2 Overview of the Linux Virtual File System
4 Original author: Richard Gooch <rgooch@atnf.csiro.au>
6 Last updated on June 24, 2007.
8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
11 This file is released under the GPLv2.
17 The Virtual File System (also known as the Virtual Filesystem Switch)
18 is the software layer in the kernel that provides the filesystem
19 interface to userspace programs. It also provides an abstraction
20 within the kernel which allows different filesystem implementations to
23 VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24 on are called from a process context. Filesystem locking is described
25 in the document Documentation/filesystems/Locking.
28 Directory Entry Cache (dcache)
29 ------------------------------
31 The VFS implements the open(2), stat(2), chmod(2), and similar system
32 calls. The pathname argument that is passed to them is used by the VFS
33 to search through the directory entry cache (also known as the dentry
34 cache or dcache). This provides a very fast look-up mechanism to
35 translate a pathname (filename) into a specific dentry. Dentries live
36 in RAM and are never saved to disc: they exist only for performance.
38 The dentry cache is meant to be a view into your entire filespace. As
39 most computers cannot fit all dentries in the RAM at the same time,
40 some bits of the cache are missing. In order to resolve your pathname
41 into a dentry, the VFS may have to resort to creating dentries along
42 the way, and then loading the inode. This is done by looking up the
49 An individual dentry usually has a pointer to an inode. Inodes are
50 filesystem objects such as regular files, directories, FIFOs and other
51 beasts. They live either on the disc (for block device filesystems)
52 or in the memory (for pseudo filesystems). Inodes that live on the
53 disc are copied into the memory when required and changes to the inode
54 are written back to disc. A single inode can be pointed to by multiple
55 dentries (hard links, for example, do this).
57 To look up an inode requires that the VFS calls the lookup() method of
58 the parent directory inode. This method is installed by the specific
59 filesystem implementation that the inode lives in. Once the VFS has
60 the required dentry (and hence the inode), we can do all those boring
61 things like open(2) the file, or stat(2) it to peek at the inode
62 data. The stat(2) operation is fairly simple: once the VFS has the
63 dentry, it peeks at the inode data and passes some of it back to
70 Opening a file requires another operation: allocation of a file
71 structure (this is the kernel-side implementation of file
72 descriptors). The freshly allocated file structure is initialized with
73 a pointer to the dentry and a set of file operation member functions.
74 These are taken from the inode data. The open() file method is then
75 called so the specific filesystem implementation can do its work. You
76 can see that this is another switch performed by the VFS. The file
77 structure is placed into the file descriptor table for the process.
79 Reading, writing and closing files (and other assorted VFS operations)
80 is done by using the userspace file descriptor to grab the appropriate
81 file structure, and then calling the required file structure method to
82 do whatever is required. For as long as the file is open, it keeps the
83 dentry in use, which in turn means that the VFS inode is still in use.
86 Registering and Mounting a Filesystem
87 =====================================
89 To register and unregister a filesystem, use the following API
94 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
97 The passed struct file_system_type describes your filesystem. When a
98 request is made to mount a filesystem onto a directory in your namespace,
99 the VFS will call the appropriate mount() method for the specific
100 filesystem. New vfsmount referring to the tree returned by ->mount()
101 will be attached to the mountpoint, so that when pathname resolution
102 reaches the mountpoint it will jump into the root of that vfsmount.
104 You can see all filesystems that are registered to the kernel in the
105 file /proc/filesystems.
108 struct file_system_type
109 -----------------------
111 This describes the filesystem. As of kernel 2.6.39, the following
114 struct file_system_type {
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;
127 name: the name of the filesystem type, such as "ext2", "iso9660",
130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
132 mount: the method to call when a new instance of this
133 filesystem should be mounted
135 kill_sb: the method to call when an instance of this filesystem
138 owner: for internal VFS use: you should initialize this to THIS_MODULE in
141 next: for internal VFS use: you should initialize this to NULL
143 s_lock_key, s_umount_key: lockdep-specific
145 The mount() method has the following arguments:
147 struct file_system_type *fs_type: describes the filesystem, partly initialized
148 by the specific filesystem code
150 int flags: mount flags
152 const char *dev_name: the device name we are mounting.
154 void *data: arbitrary mount options, usually comes as an ASCII
155 string (see "Mount Options" section)
157 The mount() method must return the root dentry of the tree requested by
158 caller. An active reference to its superblock must be grabbed and the
159 superblock must be locked. On failure it should return ERR_PTR(error).
161 The arguments match those of mount(2) and their interpretation
162 depends on filesystem type. E.g. for block filesystems, dev_name is
163 interpreted as block device name, that device is opened and if it
164 contains a suitable filesystem image the method creates and initializes
165 struct super_block accordingly, returning its root dentry to caller.
