1 .. SPDX-License-Identifier: GPL-2.0
3 =========================================
4 Overview of the Linux Virtual File System
5 =========================================
7 Original author: Richard Gooch <rgooch@atnf.csiro.au>
9 - Copyright (C) 1999 Richard Gooch
10 - Copyright (C) 2005 Pekka Enberg
16 The Virtual File System (also known as the Virtual Filesystem Switch) is
17 the software layer in the kernel that provides the filesystem interface
18 to userspace programs. It also provides an abstraction within the
19 kernel which allows different filesystem implementations to coexist.
21 VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22 are called from a process context. Filesystem locking is described in
23 the document Documentation/filesystems/locking.rst.
26 Directory Entry Cache (dcache)
27 ------------------------------
29 The VFS implements the open(2), stat(2), chmod(2), and similar system
30 calls. The pathname argument that is passed to them is used by the VFS
31 to search through the directory entry cache (also known as the dentry
32 cache or dcache). This provides a very fast look-up mechanism to
33 translate a pathname (filename) into a specific dentry. Dentries live
34 in RAM and are never saved to disc: they exist only for performance.
36 The dentry cache is meant to be a view into your entire filespace. As
37 most computers cannot fit all dentries in the RAM at the same time, some
38 bits of the cache are missing. In order to resolve your pathname into a
39 dentry, the VFS may have to resort to creating dentries along the way,
40 and then loading the inode. This is done by looking up the inode.
46 An individual dentry usually has a pointer to an inode. Inodes are
47 filesystem objects such as regular files, directories, FIFOs and other
48 beasts. They live either on the disc (for block device filesystems) or
49 in the memory (for pseudo filesystems). Inodes that live on the disc
50 are copied into the memory when required and changes to the inode are
51 written back to disc. A single inode can be pointed to by multiple
52 dentries (hard links, for example, do this).
54 To look up an inode requires that the VFS calls the lookup() method of
55 the parent directory inode. This method is installed by the specific
56 filesystem implementation that the inode lives in. Once the VFS has the
57 required dentry (and hence the inode), we can do all those boring things
58 like open(2) the file, or stat(2) it to peek at the inode data. The
59 stat(2) operation is fairly simple: once the VFS has the dentry, it
60 peeks at the inode data and passes some of it back to userspace.
66 Opening a file requires another operation: allocation of a file
67 structure (this is the kernel-side implementation of file descriptors).
68 The freshly allocated file structure is initialized with a pointer to
69 the dentry and a set of file operation member functions. These are
70 taken from the inode data. The open() file method is then called so the
71 specific filesystem implementation can do its work. You can see that
72 this is another switch performed by the VFS. The file structure is
73 placed into the file descriptor table for the process.
75 Reading, writing and closing files (and other assorted VFS operations)
76 is done by using the userspace file descriptor to grab the appropriate
77 file structure, and then calling the required file structure method to
78 do whatever is required. For as long as the file is open, it keeps the
79 dentry in use, which in turn means that the VFS inode is still in use.
82 Registering and Mounting a Filesystem
83 =====================================
85 To register and unregister a filesystem, use the following API
92 extern int register_filesystem(struct file_system_type *);
93 extern int unregister_filesystem(struct file_system_type *);
95 The passed struct file_system_type describes your filesystem. When a
96 request is made to mount a filesystem onto a directory in your
97 namespace, the VFS will call the appropriate mount() method for the
98 specific filesystem. New vfsmount referring to the tree returned by
99 ->mount() will be attached to the mountpoint, so that when pathname
100 resolution reaches the mountpoint it will jump into the root of that
103 You can see all filesystems that are registered to the kernel in the
104 file /proc/filesystems.
107 struct file_system_type
108 -----------------------
110 This describes the filesystem. The following
115 struct file_system_type {
118 int (*init_fs_context)(struct fs_context *);
119 const struct fs_parameter_spec *parameters;
120 struct dentry *(*mount) (struct file_system_type *, int,
121 const char *, void *);
122 void (*kill_sb) (struct super_block *);
123 struct module *owner;
124 struct file_system_type * next;
125 struct hlist_head fs_supers;
127 struct lock_class_key s_lock_key;
128 struct lock_class_key s_umount_key;
129 struct lock_class_key s_vfs_rename_key;
130 struct lock_class_key s_writers_key[SB_FREEZE_LEVELS];
132 struct lock_class_key i_lock_key;
133 struct lock_class_key i_mutex_key;
134 struct lock_class_key invalidate_lock_key;
135 struct lock_class_key i_mutex_dir_key;
139 the name of the filesystem type, such as "ext2", "iso9660",
143 various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
146 Initializes 'struct fs_context' ->ops and ->fs_private fields with
147 filesystem-specific data.
