap to
709 a file, the file must have a stable mapping to a block
710 device. The swap system does not go through the filesystem
711 but instead uses bmap to find out where the blocks in the file
712 are and uses those addresses directly.
713
714 invalidatepage: If a page has PagePrivate set, then invalidatepage
715 will be called when part or all of the page is to be removed
716 from the address space. This generally corresponds to either a
717 truncation, punch hole or a complete invalidation of the address
718 space (in the latter case 'offset' will always be 0 and 'length'
719 will be PAGE_SIZE). Any private data associated with the page
720 should be updated to reflect this truncation. If offset is 0 and
721 length is PAGE_SIZE, then the private data should be released,
722 because the page must be able to be completely discarded. This may
723 be done by calling the ->releasepage function, but in this case the
724 release MUST succeed.
725
726 releasepage: releasepage is called on PagePrivate pages to indicate
727 that the page should be freed if possible. ->releasepage
728 should remove any private data from the page and clear the
729 PagePrivate flag. If releasepage() fails for some reason, it must
730 indicate failure with a 0 return value.
731 releasepage() is used in two distinct though related cases. The
732 first is when the VM finds a clean page with no active users and
733 wants to make it a free page. If ->releasepage succeeds, the
734 page will be removed from the address_space and become free.
735
736 The second case is when a request has been made to invalidate
737 some or all pages in an address_space. This can happen
738 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
739 filesystem explicitly requesting it as nfs and 9fs do (when
740 they believe the cache may be out of date with storage) by
741 calling invalidate_inode_pages2().
742 If the filesystem makes such a call, and needs to be certain
743 that all pages are invalidated, then its releasepage will
744 need to ensure this. Possibly it can clear the PageUptodate
745 bit if it cannot free private data yet.
746
747 freepage: freepage is called once the page is no longer visible in
748 the page cache in order to allow the cleanup of any private
749 data. Since it may be called by the memory reclaimer, it
750 should not assume that the original address_space mapping still
751 exists, and it should not block.
752
753 direct_IO: called by the generic read/write routines to perform
754 direct_IO - that is IO requests which bypass the page cache
755 and transfer data directly between the storage and the
756 application's address space.
757
758 isolate_page: Called by the VM when isolating a movable non-lru page.
759 If page is successfully isolated, VM marks the page as PG_isolated
760 via __SetPageIsolated.
761
762 migrate_page: This is used to compact the physical memory usage.
763 If the VM wants to relocate a page (maybe off a memory card
764 that is signalling imminent failure) it will pass a new page
765 and an old page to this function. migrate_page should
766 transfer any private data across and update any references
767 that it has to the page.
768
769 putback_page: Called by the VM when isolated page's migration fails.
770
771 launder_page: Called before freeing a page - it writes back the dirty page. To
772 prevent redirtying the page, it is kept locked during the whole
773 operation.
774
775 is_partially_uptodate: Called by the VM when reading a file through the
776 pagecache when the underlying blocksize != pagesize. If the required
777 block is up to date then the read can complete without needing the IO
778 to bring the whole page up to date.
779
780 is_dirty_writeback: Called by the VM when attempting to reclaim a page.
781 The VM uses dirty and writeback information to determine if it needs
782 to stall to allow flushers a chance to complete some IO. Ordinarily
783 it can use PageDirty and PageWriteback but some filesystems have
784 more complex state (unstable pages in NFS prevent reclaim) or
785 do not set those flags due to locking problems. This callback
786 allows a filesystem to indicate to the VM if a page should be
787 treated as dirty or writeback for the purposes of stalling.
788
789 error_remove_page: normally set to generic_error_remove_page if truncation
790 is ok for this address space. Used for memory failure handling.
791 Setting this implies you deal with pages going away under you,
792 unless you have them locked or reference counts increased.
793
794 swap_activate: Called when swapon is used on a file to allocate
795 space if necessary and pin the block lookup information in
796 memory. A return value of zero indicates success,
797 in which case this file can be used to back swapspace. The
798 swapspace operations will be proxied to this address space's
799 ->swap_{out,in} methods.
800
801 swap_deactivate: Called during swapoff on files where swap_activate
802 was successful.
803
804
805The File Object
806===============
807
808A file object represents a file opened by a process.