167 ->mount() may choose to return a subtree of existing filesystem - it
168 doesn't have to create a new one. The main result from the caller's
169 point of view is a reference to dentry at the root of (sub)tree to
170 be attached; creation of new superblock is a common side effect.
172 The most interesting member of the superblock structure that the
173 mount() method fills in is the "s_op" field. This is a pointer to
174 a "struct super_operations" which describes the next level of the
175 filesystem implementation.
177 Usually, a filesystem uses one of the generic mount() implementations
178 and provides a fill_super() callback instead. The generic variants are:
180 mount_bdev: mount a filesystem residing on a block device
182 mount_nodev: mount a filesystem that is not backed by a device
184 mount_single: mount a filesystem which shares the instance between
187 A fill_super() callback implementation has the following arguments:
189 struct super_block *sb: the superblock structure. The callback
190 must initialize this properly.
192 void *data: arbitrary mount options, usually comes as an ASCII
193 string (see "Mount Options" section)
195 int silent: whether or not to be silent on error
198 The Superblock Object
199 =====================
201 A superblock object represents a mounted filesystem.
204 struct super_operations
205 -----------------------
207 This describes how the VFS can manipulate the superblock of your
208 filesystem. As of kernel 2.6.22, the following members are defined:
210 struct super_operations {
211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
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 *);
227 int (*show_options)(struct seq_file *, struct dentry *);
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);
235 All methods are called without any locks being held, unless otherwise
236 noted. This means that most methods can block safely. All methods are
237 only called from a process context (i.e. not from an interrupt handler
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.
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
251 dirty_inode: this method is called by the VFS to mark an inode dirty.
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.
257 drop_inode: called when the last access to the inode is dropped,
258 with the inode->i_lock spinlock held.
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)
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
270 delete_inode: called when the VFS wants to delete an inode
272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
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.
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).
283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
286 statfs: called when the VFS needs to get filesystem statistics.
288 remount_fs: called when the filesystem is remounted. This is called
289 with the kernel lock held
291 clear_inode: called then the VFS clears the inode. Optional
293 umount_begin: called when the VFS is unmounting a filesystem.
295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
298 quota_read: called by the VFS to read from filesystem quota file.
300 quota_write: called by the VFS to write to filesystem quota file.
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.
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.
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.
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
322 Whoever sets up the inode is responsible for filling in the "i_op" field. This
323 is a pointer to a "struct inode_operations" which describes the methods that
324 can be performed on individual inodes.
326 struct xattr_handlers
327 ---------------------
329 On filesystems that support extended attributes (xattrs), the s_xattr
330 superblock field points to a NULL-terminated array of xattr handlers. Extended
331 attributes are name:value pairs.
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.
336 prefix: Indicates that the handler matches all attributes with the specified
337 name prefix (such as "user."); the name field must be NULL.
339 list: Determine if attributes matching this xattr handler should be listed
340 for a particular dentry. Used by some listxattr implementations like
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.
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.
351 When none of the xattr handlers of a filesystem match the specified attribute
352 name or when a filesystem doesn't support extended attributes, the various
353 *xattr(2) system calls return -EOPNOTSUPP.
359 An inode object represents an object within the filesystem.
362 struct inode_operations
363 -----------------------
365 This describes how the VFS can manipulate an inode in your
366 filesystem. As of kernel 2.6.22, the following members are defined:
368 struct 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) (const struct path *, struct kstat *, u32, unsigned int);
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);
390 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
393 Again, all methods are called without any locks being held, unless
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
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
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
420 unlink: called by the unlink(2) system call. Only required if you
421 want to support deleting inodes
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
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
431 rmdir: called by the rmdir(2) system call. Only required if you want
432 to support deleting subdirectories
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
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.
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.
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).
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
474 permission: called by the VFS to check for access rights on a POSIX-like
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.
481 If a situation is encountered that rcu-walk cannot handle, return
482 -ECHILD and it will be called again in ref-walk mode.
484 setattr: called by the VFS to set attributes for a file. This method
485 is called by chmod(2) and related system calls.
487 getattr: called by the VFS to get attributes of a file. This method
488 is called by stat(2) and related system calls.
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.
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.
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 wants to leave actual opening to the
500 caller (e.g. if the file turned out to be a symlink, device, or just
501 something filesystem won't do atomic open for), it may signal this by
502 returning finish_no_open(file, dentry). This method is only called if
503 the last component is negative or needs lookup. Cached positive dentries
504 are still handled by f_op->open(). If the file was created,
505 FMODE_CREATED flag should be set in file->f_mode. In case of O_EXCL
506 the method must only succeed if the file didn't exist and hence FMODE_CREATED
507 shall always be set on success.