150 Pointer to the array of filesystem parameters descriptors
151 'struct fs_parameter_spec'.
152 More info in Documentation/filesystems/mount_api.rst.
155 the method to call when a new instance of this filesystem should
159 the method to call when an instance of this filesystem should be
164 for internal VFS use: you should initialize this to THIS_MODULE
168 for internal VFS use: you should initialize this to NULL
171 for internal VFS use: hlist of filesystem instances (superblocks)
173 s_lock_key, s_umount_key, s_vfs_rename_key, s_writers_key,
174 i_lock_key, i_mutex_key, invalidate_lock_key, i_mutex_dir_key: lockdep-specific
176 The mount() method has the following arguments:
178 ``struct file_system_type *fs_type``
179 describes the filesystem, partly initialized by the specific
185 ``const char *dev_name``
186 the device name we are mounting.
189 arbitrary mount options, usually comes as an ASCII string (see
190 "Mount Options" section)
192 The mount() method must return the root dentry of the tree requested by
193 caller. An active reference to its superblock must be grabbed and the
194 superblock must be locked. On failure it should return ERR_PTR(error).
196 The arguments match those of mount(2) and their interpretation depends
197 on filesystem type. E.g. for block filesystems, dev_name is interpreted
198 as block device name, that device is opened and if it contains a
199 suitable filesystem image the method creates and initializes struct
200 super_block accordingly, returning its root dentry to caller.
202 ->mount() may choose to return a subtree of existing filesystem - it
203 doesn't have to create a new one. The main result from the caller's
204 point of view is a reference to dentry at the root of (sub)tree to be
205 attached; creation of new superblock is a common side effect.
207 The most interesting member of the superblock structure that the mount()
208 method fills in is the "s_op" field. This is a pointer to a "struct
209 super_operations" which describes the next level of the filesystem
212 Usually, a filesystem uses one of the generic mount() implementations
213 and provides a fill_super() callback instead. The generic variants are:
216 mount a filesystem residing on a block device
219 mount a filesystem that is not backed by a device
222 mount a filesystem which shares the instance between all mounts
224 A fill_super() callback implementation has the following arguments:
226 ``struct super_block *sb``
227 the superblock structure. The callback must initialize this
231 arbitrary mount options, usually comes as an ASCII string (see
232 "Mount Options" section)
235 whether or not to be silent on error
238 The Superblock Object
239 =====================
241 A superblock object represents a mounted filesystem.
244 struct super_operations
245 -----------------------
247 This describes how the VFS can manipulate the superblock of your
248 filesystem. The following members are defined:
252 struct super_operations {
253 struct inode *(*alloc_inode)(struct super_block *sb);
254 void (*destroy_inode)(struct inode *);
255 void (*free_inode)(struct inode *);
257 void (*dirty_inode) (struct inode *, int flags);
258 int (*write_inode) (struct inode *, struct writeback_control *wbc);
259 int (*drop_inode) (struct inode *);
260 void (*evict_inode) (struct inode *);
261 void (*put_super) (struct super_block *);
262 int (*sync_fs)(struct super_block *sb, int wait);
263 int (*freeze_super) (struct super_block *sb,
264 enum freeze_holder who);
265 int (*freeze_fs) (struct super_block *);
266 int (*thaw_super) (struct super_block *sb,
267 enum freeze_wholder who);
268 int (*unfreeze_fs) (struct super_block *);
269 int (*statfs) (struct dentry *, struct kstatfs *);
270 int (*remount_fs) (struct super_block *, int *, char *);
271 void (*umount_begin) (struct super_block *);
273 int (*show_options)(struct seq_file *, struct dentry *);
274 int (*show_devname)(struct seq_file *, struct dentry *);
275 int (*show_path)(struct seq_file *, struct dentry *);
276 int (*show_stats)(struct seq_file *, struct dentry *);
278 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
279 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
280 struct dquot **(*get_dquots)(struct inode *);
282 long (*nr_cached_objects)(struct super_block *,
283 struct shrink_control *);
284 long (*free_cached_objects)(struct super_block *,
285 struct shrink_control *);
288 All methods are called without any locks being held, unless otherwise
289 noted. This means that most methods can block safely. All methods are
290 only called from a process context (i.e. not from an interrupt handler
294 this method is called by alloc_inode() to allocate memory for
295 struct inode and initialize it. If this function is not
296 defined, a simple 'struct inode' is allocated. Normally
297 alloc_inode will be used to allocate a larger structure which
298 contains a 'struct inode' embedded within it.