809
810
811struct file_operations
812----------------------
813
814This describes how the VFS can manipulate an open file. As of kernel
8154.1, the following members are defined:
816
817struct file_operations {
818 struct module *owner;
819 loff_t (*llseek) (struct file *, loff_t, int);
820 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
821 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
822 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
823 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
824 int (*iterate) (struct file *, struct dir_context *);
825 unsigned int (*poll) (struct file *, struct poll_table_struct *);
826 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
827 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
828 int (*mmap) (struct file *, struct vm_area_struct *);
829 int (*mremap)(struct file *, struct vm_area_struct *);
830 int (*open) (struct inode *, struct file *);
831 int (*flush) (struct file *, fl_owner_t id);
832 int (*release) (struct inode *, struct file *);
833 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
834 int (*fasync) (int, struct file *, int);
835 int (*lock) (struct file *, int, struct file_lock *);
836 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
837 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
838 int (*check_flags)(int);
839 int (*flock) (struct file *, int, struct file_lock *);
840 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
841 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
842 int (*setlease)(struct file *, long, struct file_lock **, void **);
843 long (*fallocate)(struct file *file, int mode, loff_t offset,
844 loff_t len);
845 void (*show_fdinfo)(struct seq_file *m, struct file *f);
846#ifndef CONFIG_MMU
847 unsigned (*mmap_capabilities)(struct file *);
848#endif
849};
850
851Again, all methods are called without any locks being held, unless
852otherwise noted.
853
854 llseek: called when the VFS needs to move the file position index
855
856 read: called by read(2) and related system calls
857
858 read_iter: possibly asynchronous read with iov_iter as destination
859
860 write: called by write(2) and related system calls
861
862 write_iter: possibly asynchronous write with iov_iter as source
863
864 iterate: called when the VFS needs to read the directory contents
865
866 poll: called by the VFS when a process wants to check if there is
867 activity on this file and (optionally) go to sleep until there
868 is activity. Called by the select(2) and poll(2) system calls
869
870 unlocked_ioctl: called by the ioctl(2) system call.
871
872 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
873 are used on 64 bit kernels.
874
875 mmap: called by the mmap(2) system call
876
877 open: called by the VFS when an inode should be opened. When the VFS
878 opens a file, it creates a new "struct file". It then calls the
879 open method for the newly allocated file structure. You might
880 think that the open method really belongs in
881 "struct inode_operations", and you may be right. I think it's
882 done the way it is because it makes filesystems simpler to
883 implement. The open() method is a good place to initialize the
884 "private_data" member in the file structure if you want to point
885 to a device structure
886
887 flush: called by the close(2) system call to flush a file
888
889 release: called when the last reference to an open file is closed
890
891 fsync: called by the fsync(2) system call
892
893 fasync: called by the fcntl(2) system call when asynchronous
894 (non-blocking) mode is enabled for a file
895
896 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
897 commands
898
899 get_unmapped_area: called by the mmap(2) system call
900
901 check_flags: called by the fcntl(2) system call for F_SETFL command
902
903 flock: called by the flock(2) system call
904
905 splice_write: called by the VFS to splice data from a pipe to a file. This
906 method is used by the splice(2) system call
907
908 splice_read: called by the VFS to splice data from file to a pipe. This
909 method is used by the splice(2) system call
910
911 setlease: called by the VFS to set or release a file lock lease. setlease
912 implementations should call generic_setlease to record or remove
913 the lease in the inode after setting it.
914
915 fallocate: called by the VFS to preallocate blocks or punch a hole.
916
917Note that the file operations are implemented by the specific
918filesystem in which the inode resides. When opening a device node
919(character or block special) most filesystems will call special
920support routines in the VFS which will locate the required device
921driver information. These support routines replace the filesystem file
922operations with those for the device driver, and then proceed to call
923the new open() method for the file. This is how opening a device file
924in the filesystem eventually ends up calling the device driver open()
925method.
926
927
928Directory Entry Cache (dcache)
929==============================
930
931
932struct dentry_operations
933------------------------
934
935This describes how a filesystem can overload the standard dentry
936operations. Dentries and the dcache are the domain of the VFS and the
937individual filesystem implementations. Device drivers have no business
938here. These methods may be set to NULL, as they are either optional or
939the VFS uses a default. As of kernel 2.6.22, the following members are
940defined:
941
942struct dentry_operations {
943 int (*d_revalidate)(struct dentry *, unsigned int);
944 int (*d_weak_revalidate)(struct dentry *, unsigned int);
945 int (*d_hash)(const struct dentry *, struct qstr *);
946 int (*d_compare)(const struct dentry *,
947 unsigned int, const char *, const struct qstr *);
948 int (*d_delete)(const struct dentry *);
949 int (*d_init)(struct dentry *);
950 void (*d_release)(struct dentry *);
951 void (*d_iput)(struct dentry *, struct inode *);
952 char *(*d_dname)(struct dentry *, char *, int);
953 struct vfsmount *(*d_automount)(struct path *);
954 int (*d_manage)(const struct path *, bool);
955 struct dentry *(*d_real)(struct dentry *, const struct inode *,
956 unsigned int);
957};
958
959 d_revalidate: called when the VFS needs to revalidate a dentry. This
960 is called whenever a name look-up finds a dentry in the
961 dcache. Most local filesystems leave this as NULL, because all their
962 dentries in the dcache are valid. Network filesystems are different
963 since things can change on the server without the client necessarily
964 being aware of it.