509 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
510 atomically creating, opening and unlinking a file in given directory.
512 The Address Space Object
513 ========================
515 The address space object is used to group and manage pages in the page
516 cache. It can be used to keep track of the pages in a file (or
517 anything else) and also track the mapping of sections of the file into
518 process address spaces.
520 There are a number of distinct yet related services that an
521 address-space can provide. These include communicating memory
522 pressure, page lookup by address, and keeping track of pages tagged as
525 The first can be used independently to the others. The VM can try to
526 either write dirty pages in order to clean them, or release clean
527 pages in order to reuse them. To do this it can call the ->writepage
528 method on dirty pages, and ->releasepage on clean pages with
529 PagePrivate set. Clean pages without PagePrivate and with no external
530 references will be released without notice being given to the
533 To achieve this functionality, pages need to be placed on an LRU with
534 lru_cache_add and mark_page_active needs to be called whenever the
537 Pages are normally kept in a radix tree index by ->index. This tree
538 maintains information about the PG_Dirty and PG_Writeback status of
539 each page, so that pages with either of these flags can be found
542 The Dirty tag is primarily used by mpage_writepages - the default
543 ->writepages method. It uses the tag to find dirty pages to call
544 ->writepage on. If mpage_writepages is not used (i.e. the address
545 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
546 almost unused. write_inode_now and sync_inode do use it (through
547 __sync_single_inode) to check if ->writepages has been successful in
548 writing out the whole address_space.
550 The Writeback tag is used by filemap*wait* and sync_page* functions,
551 via filemap_fdatawait_range, to wait for all writeback to complete.
553 An address_space handler may attach extra information to a page,
554 typically using the 'private' field in the 'struct page'. If such
555 information is attached, the PG_Private flag should be set. This will
556 cause various VM routines to make extra calls into the address_space
557 handler to deal with that data.
559 An address space acts as an intermediate between storage and
560 application. Data is read into the address space a whole page at a
561 time, and provided to the application either by copying of the page,
562 or by memory-mapping the page.
563 Data is written into the address space by the application, and then
564 written-back to storage typically in whole pages, however the
565 address_space has finer control of write sizes.
567 The read process essentially only requires 'readpage'. The write
568 process is more complicated and uses write_begin/write_end or
569 set_page_dirty to write data into the address_space, and writepage
570 and writepages to writeback data to storage.
572 Adding and removing pages to/from an address_space is protected by the
575 When data is written to a page, the PG_Dirty flag should be set. It
576 typically remains set until writepage asks for it to be written. This
577 should clear PG_Dirty and set PG_Writeback. It can be actually
578 written at any point after PG_Dirty is clear. Once it is known to be
579 safe, PG_Writeback is cleared.
581 Writeback makes use of a writeback_control structure to direct the
582 operations. This gives the the writepage and writepages operations some
583 information about the nature of and reason for the writeback request,
584 and the constraints under which it is being done. It is also used to
585 return information back to the caller about the result of a writepage or
588 Handling errors during writeback
589 --------------------------------
590 Most applications that do buffered I/O will periodically call a file
591 synchronization call (fsync, fdatasync, msync or sync_file_range) to
592 ensure that data written has made it to the backing store. When there
593 is an error during writeback, they expect that error to be reported when
594 a file sync request is made. After an error has been reported on one
595 request, subsequent requests on the same file descriptor should return
596 0, unless further writeback errors have occurred since the previous file
599 Ideally, the kernel would report errors only on file descriptions on
600 which writes were done that subsequently failed to be written back. The
601 generic pagecache infrastructure does not track the file descriptions
602 that have dirtied each individual page however, so determining which
603 file descriptors should get back an error is not possible.
605 Instead, the generic writeback error tracking infrastructure in the
606 kernel settles for reporting errors to fsync on all file descriptions
607 that were open at the time that the error occurred. In a situation with
608 multiple writers, all of them will get back an error on a subsequent fsync,
609 even if all of the writes done through that particular file descriptor
610 succeeded (or even if there were no writes on that file descriptor at all).
612 Filesystems that wish to use this infrastructure should call
613 mapping_set_error to record the error in the address_space when it
614 occurs. Then, after writing back data from the pagecache in their
615 file->fsync operation, they should call file_check_and_advance_wb_err to
616 ensure that the struct file's error cursor has advanced to the correct
617 point in the stream of errors emitted by the backing device(s).