301 this method is called by destroy_inode() to release resources
302 allocated for struct inode. It is only required if
303 ->alloc_inode was defined and simply undoes anything done by
307 this method is called from RCU callback. If you use call_rcu()
308 in ->destroy_inode to free 'struct inode' memory, then it's
309 better to release memory in this method.
312 this method is called by the VFS when an inode is marked dirty.
313 This is specifically for the inode itself being marked dirty,
314 not its data. If the update needs to be persisted by fdatasync(),
315 then I_DIRTY_DATASYNC will be set in the flags argument.
316 I_DIRTY_TIME will be set in the flags in case lazytime is enabled
317 and struct inode has times updated since the last ->dirty_inode
321 this method is called when the VFS needs to write an inode to
322 disc. The second parameter indicates whether the write should
323 be synchronous or not, not all filesystems check this flag.
326 called when the last access to the inode is dropped, with the
327 inode->i_lock spinlock held.
329 This method should be either NULL (normal UNIX filesystem
330 semantics) or "generic_delete_inode" (for filesystems that do
331 not want to cache inodes - causing "delete_inode" to always be
332 called regardless of the value of i_nlink)
334 The "generic_delete_inode()" behavior is equivalent to the old
335 practice of using "force_delete" in the put_inode() case, but
336 does not have the races that the "force_delete()" approach had.
339 called when the VFS wants to evict an inode. Caller does
340 *not* evict the pagecache or inode-associated metadata buffers;
341 the method has to use truncate_inode_pages_final() to get rid
342 of those. Caller makes sure async writeback cannot be running for
343 the inode while (or after) ->evict_inode() is called. Optional.
346 called when the VFS wishes to free the superblock
347 (i.e. unmount). This is called with the superblock lock held
350 called when VFS is writing out all dirty data associated with a
351 superblock. The second parameter indicates whether the method
352 should wait until the write out has been completed. Optional.
355 Called instead of ->freeze_fs callback if provided.
356 Main difference is that ->freeze_super is called without taking
357 down_write(&sb->s_umount). If filesystem implements it and wants
358 ->freeze_fs to be called too, then it has to call ->freeze_fs
359 explicitly from this callback. Optional.
362 called when VFS is locking a filesystem and forcing it into a
363 consistent state. This method is currently used by the Logical
364 Volume Manager (LVM) and ioctl(FIFREEZE). Optional.
367 called when VFS is unlocking a filesystem and making it writable
368 again after ->freeze_super. Optional.
371 called when VFS is unlocking a filesystem and making it writable
372 again after ->freeze_fs. Optional.
375 called when the VFS needs to get filesystem statistics.
378 called when the filesystem is remounted. This is called with
382 called when the VFS is unmounting a filesystem.
385 called by the VFS to show mount options for /proc/<pid>/mounts
386 and /proc/<pid>/mountinfo.
387 (see "Mount Options" section)
390 Optional. Called by the VFS to show device name for
391 /proc/<pid>/{mounts,mountinfo,mountstats}. If not provided then
392 '(struct mount).mnt_devname' will be used.
395 Optional. Called by the VFS (for /proc/<pid>/mountinfo) to show
396 the mount root dentry path relative to the filesystem root.
399 Optional. Called by the VFS (for /proc/<pid>/mountstats) to show
400 filesystem-specific mount statistics.
403 called by the VFS to read from filesystem quota file.
406 called by the VFS to write to filesystem quota file.
409 called by quota to get 'struct dquot' array for a particular inode.
412 ``nr_cached_objects``
413 called by the sb cache shrinking function for the filesystem to
414 return the number of freeable cached objects it contains.
417 ``free_cache_objects``
418 called by the sb cache shrinking function for the filesystem to
419 scan the number of objects indicated to try to free them.