965
966 This function should return a positive value if the dentry is still
967 valid, and zero or a negative error code if it isn't.
968
969 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
970 If in rcu-walk mode, the filesystem must revalidate the dentry without
971 blocking or storing to the dentry, d_parent and d_inode should not be
972 used without care (because they can change and, in d_inode case, even
973 become NULL under us).
974
975 If a situation is encountered that rcu-walk cannot handle, return
976 -ECHILD and it will be called again in ref-walk mode.
977
978 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
979 This is called when a path-walk ends at dentry that was not acquired by
980 doing a lookup in the parent directory. This includes "/", "." and "..",
981 as well as procfs-style symlinks and mountpoint traversal.
982
983 In this case, we are less concerned with whether the dentry is still
984 fully correct, but rather that the inode is still valid. As with
985 d_revalidate, most local filesystems will set this to NULL since their
986 dcache entries are always valid.
987
988 This function has the same return code semantics as d_revalidate.
989
990 d_weak_revalidate is only called after leaving rcu-walk mode.
991
992 d_hash: called when the VFS adds a dentry to the hash table. The first
993 dentry passed to d_hash is the parent directory that the name is
994 to be hashed into.
995
996 Same locking and synchronisation rules as d_compare regarding
997 what is safe to dereference etc.
998
999 d_compare: called to compare a dentry name with a given name. The first
1000 dentry is the parent of the dentry to be compared, the second is
1001 the child dentry. len and name string are properties of the dentry
1002 to be compared. qstr is the name to compare it with.
1003
1004 Must be constant and idempotent, and should not take locks if
1005 possible, and should not or store into the dentry.
1006 Should not dereference pointers outside the dentry without
1007 lots of care (eg. d_parent, d_inode, d_name should not be used).
1008
1009 However, our vfsmount is pinned, and RCU held, so the dentries and
1010 inodes won't disappear, neither will our sb or filesystem module.
1011 ->d_sb may be used.
1012
1013 It is a tricky calling convention because it needs to be called under
1014 "rcu-walk", ie. without any locks or references on things.
1015
1016 d_delete: called when the last reference to a dentry is dropped and the
1017 dcache is deciding whether or not to cache it. Return 1 to delete
1018 immediately, or 0 to cache the dentry. Default is NULL which means to
1019 always cache a reachable dentry. d_delete must be constant and
1020 idempotent.
1021
1022 d_init: called when a dentry is allocated
1023
1024 d_release: called when a dentry is really deallocated
1025
1026 d_iput: called when a dentry loses its inode (just prior to its
1027 being deallocated). The default when this is NULL is that the
1028 VFS calls iput(). If you define this method, you must call
1029 iput() yourself
1030
1031 d_dname: called when the pathname of a dentry should be generated.
1032 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1033 pathname generation. (Instead of doing it when dentry is created,
1034 it's done only when the path is needed.). Real filesystems probably
1035 dont want to use it, because their dentries are present in global
1036 dcache hash, so their hash should be an invariant. As no lock is
1037 held, d_dname() should not try to modify the dentry itself, unless
1038 appropriate SMP safety is used. CAUTION : d_path() logic is quite
1039 tricky. The correct way to return for example "Hello" is to put it
1040 at the end of the buffer, and returns a pointer to the first char.
1041 dynamic_dname() helper function is provided to take care of this.
1042
1043 Example :
1044
1045 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1046 {
1047 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1048 dentry->d_inode->i_ino);
1049 }
1050
1051 d_automount: called when an automount dentry is to be traversed (optional).
1052 This should create a new VFS mount record and return the record to the
1053 caller. The caller is supplied with a path parameter giving the
1054 automount directory to describe the automount target and the parent
1055 VFS mount record to provide inheritable mount parameters. NULL should
1056 be returned if someone else managed to make the automount first. If
1057 the vfsmount creation failed, then an error code should be returned.
1058 If -EISDIR is returned, then the directory will be treated as an
1059 ordinary directory and returned to pathwalk to continue walking.
1060
1061 If a vfsmount is returned, the caller will attempt to mount it on the
1062 mountpoint and will remove the vfsmount from its expiration list in
1063 the case of failure. The vfsmount should be returned with 2 refs on
1064 it to prevent automatic expiration - the caller will clean up the
1065 additional ref.
1066
1067 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1068 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1069 inode being added.
1070
1071 d_manage: called to allow the filesystem to manage the transition from a
1072 dentry (optional). This allows autofs, for example, to hold up clients
1073 waiting to explore behind a 'mountpoint' whilst letting the daemon go
1074 past and construct the subtree there. 0 should be returned to let the
1075 calling process continue. -EISDIR can be returned to tell pathwalk to
1076 use this directory as an ordinary directory and to ignore anything
1077 mounted on it and not to check the automount flag. Any other error
1078 code will abort pathwalk completely.