619 struct address_space_operations
620 -------------------------------
622 This describes how the VFS can manipulate mapping of a file to page cache in
623 your filesystem. The following members are defined:
625 struct address_space_operations {
626 int (*writepage)(struct page *page, struct writeback_control *wbc);
627 int (*readpage)(struct file *, struct page *);
628 int (*writepages)(struct address_space *, struct writeback_control *);
629 int (*set_page_dirty)(struct page *page);
630 int (*readpages)(struct file *filp, struct address_space *mapping,
631 struct list_head *pages, unsigned nr_pages);
632 int (*write_begin)(struct file *, struct address_space *mapping,
633 loff_t pos, unsigned len, unsigned flags,
634 struct page **pagep, void **fsdata);
635 int (*write_end)(struct file *, struct address_space *mapping,
636 loff_t pos, unsigned len, unsigned copied,
637 struct page *page, void *fsdata);
638 sector_t (*bmap)(struct address_space *, sector_t);
639 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
640 int (*releasepage) (struct page *, int);
641 void (*freepage)(struct page *);
642 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
643 /* isolate a page for migration */
644 bool (*isolate_page) (struct page *, isolate_mode_t);
645 /* migrate the contents of a page to the specified target */
646 int (*migratepage) (struct page *, struct page *);
647 /* put migration-failed page back to right list */
648 void (*putback_page) (struct page *);
649 int (*launder_page) (struct page *);
651 int (*is_partially_uptodate) (struct page *, unsigned long,
653 void (*is_dirty_writeback) (struct page *, bool *, bool *);
654 int (*error_remove_page) (struct mapping *mapping, struct page *page);
655 int (*swap_activate)(struct file *);
656 int (*swap_deactivate)(struct file *);
659 writepage: called by the VM to write a dirty page to backing store.
660 This may happen for data integrity reasons (i.e. 'sync'), or
661 to free up memory (flush). The difference can be seen in
663 The PG_Dirty flag has been cleared and PageLocked is true.
664 writepage should start writeout, should set PG_Writeback,
665 and should make sure the page is unlocked, either synchronously
666 or asynchronously when the write operation completes.
668 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
669 try too hard if there are problems, and may choose to write out
670 other pages from the mapping if that is easier (e.g. due to
671 internal dependencies). If it chooses not to start writeout, it
672 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
673 calling ->writepage on that page.
675 See the file "Locking" for more details.
677 readpage: called by the VM to read a page from backing store.
678 The page will be Locked when readpage is called, and should be
679 unlocked and marked uptodate once the read completes.
680 If ->readpage discovers that it needs to unlock the page for
681 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
682 In this case, the page will be relocated, relocked and if
683 that all succeeds, ->readpage will be called again.
685 writepages: called by the VM to write out pages associated with the
686 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
687 the writeback_control will specify a range of pages that must be
688 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
689 and that many pages should be written if possible.
690 If no ->writepages is given, then mpage_writepages is used
691 instead. This will choose pages from the address space that are
692 tagged as DIRTY and will pass them to ->writepage.
694 set_page_dirty: called by the VM to set a page dirty.
695 This is particularly needed if an address space attaches
696 private data to a page, and that data needs to be updated when
697 a page is dirtied. This is called, for example, when a memory
698 mapped page gets modified.
699 If defined, it should set the PageDirty flag, and the
700 PAGECACHE_TAG_DIRTY tag in the radix tree.
702 readpages: called by the VM to read pages associated with the address_space
703 object. This is essentially just a vector version of
704 readpage. Instead of just one page, several pages are
706 readpages is only used for read-ahead, so read errors are
707 ignored. If anything goes wrong, feel free to give up.
710 Called by the generic buffered write code to ask the filesystem to
711 prepare to write len bytes at the given offset in the file. The
712 address_space should check that the write will be able to complete,
713 by allocating space if necessary and doing any other internal
714 housekeeping. If the write will update parts of any basic-blocks on
715 storage, then those blocks should be pre-read (if they haven't been
716 read already) so that the updated blocks can be written out properly.
718 The filesystem must return the locked pagecache page for the specified
719 offset, in *pagep, for the caller to write into.
721 It must be able to cope with short writes (where the length passed to
722 write_begin is greater than the number of bytes copied into the page).
724 flags is a field for AOP_FLAG_xxx flags, described in
727 A void * may be returned in fsdata, which then gets passed into
730 Returns 0 on success; < 0 on failure (which is the error code), in
731 which case write_end is not called.
733 write_end: After a successful write_begin, and data copy, write_end must
734 be called. len is the original len passed to write_begin, and copied
735 is the amount that was able to be copied.
737 The filesystem must take care of unlocking the page and releasing it
738 refcount, and updating i_size.
740 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
741 that were able to be copied into pagecache.