420 Optional, but any filesystem implementing this method needs to
421 also implement ->nr_cached_objects for it to be called
424 We can't do anything with any errors that the filesystem might
425 encountered, hence the void return type. This will never be
426 called if the VM is trying to reclaim under GFP_NOFS conditions,
427 hence this method does not need to handle that situation itself.
429 Implementations must include conditional reschedule calls inside
430 any scanning loop that is done. This allows the VFS to
431 determine appropriate scan batch sizes without having to worry
432 about whether implementations will cause holdoff problems due to
433 large scan batch sizes.
435 Whoever sets up the inode is responsible for filling in the "i_op"
436 field. This is a pointer to a "struct inode_operations" which describes
437 the methods that can be performed on individual inodes.
440 struct xattr_handlers
441 ---------------------
443 On filesystems that support extended attributes (xattrs), the s_xattr
444 superblock field points to a NULL-terminated array of xattr handlers.
445 Extended attributes are name:value pairs.
448 Indicates that the handler matches attributes with the specified
449 name (such as "system.posix_acl_access"); the prefix field must
453 Indicates that the handler matches all attributes with the
454 specified name prefix (such as "user."); the name field must be
458 Determine if attributes matching this xattr handler should be
459 listed for a particular dentry. Used by some listxattr
460 implementations like generic_listxattr.
463 Called by the VFS to get the value of a particular extended
464 attribute. This method is called by the getxattr(2) system
468 Called by the VFS to set the value of a particular extended
469 attribute. When the new value is NULL, called to remove a
470 particular extended attribute. This method is called by the
471 setxattr(2) and removexattr(2) system calls.
473 When none of the xattr handlers of a filesystem match the specified
474 attribute name or when a filesystem doesn't support extended attributes,
475 the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
481 An inode object represents an object within the filesystem.
484 struct inode_operations
485 -----------------------
487 This describes how the VFS can manipulate an inode in your filesystem.
488 As of kernel 2.6.22, the following members are defined:
492 struct inode_operations {
493 int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t, bool);
494 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
495 int (*link) (struct dentry *,struct inode *,struct dentry *);
496 int (*unlink) (struct inode *,struct dentry *);
497 int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
498 int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
499 int (*rmdir) (struct inode *,struct dentry *);
500 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
501 int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
502 struct inode *, struct dentry *, unsigned int);
503 int (*readlink) (struct dentry *, char __user *,int);
504 const char *(*get_link) (struct dentry *, struct inode *,
505 struct delayed_call *);
506 int (*permission) (struct mnt_idmap *, struct inode *, int);
507 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
508 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
509 int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
510 ssize_t (*listxattr) (struct dentry *, char *, size_t);
511 void (*update_time)(struct inode *, struct timespec *, int);
512 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
513 unsigned open_flag, umode_t create_mode);
514 int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
515 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
516 int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int);
517 int (*fileattr_set)(struct mnt_idmap *idmap,
518 struct dentry *dentry, struct fileattr *fa);
519 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
522 Again, all methods are called without any locks being held, unless
526 called by the open(2) and creat(2) system calls. Only required
527 if you want to support regular files. The dentry you get should
528 not have an inode (i.e. it should be a negative dentry). Here
529 you will probably call d_instantiate() with the dentry and the
533 called when the VFS needs to look up an inode in a parent
534 directory. The name to look for is found in the dentry. This
535 method must call d_add() to insert the found inode into the
536 dentry. The "i_count" field in the inode structure should be
537 incremented. If the named inode does not exist a NULL inode
538 should be inserted into the dentry (this is called a negative
539 dentry). Returning an error code from this routine must only be
540 done on a real error, otherwise creating inodes with system
541 calls like create(2), mknod(2), mkdir(2) and so on will fail.
542 If you wish to overload the dentry methods then you should
543 initialise the "d_dop" field in the dentry; this is a pointer to
544 a struct "dentry_operations". This method is called with the
545 directory inode semaphore held
548 called by the link(2) system call. Only required if you want to
549 support hard links. You will probably need to call
550 d_instantiate() just as you would in the create() method
553 called by the unlink(2) system call. Only required if you want
554 to support deleting inodes
557 called by the symlink(2) system call. Only required if you want
558 to support symlinks. You will probably need to call
559 d_instantiate() just as you would in the create() method
562 called by the mkdir(2) system call. Only required if you want
563 to support creating subdirectories. You will probably need to
564 call d_instantiate() just as you would in the create() method
567 called by the rmdir(2) system call. Only required if you want
568 to support deleting subdirectories
571 called by the mknod(2) system call to create a device (char,
572 block) inode or a named pipe (FIFO) or socket. Only required if
573 you want to support creating these types of inodes. You will
574 probably need to call d_instantiate() just as you would in the
578 called by the rename(2) system call to rename the object to have
579 the parent and name given by the second inode and dentry.