1079
1080 If the 'rcu_walk' parameter is true, then the caller is doing a
1081 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
1082 and the caller can be asked to leave it and call again by returning
1083 -ECHILD. -EISDIR may also be returned to tell pathwalk to
1084 ignore d_automount or any mounts.
1085
1086 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1087 dentry being transited from.
1088
1089 d_real: overlay/union type filesystems implement this method to return one of
1090 the underlying dentries hidden by the overlay. It is used in three
1091 different modes:
1092
1093 Called from open it may need to copy-up the file depending on the
1094 supplied open flags. This mode is selected with a non-zero flags
1095 argument. In this mode the d_real method can return an error.
1096
1097 Called from file_dentry() it returns the real dentry matching the inode
1098 argument. The real dentry may be from a lower layer already copied up,
1099 but still referenced from the file. This mode is selected with a
1100 non-NULL inode argument. This will always succeed.
1101
1102 With NULL inode and zero flags the topmost real underlying dentry is
1103 returned. This will always succeed.
1104
1105 This method is never called with both non-NULL inode and non-zero flags.
1106
1107Each dentry has a pointer to its parent dentry, as well as a hash list
1108of child dentries. Child dentries are basically like files in a
1109directory.
1110
1111
1112Directory Entry Cache API
1113--------------------------
1114
1115There are a number of functions defined which permit a filesystem to
1116manipulate dentries:
1117
1118 dget: open a new handle for an existing dentry (this just increments
1119 the usage count)
1120
1121 dput: close a handle for a dentry (decrements the usage count). If
1122 the usage count drops to 0, and the dentry is still in its
1123 parent's hash, the "d_delete" method is called to check whether
1124 it should be cached. If it should not be cached, or if the dentry
1125 is not hashed, it is deleted. Otherwise cached dentries are put
1126 into an LRU list to be reclaimed on memory shortage.
1127
1128 d_drop: this unhashes a dentry from its parents hash list. A
1129 subsequent call to dput() will deallocate the dentry if its
1130 usage count drops to 0
1131
1132 d_delete: delete a dentry. If there are no other open references to
1133 the dentry then the dentry is turned into a negative dentry
1134 (the d_iput() method is called). If there are other
1135 references, then d_drop() is called instead
1136
1137 d_add: add a dentry to its parents hash list and then calls
1138 d_instantiate()
1139
1140 d_instantiate: add a dentry to the alias hash list for the inode and
1141 updates the "d_inode" member. The "i_count" member in the
1142 inode structure should be set/incremented. If the inode
1143 pointer is NULL, the dentry is called a "negative
1144 dentry". This function is commonly called when an inode is
1145 created for an existing negative dentry
1146
1147 d_lookup: look up a dentry given its parent and path name component
1148 It looks up the child of that given name from the dcache
1149 hash table. If it is found, the reference count is incremented
1150 and the dentry is returned. The caller must use dput()
1151 to free the dentry when it finishes using it.
1152
1153Mount Options
1154=============
1155
1156Parsing options
1157---------------
1158
1159On mount and remount the filesystem is passed a string containing a
1160comma separated list of mount options. The options can have either of
1161these forms:
1162
1163 option
1164 option=value
1165
1166The <linux/parser.h> header defines an API that helps parse these
1167options. There are plenty of examples on how to use it in existing
1168filesystems.
1169
1170Showing options
1171---------------
1172
1173If a filesystem accepts mount options, it must define show_options()
1174to show all the currently active options. The rules are:
1175
1176 - options MUST be shown which are not default or their values differ
1177 from the default
1178
1179 - options MAY be shown which are enabled by default or have their
1180 default value
1181
1182Options used only internally between a mount helper and the kernel
1183(such as file descriptors), or which only have an effect during the
1184mounting (such as ones controlling the creation of a journal) are exempt
1185from the above rules.
1186
1187The underlying reason for the above rules is to make sure, that a
1188mount can be accurately replicated (e.g. umounting and mounting again)
1189based on the information found in /proc/mounts.
1190
1191A simple method of saving options at mount/remount time and showing
1192them is provided with the save_mount_options() and
1193generic_show_options() helper functions. Please note, that using
1194these may have drawbacks. For more info see header comments for these
1195functions in fs/namespace.c.
1196
1197Resources
1198=========
1199
1200(Note some of these resources are not up-to-date with the latest kernel
1201 version.)
1202
1203Creating Linux virtual filesystems. 2002
1204 <http://lwn.net/Articles/13325/>
1205
1206The Linux Virtual File-system Layer by Neil Brown. 1999
1207 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1208
1209A tour of the Linux VFS by Michael K. Johnson. 1996
1210 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1211
1212A small trail through the Linux kernel by Andries Brouwer. 2001
1213 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>