743 bmap: called by the VFS to map a logical block offset within object to
744 physical block number. This method is used by the FIBMAP
745 ioctl and for working with swap-files. To be able to swap to
746 a file, the file must have a stable mapping to a block
747 device. The swap system does not go through the filesystem
748 but instead uses bmap to find out where the blocks in the file
749 are and uses those addresses directly.
751 invalidatepage: If a page has PagePrivate set, then invalidatepage
752 will be called when part or all of the page is to be removed
753 from the address space. This generally corresponds to either a
754 truncation, punch hole or a complete invalidation of the address
755 space (in the latter case 'offset' will always be 0 and 'length'
756 will be PAGE_SIZE). Any private data associated with the page
757 should be updated to reflect this truncation. If offset is 0 and
758 length is PAGE_SIZE, then the private data should be released,
759 because the page must be able to be completely discarded. This may
760 be done by calling the ->releasepage function, but in this case the
761 release MUST succeed.
763 releasepage: releasepage is called on PagePrivate pages to indicate
764 that the page should be freed if possible. ->releasepage
765 should remove any private data from the page and clear the
766 PagePrivate flag. If releasepage() fails for some reason, it must
767 indicate failure with a 0 return value.
768 releasepage() is used in two distinct though related cases. The
769 first is when the VM finds a clean page with no active users and
770 wants to make it a free page. If ->releasepage succeeds, the
771 page will be removed from the address_space and become free.
773 The second case is when a request has been made to invalidate
774 some or all pages in an address_space. This can happen
775 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
776 filesystem explicitly requesting it as nfs and 9fs do (when
777 they believe the cache may be out of date with storage) by
778 calling invalidate_inode_pages2().
779 If the filesystem makes such a call, and needs to be certain
780 that all pages are invalidated, then its releasepage will
781 need to ensure this. Possibly it can clear the PageUptodate
782 bit if it cannot free private data yet.
784 freepage: freepage is called once the page is no longer visible in
785 the page cache in order to allow the cleanup of any private
786 data. Since it may be called by the memory reclaimer, it
787 should not assume that the original address_space mapping still
788 exists, and it should not block.
790 direct_IO: called by the generic read/write routines to perform
791 direct_IO - that is IO requests which bypass the page cache
792 and transfer data directly between the storage and the
793 application's address space.
795 isolate_page: Called by the VM when isolating a movable non-lru page.
796 If page is successfully isolated, VM marks the page as PG_isolated
797 via __SetPageIsolated.
799 migrate_page: This is used to compact the physical memory usage.
800 If the VM wants to relocate a page (maybe off a memory card
801 that is signalling imminent failure) it will pass a new page
802 and an old page to this function. migrate_page should
803 transfer any private data across and update any references
804 that it has to the page.
806 putback_page: Called by the VM when isolated page's migration fails.
808 launder_page: Called before freeing a page - it writes back the dirty page. To
809 prevent redirtying the page, it is kept locked during the whole
812 is_partially_uptodate: Called by the VM when reading a file through the
813 pagecache when the underlying blocksize != pagesize. If the required
814 block is up to date then the read can complete without needing the IO
815 to bring the whole page up to date.
817 is_dirty_writeback: Called by the VM when attempting to reclaim a page.
818 The VM uses dirty and writeback information to determine if it needs
819 to stall to allow flushers a chance to complete some IO. Ordinarily
820 it can use PageDirty and PageWriteback but some filesystems have
821 more complex state (unstable pages in NFS prevent reclaim) or
822 do not set those flags due to locking problems. This callback
823 allows a filesystem to indicate to the VM if a page should be
824 treated as dirty or writeback for the purposes of stalling.
826 error_remove_page: normally set to generic_error_remove_page if truncation
827 is ok for this address space. Used for memory failure handling.
828 Setting this implies you deal with pages going away under you,
829 unless you have them locked or reference counts increased.
831 swap_activate: Called when swapon is used on a file to allocate
832 space if necessary and pin the block lookup information in
833 memory. A return value of zero indicates success,
834 in which case this file can be used to back swapspace.
836 swap_deactivate: Called during swapoff on files where swap_activate
843 A file object represents a file opened by a process. This is also known
844 as an "open file description" in POSIX parlance.