581 The filesystem must return -EINVAL for any unsupported or
582 unknown flags. Currently the following flags are implemented:
583 (1) RENAME_NOREPLACE: this flag indicates that if the target of
584 the rename exists the rename should fail with -EEXIST instead of
585 replacing the target. The VFS already checks for existence, so
586 for local filesystems the RENAME_NOREPLACE implementation is
587 equivalent to plain rename.
588 (2) RENAME_EXCHANGE: exchange source and target. Both must
589 exist; this is checked by the VFS. Unlike plain rename, source
590 and target may be of different type.
593 called by the VFS to follow a symbolic link to the inode it
594 points to. Only required if you want to support symbolic links.
595 This method returns the symlink body to traverse (and possibly
596 resets the current position with nd_jump_link()). If the body
597 won't go away until the inode is gone, nothing else is needed;
598 if it needs to be otherwise pinned, arrange for its release by
599 having get_link(..., ..., done) do set_delayed_call(done,
600 destructor, argument). In that case destructor(argument) will
601 be called once VFS is done with the body you've returned. May
602 be called in RCU mode; that is indicated by NULL dentry
603 argument. If request can't be handled without leaving RCU mode,
604 have it return ERR_PTR(-ECHILD).
606 If the filesystem stores the symlink target in ->i_link, the
607 VFS may use it directly without calling ->get_link(); however,
608 ->get_link() must still be provided. ->i_link must not be
609 freed until after an RCU grace period. Writing to ->i_link
610 post-iget() time requires a 'release' memory barrier.
613 this is now just an override for use by readlink(2) for the
614 cases when ->get_link uses nd_jump_link() or object is not in
615 fact a symlink. Normally filesystems should only implement
616 ->get_link for symlinks and readlink(2) will automatically use
620 called by the VFS to check for access rights on a POSIX-like
623 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
624 rcu-walk mode, the filesystem must check the permission without
625 blocking or storing to the inode.
627 If a situation is encountered that rcu-walk cannot handle,
629 -ECHILD and it will be called again in ref-walk mode.
632 called by the VFS to set attributes for a file. This method is
633 called by chmod(2) and related system calls.
636 called by the VFS to get attributes of a file. This method is
637 called by stat(2) and related system calls.
640 called by the VFS to list all extended attributes for a given
641 file. This method is called by the listxattr(2) system call.
644 called by the VFS to update a specific time or the i_version of
645 an inode. If this is not defined the VFS will update the inode
646 itself and call mark_inode_dirty_sync.
649 called on the last component of an open. Using this optional
650 method the filesystem can look up, possibly create and open the
651 file in one atomic operation. If it wants to leave actual
652 opening to the caller (e.g. if the file turned out to be a
653 symlink, device, or just something filesystem won't do atomic
654 open for), it may signal this by returning finish_no_open(file,
655 dentry). This method is only called if the last component is
656 negative or needs lookup. Cached positive dentries are still
657 handled by f_op->open(). If the file was created, FMODE_CREATED
658 flag should be set in file->f_mode. In case of O_EXCL the
659 method must only succeed if the file didn't exist and hence
660 FMODE_CREATED shall always be set on success.
663 called in the end of O_TMPFILE open(). Optional, equivalent to
664 atomically creating, opening and unlinking a file in given
665 directory. On success needs to return with the file already
666 open; this can be done by calling finish_open_simple() right at
670 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
671 retrieve miscellaneous file flags and attributes. Also called
672 before the relevant SET operation to check what is being changed
673 (in this case with i_rwsem locked exclusive). If unset, then
674 fall back to f_op->ioctl().
677 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
678 change miscellaneous file flags and attributes. Callers hold
679 i_rwsem exclusive. If unset, then fall back to f_op->ioctl().
682 The Address Space Object
683 ========================
685 The address space object is used to group and manage pages in the page
686 cache. It can be used to keep track of the pages in a file (or anything
687 else) and also track the mapping of sections of the file into process
690 There are a number of distinct yet related services that an
691 address-space can provide. These include communicating memory pressure,
692 page lookup by address, and keeping track of pages tagged as Dirty or
695 The first can be used independently to the others. The VM can try to
696 either write dirty pages in order to clean them, or release clean pages
697 in order to reuse them. To do this it can call the ->writepage method
698 on dirty pages, and ->release_folio on clean folios with the private
699 flag set. Clean pages without PagePrivate and with no external references
700 will be released without notice being given to the address_space.
702 To achieve this functionality, pages need to be placed on an LRU with
703 lru_cache_add and mark_page_active needs to be called whenever the page
706 Pages are normally kept in a radix tree index by ->index. This tree
707 maintains information about the PG_Dirty and PG_Writeback status of each
708 page, so that pages with either of these flags can be found quickly.
710 The Dirty tag is primarily used by mpage_writepages - the default
711 ->writepages method. It uses the tag to find dirty pages to call
712 ->writepage on. If mpage_writepages is not used (i.e. the address
713 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
714 unused. write_inode_now and sync_inode do use it (through
715 __sync_single_inode) to check if ->writepages has been successful in
716 writing out the whole address_space.
718 The Writeback tag is used by filemap*wait* and sync_page* functions, via
719 filemap_fdatawait_range, to wait for all writeback to complete.
721 An address_space handler may attach extra information to a page,
722 typically using the 'private' field in the 'struct page'. If such
723 information is attached, the PG_Private flag should be set. This will
724 cause various VM routines to make extra calls into the address_space
725 handler to deal with that data.
727 An address space acts as an intermediate between storage and
728 application. Data is read into the address space a whole page at a
729 time, and provided to the application either by copying of the page, or
730 by memory-mapping the page. Data is written into the address space by
731 the application, and then written-back to storage typically in whole
732 pages, however the address_space has finer control of write sizes.
734 The read process essentially only requires 'read_folio'. The write
735 process is more complicated and uses write_begin/write_end or
736 dirty_folio to write data into the address_space, and writepage and
737 writepages to writeback data to storage.
739 Adding and removing pages to/from an address_space is protected by the
742 When data is written to a page, the PG_Dirty flag should be set. It
743 typically remains set until writepage asks for it to be written. This
744 should clear PG_Dirty and set PG_Writeback. It can be actually written
745 at any point after PG_Dirty is clear. Once it is known to be safe,
746 PG_Writeback is cleared.
748 Writeback makes use of a writeback_control structure to direct the
749 operations. This gives the writepage and writepages operations some
750 information about the nature of and reason for the writeback request,
751 and the constraints under which it is being done. It is also used to
752 return information back to the caller about the result of a writepage or
756 Handling errors during writeback
757 --------------------------------
759 Most applications that do buffered I/O will periodically call a file
760 synchronization call (fsync, fdatasync, msync or sync_file_range) to
761 ensure that data written has made it to the backing store. When there
762 is an error during writeback, they expect that error to be reported when
763 a file sync request is made. After an error has been reported on one
764 request, subsequent requests on the same file descriptor should return
765 0, unless further writeback errors have occurred since the previous file
768 Ideally, the kernel would report errors only on file descriptions on
769 which writes were done that subsequently failed to be written back. The
770 generic pagecache infrastructure does not track the file descriptions
771 that have dirtied each individual page however, so determining which
772 file descriptors should get back an error is not possible.
774 Instead, the generic writeback error tracking infrastructure in the
775 kernel settles for reporting errors to fsync on all file descriptions
776 that were open at the time that the error occurred. In a situation with
777 multiple writers, all of them will get back an error on a subsequent
778 fsync, even if all of the writes done through that particular file
779 descriptor succeeded (or even if there were no writes on that file
782 Filesystems that wish to use this infrastructure should call
783 mapping_set_error to record the error in the address_space when it
784 occurs. Then, after writing back data from the pagecache in their
785 file->fsync operation, they should call file_check_and_advance_wb_err to
786 ensure that the struct file's error cursor has advanced to the correct
787 point in the stream of errors emitted by the backing device(s).