847 struct file_operations
848 ----------------------
850 This describes how the VFS can manipulate an open file. As of kernel
851 4.18, the following members are defined:
853 struct file_operations {
854 struct module *owner;
855 loff_t (*llseek) (struct file *, loff_t, int);
856 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
857 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
858 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
859 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
860 int (*iterate) (struct file *, struct dir_context *);
861 int (*iterate_shared) (struct file *, struct dir_context *);
862 __poll_t (*poll) (struct file *, struct poll_table_struct *);
863 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
864 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
865 int (*mmap) (struct file *, struct vm_area_struct *);
866 int (*open) (struct inode *, struct file *);
867 int (*flush) (struct file *, fl_owner_t id);
868 int (*release) (struct inode *, struct file *);
869 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
870 int (*fasync) (int, struct file *, int);
871 int (*lock) (struct file *, int, struct file_lock *);
872 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
873 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
874 int (*check_flags)(int);
875 int (*flock) (struct file *, int, struct file_lock *);
876 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
877 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
878 int (*setlease)(struct file *, long, struct file_lock **, void **);
879 long (*fallocate)(struct file *file, int mode, loff_t offset,
881 void (*show_fdinfo)(struct seq_file *m, struct file *f);
883 unsigned (*mmap_capabilities)(struct file *);
885 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
886 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
887 struct file *file_out, loff_t pos_out,
888 loff_t len, unsigned int remap_flags);
889 int (*fadvise)(struct file *, loff_t, loff_t, int);
892 Again, all methods are called without any locks being held, unless
895 llseek: called when the VFS needs to move the file position index
897 read: called by read(2) and related system calls
899 read_iter: possibly asynchronous read with iov_iter as destination
901 write: called by write(2) and related system calls
903 write_iter: possibly asynchronous write with iov_iter as source
905 iterate: called when the VFS needs to read the directory contents
907 iterate_shared: called when the VFS needs to read the directory contents
908 when filesystem supports concurrent dir iterators
910 poll: called by the VFS when a process wants to check if there is
911 activity on this file and (optionally) go to sleep until there
912 is activity. Called by the select(2) and poll(2) system calls
914 unlocked_ioctl: called by the ioctl(2) system call.
916 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
917 are used on 64 bit kernels.
919 mmap: called by the mmap(2) system call
921 open: called by the VFS when an inode should be opened. When the VFS
922 opens a file, it creates a new "struct file". It then calls the
923 open method for the newly allocated file structure. You might
924 think that the open method really belongs in
925 "struct inode_operations", and you may be right. I think it's
926 done the way it is because it makes filesystems simpler to
927 implement. The open() method is a good place to initialize the
928 "private_data" member in the file structure if you want to point
929 to a device structure
931 flush: called by the close(2) system call to flush a file
933 release: called when the last reference to an open file is closed
935 fsync: called by the fsync(2) system call. Also see the section above
936 entitled "Handling errors during writeback".
938 fasync: called by the fcntl(2) system call when asynchronous
939 (non-blocking) mode is enabled for a file
941 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
944 get_unmapped_area: called by the mmap(2) system call
946 check_flags: called by the fcntl(2) system call for F_SETFL command
948 flock: called by the flock(2) system call
950 splice_write: called by the VFS to splice data from a pipe to a file. This
951 method is used by the splice(2) system call
953 splice_read: called by the VFS to splice data from file to a pipe. This
954 method is used by the splice(2) system call
956 setlease: called by the VFS to set or release a file lock lease. setlease
957 implementations should call generic_setlease to record or remove
958 the lease in the inode after setting it.
960 fallocate: called by the VFS to preallocate blocks or punch a hole.
962 copy_file_range: called by the copy_file_range(2) system call.
964 remap_file_range: called by the ioctl(2) system call for FICLONERANGE and
965 FICLONE and FIDEDUPERANGE commands to remap file ranges. An
966 implementation should remap len bytes at pos_in of the source file into
967 the dest file at pos_out. Implementations must handle callers passing
968 in len == 0; this means "remap to the end of the source file". The
969 return value should the number of bytes remapped, or the usual
970 negative error code if errors occurred before any bytes were remapped.
971 The remap_flags parameter accepts REMAP_FILE_* flags. If
972 REMAP_FILE_DEDUP is set then the implementation must only remap if the
973 requested file ranges have identical contents. If REMAP_CAN_SHORTEN is
974 set, the caller is ok with the implementation shortening the request
975 length to satisfy alignment or EOF requirements (or any other reason).
977 fadvise: possibly called by the fadvise64() system call.
979 Note that the file operations are implemented by the specific
980 filesystem in which the inode resides. When opening a device node
981 (character or block special) most filesystems will call special
982 support routines in the VFS which will locate the required device
983 driver information. These support routines replace the filesystem file
984 operations with those for the device driver, and then proceed to call
985 the new open() method for the file. This is how opening a device file
986 in the filesystem eventually ends up calling the device driver open()
990 Directory Entry Cache (dcache)
991 ==============================
994 struct dentry_operations
995 ------------------------
997 This describes how a filesystem can overload the standard dentry
998 operations. Dentries and the dcache are the domain of the VFS and the
999 individual filesystem implementations. Device drivers have no business
1000 here. These methods may be set to NULL, as they are either optional or
1001 the VFS uses a default. As of kernel 2.6.22, the following members are
1004 struct dentry_operations {
1005 int (*d_revalidate)(struct dentry *, unsigned int);
1006 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1007 int (*d_hash)(const struct dentry *, struct qstr *);
1008 int (*d_compare)(const struct dentry *,
1009 unsigned int, const char *, const struct qstr *);
1010 int (*d_delete)(const struct dentry *);
1011 int (*d_init)(struct dentry *);
1012 void (*d_release)(struct dentry *);
1013 void (*d_iput)(struct dentry *, struct inode *);
1014 char *(*d_dname)(struct dentry *, char *, int);
1015 struct vfsmount *(*d_automount)(struct path *);
1016 int (*d_manage)(const struct path *, bool);
1017 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1020 d_revalidate: called when the VFS needs to revalidate a dentry. This
1021 is called whenever a name look-up finds a dentry in the
1022 dcache. Most local filesystems leave this as NULL, because all their
1023 dentries in the dcache are valid. Network filesystems are different
1024 since things can change on the server without the client necessarily
1027 This function should return a positive value if the dentry is still
1028 valid, and zero or a negative error code if it isn't.
1030 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
1031 If in rcu-walk mode, the filesystem must revalidate the dentry without
1032 blocking or storing to the dentry, d_parent and d_inode should not be
1033 used without care (because they can change and, in d_inode case, even
1034 become NULL under us).
1036 If a situation is encountered that rcu-walk cannot handle, return
1037 -ECHILD and it will be called again in ref-walk mode.
1039 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
1040 This is called when a path-walk ends at dentry that was not acquired by
1041 doing a lookup in the parent directory. This includes "/", "." and "..",
1042 as well as procfs-style symlinks and mountpoint traversal.
1044 In this case, we are less concerned with whether the dentry is still
1045 fully correct, but rather that the inode is still valid. As with
1046 d_revalidate, most local filesystems will set this to NULL since their
1047 dcache entries are always valid.
1049 This function has the same return code semantics as d_revalidate.
1051 d_weak_revalidate is only called after leaving rcu-walk mode.
1053 d_hash: called when the VFS adds a dentry to the hash table. The first
1054 dentry passed to d_hash is the parent directory that the name is
1057 Same locking and synchronisation rules as d_compare regarding
1058 what is safe to dereference etc.
1060 d_compare: called to compare a dentry name with a given name. The first
1061 dentry is the parent of the dentry to be compared, the second is
1062 the child dentry. len and name string are properties of the dentry
1063 to be compared. qstr is the name to compare it with.
1065 Must be constant and idempotent, and should not take locks if
1066 possible, and should not or store into the dentry.
1067 Should not dereference pointers outside the dentry without
1068 lots of care (eg. d_parent, d_inode, d_name should not be used).
1070 However, our vfsmount is pinned, and RCU held, so the dentries and
1071 inodes won't disappear, neither will our sb or filesystem module.
1074 It is a tricky calling convention because it needs to be called under
1075 "rcu-walk", ie. without any locks or references on things.
1077 d_delete: called when the last reference to a dentry is dropped and the
1078 dcache is deciding whether or not to cache it. Return 1 to delete
1079 immediately, or 0 to cache the dentry. Default is NULL which means to
1080 always cache a reachable dentry. d_delete must be constant and
1083 d_init: called when a dentry is allocated
1085 d_release: called when a dentry is really deallocated
1087 d_iput: called when a dentry loses its inode (just prior to its
1088 being deallocated). The default when this is NULL is that the
1089 VFS calls iput(). If you define this method, you must call
1092 d_dname: called when the pathname of a dentry should be generated.
1093 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1094 pathname generation. (Instead of doing it when dentry is created,
1095 it's done only when the path is needed.). Real filesystems probably
1096 dont want to use it, because their dentries are present in global
1097 dcache hash, so their hash should be an invariant. As no lock is
1098 held, d_dname() should not try to modify the dentry itself, unless
1099 appropriate SMP safety is used. CAUTION : d_path() logic is quite
1100 tricky. The correct way to return for example "Hello" is to put it
1101 at the end of the buffer, and returns a pointer to the first char.
1102 dynamic_dname() helper function is provided to take care of this.
1106 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1108 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1109 dentry->d_inode->i_ino);
1112 d_automount: called when an automount dentry is to be traversed (optional).
1113 This should create a new VFS mount record and return the record to the
1114 caller. The caller is supplied with a path parameter giving the
1115 automount directory to describe the automount target and the parent
1116 VFS mount record to provide inheritable mount parameters. NULL should
1117 be returned if someone else managed to make the automount first. If
1118 the vfsmount creation failed, then an error code should be returned.