790 struct address_space_operations
791 -------------------------------
793 This describes how the VFS can manipulate mapping of a file to page
794 cache in your filesystem. The following members are defined:
798 struct address_space_operations {
799 int (*writepage)(struct page *page, struct writeback_control *wbc);
800 int (*read_folio)(struct file *, struct folio *);
801 int (*writepages)(struct address_space *, struct writeback_control *);
802 bool (*dirty_folio)(struct address_space *, struct folio *);
803 void (*readahead)(struct readahead_control *);
804 int (*write_begin)(struct file *, struct address_space *mapping,
805 loff_t pos, unsigned len,
806 struct page **pagep, void **fsdata);
807 int (*write_end)(struct file *, struct address_space *mapping,
808 loff_t pos, unsigned len, unsigned copied,
809 struct page *page, void *fsdata);
810 sector_t (*bmap)(struct address_space *, sector_t);
811 void (*invalidate_folio) (struct folio *, size_t start, size_t len);
812 bool (*release_folio)(struct folio *, gfp_t);
813 void (*free_folio)(struct folio *);
814 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
815 int (*migrate_folio)(struct mapping *, struct folio *dst,
816 struct folio *src, enum migrate_mode);
817 int (*launder_folio) (struct folio *);
819 bool (*is_partially_uptodate) (struct folio *, size_t from,
821 void (*is_dirty_writeback)(struct folio *, bool *, bool *);
822 int (*error_remove_page) (struct mapping *mapping, struct page *page);
823 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
824 int (*swap_deactivate)(struct file *);
825 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
829 called by the VM to write a dirty page to backing store. This
830 may happen for data integrity reasons (i.e. 'sync'), or to free
831 up memory (flush). The difference can be seen in
832 wbc->sync_mode. The PG_Dirty flag has been cleared and
833 PageLocked is true. writepage should start writeout, should set
834 PG_Writeback, and should make sure the page is unlocked, either
835 synchronously or asynchronously when the write operation
838 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
839 try too hard if there are problems, and may choose to write out
840 other pages from the mapping if that is easier (e.g. due to
841 internal dependencies). If it chooses not to start writeout, it
842 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
843 keep calling ->writepage on that page.
845 See the file "Locking" for more details.
848 Called by the page cache to read a folio from the backing store.
849 The 'file' argument supplies authentication information to network
850 filesystems, and is generally not used by block based filesystems.
851 It may be NULL if the caller does not have an open file (eg if
852 the kernel is performing a read for itself rather than on behalf
853 of a userspace process with an open file).
855 If the mapping does not support large folios, the folio will
856 contain a single page. The folio will be locked when read_folio
857 is called. If the read completes successfully, the folio should
858 be marked uptodate. The filesystem should unlock the folio
859 once the read has completed, whether it was successful or not.
860 The filesystem does not need to modify the refcount on the folio;
861 the page cache holds a reference count and that will not be
862 released until the folio is unlocked.
864 Filesystems may implement ->read_folio() synchronously.
865 In normal operation, folios are read through the ->readahead()
866 method. Only if this fails, or if the caller needs to wait for
867 the read to complete will the page cache call ->read_folio().
868 Filesystems should not attempt to perform their own readahead
869 in the ->read_folio() operation.
871 If the filesystem cannot perform the read at this time, it can
872 unlock the folio, do whatever action it needs to ensure that the
873 read will succeed in the future and return AOP_TRUNCATED_PAGE.
874 In this case, the caller should look up the folio, lock it,
875 and call ->read_folio again.
877 Callers may invoke the ->read_folio() method directly, but using
878 read_mapping_folio() will take care of locking, waiting for the
879 read to complete and handle cases such as AOP_TRUNCATED_PAGE.
882 called by the VM to write out pages associated with the
883 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
884 the writeback_control will specify a range of pages that must be
885 written out. If it is WB_SYNC_NONE, then a nr_to_write is
886 given and that many pages should be written if possible. If no
887 ->writepages is given, then mpage_writepages is used instead.
888 This will choose pages from the address space that are tagged as
889 DIRTY and will pass them to ->writepage.
892 called by the VM to mark a folio as dirty. This is particularly
893 needed if an address space attaches private data to a folio, and
894 that data needs to be updated when a folio is dirtied. This is
895 called, for example, when a memory mapped page gets modified.