1119 If -EISDIR is returned, then the directory will be treated as an
1120 ordinary directory and returned to pathwalk to continue walking.
1122 If a vfsmount is returned, the caller will attempt to mount it on the
1123 mountpoint and will remove the vfsmount from its expiration list in
1124 the case of failure. The vfsmount should be returned with 2 refs on
1125 it to prevent automatic expiration - the caller will clean up the
1128 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1129 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1132 d_manage: called to allow the filesystem to manage the transition from a
1133 dentry (optional). This allows autofs, for example, to hold up clients
1134 waiting to explore behind a 'mountpoint' while letting the daemon go
1135 past and construct the subtree there. 0 should be returned to let the
1136 calling process continue. -EISDIR can be returned to tell pathwalk to
1137 use this directory as an ordinary directory and to ignore anything
1138 mounted on it and not to check the automount flag. Any other error
1139 code will abort pathwalk completely.
1141 If the 'rcu_walk' parameter is true, then the caller is doing a
1142 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
1143 and the caller can be asked to leave it and call again by returning
1144 -ECHILD. -EISDIR may also be returned to tell pathwalk to
1145 ignore d_automount or any mounts.
1147 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1148 dentry being transited from.
1150 d_real: overlay/union type filesystems implement this method to return one of
1151 the underlying dentries hidden by the overlay. It is used in two
1154 Called from file_dentry() it returns the real dentry matching the inode
1155 argument. The real dentry may be from a lower layer already copied up,
1156 but still referenced from the file. This mode is selected with a
1157 non-NULL inode argument.
1159 With NULL inode the topmost real underlying dentry is returned.
1161 Each dentry has a pointer to its parent dentry, as well as a hash list
1162 of child dentries. Child dentries are basically like files in a
1166 Directory Entry Cache API
1167 --------------------------
1169 There are a number of functions defined which permit a filesystem to
1170 manipulate dentries:
1172 dget: open a new handle for an existing dentry (this just increments
1175 dput: close a handle for a dentry (decrements the usage count). If
1176 the usage count drops to 0, and the dentry is still in its
1177 parent's hash, the "d_delete" method is called to check whether
1178 it should be cached. If it should not be cached, or if the dentry
1179 is not hashed, it is deleted. Otherwise cached dentries are put
1180 into an LRU list to be reclaimed on memory shortage.
1182 d_drop: this unhashes a dentry from its parents hash list. A
1183 subsequent call to dput() will deallocate the dentry if its
1184 usage count drops to 0
1186 d_delete: delete a dentry. If there are no other open references to
1187 the dentry then the dentry is turned into a negative dentry
1188 (the d_iput() method is called). If there are other
1189 references, then d_drop() is called instead
1191 d_add: add a dentry to its parents hash list and then calls
1194 d_instantiate: add a dentry to the alias hash list for the inode and
1195 updates the "d_inode" member. The "i_count" member in the
1196 inode structure should be set/incremented. If the inode
1197 pointer is NULL, the dentry is called a "negative
1198 dentry". This function is commonly called when an inode is
1199 created for an existing negative dentry
1201 d_lookup: look up a dentry given its parent and path name component
1202 It looks up the child of that given name from the dcache
1203 hash table. If it is found, the reference count is incremented
1204 and the dentry is returned. The caller must use dput()
1205 to free the dentry when it finishes using it.
1213 On mount and remount the filesystem is passed a string containing a
1214 comma separated list of mount options. The options can have either of
1220 The <linux/parser.h> header defines an API that helps parse these
1221 options. There are plenty of examples on how to use it in existing
1227 If a filesystem accepts mount options, it must define show_options()
1228 to show all the currently active options. The rules are:
1230 - options MUST be shown which are not default or their values differ
1233 - options MAY be shown which are enabled by default or have their
1236 Options used only internally between a mount helper and the kernel
1237 (such as file descriptors), or which only have an effect during the
1238 mounting (such as ones controlling the creation of a journal) are exempt
1239 from the above rules.
1241 The underlying reason for the above rules is to make sure, that a
1242 mount can be accurately replicated (e.g. umounting and mounting again)
1243 based on the information found in /proc/mounts.
1248 (Note some of these resources are not up-to-date with the latest kernel
1251 Creating Linux virtual filesystems. 2002
1252 <http://lwn.net/Articles/13325/>
1254 The Linux Virtual File-system Layer by Neil Brown. 1999
1255 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1257 A tour of the Linux VFS by Michael K. Johnson. 1996
1258 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1260 A small trail through the Linux kernel by Andries Brouwer. 2001
1261 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
git.samba.org / sfrench / cifs-2.6.git / blob