896 If defined, it should set the folio dirty flag, and the
897 PAGECACHE_TAG_DIRTY search mark in i_pages.
900 Called by the VM to read pages associated with the address_space
901 object. The pages are consecutive in the page cache and are
902 locked. The implementation should decrement the page refcount
903 after starting I/O on each page. Usually the page will be
904 unlocked by the I/O completion handler. The set of pages are
905 divided into some sync pages followed by some async pages,
906 rac->ra->async_size gives the number of async pages. The
907 filesystem should attempt to read all sync pages but may decide
908 to stop once it reaches the async pages. If it does decide to
909 stop attempting I/O, it can simply return. The caller will
910 remove the remaining pages from the address space, unlock them
911 and decrement the page refcount. Set PageUptodate if the I/O
912 completes successfully. Setting PageError on any page will be
913 ignored; simply unlock the page if an I/O error occurs.
916 Called by the generic buffered write code to ask the filesystem
917 to prepare to write len bytes at the given offset in the file.
918 The address_space should check that the write will be able to
919 complete, by allocating space if necessary and doing any other
920 internal housekeeping. If the write will update parts of any
921 basic-blocks on storage, then those blocks should be pre-read
922 (if they haven't been read already) so that the updated blocks
923 can be written out properly.
925 The filesystem must return the locked pagecache page for the
926 specified offset, in ``*pagep``, for the caller to write into.
928 It must be able to cope with short writes (where the length
929 passed to write_begin is greater than the number of bytes copied
932 A void * may be returned in fsdata, which then gets passed into
935 Returns 0 on success; < 0 on failure (which is the error code),
936 in which case write_end is not called.
939 After a successful write_begin, and data copy, write_end must be
940 called. len is the original len passed to write_begin, and
941 copied is the amount that was able to be copied.
943 The filesystem must take care of unlocking the page and
944 releasing it refcount, and updating i_size.
946 Returns < 0 on failure, otherwise the number of bytes (<=
947 'copied') that were able to be copied into pagecache.
950 called by the VFS to map a logical block offset within object to
951 physical block number. This method is used by the FIBMAP ioctl
952 and for working with swap-files. To be able to swap to a file,
953 the file must have a stable mapping to a block device. The swap
954 system does not go through the filesystem but instead uses bmap
955 to find out where the blocks in the file are and uses those
959 If a folio has private data, then invalidate_folio will be
960 called when part or all of the folio is to be removed from the
961 address space. This generally corresponds to either a
962 truncation, punch hole or a complete invalidation of the address
963 space (in the latter case 'offset' will always be 0 and 'length'
964 will be folio_size()). Any private data associated with the folio
965 should be updated to reflect this truncation. If offset is 0
966 and length is folio_size(), then the private data should be
967 released, because the folio must be able to be completely
968 discarded. This may be done by calling the ->release_folio
969 function, but in this case the release MUST succeed.
972 release_folio is called on folios with private data to tell the
973 filesystem that the folio is about to be freed. ->release_folio
974 should remove any private data from the folio and clear the
975 private flag. If release_folio() fails, it should return false.
976 release_folio() is used in two distinct though related cases.
977 The first is when the VM wants to free a clean folio with no
978 active users. If ->release_folio succeeds, the folio will be
979 removed from the address_space and be freed.
981 The second case is when a request has been made to invalidate
982 some or all folios in an address_space. This can happen
983 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
984 filesystem explicitly requesting it as nfs and 9p do (when they
985 believe the cache may be out of date with storage) by calling
986 invalidate_inode_pages2(). If the filesystem makes such a call,
987 and needs to be certain that all folios are invalidated, then
988 its release_folio will need to ensure this. Possibly it can
989 clear the uptodate flag if it cannot free private data yet.
992 free_folio is called once the folio is no longer visible in the
993 page cache in order to allow the cleanup of any private data.
994 Since it may be called by the memory reclaimer, it should not
995 assume that the original address_space mapping still exists, and
999 called by the generic read/write routines to perform direct_IO -
1000 that is IO requests which bypass the page cache and transfer
1001 data directly between the storage and the application's address
1005 This is used to compact the physical memory usage. If the VM
1006 wants to relocate a folio (maybe from a memory device that is
1007 signalling imminent failure) it will pass a new folio and an old
1008 folio to this function. migrate_folio should transfer any private
1009 data across and update any references that it has to the folio.
1012 Called before freeing a folio - it writes back the dirty folio.
1013 To prevent redirtying the folio, it is kept locked during the
1016 ``is_partially_uptodate``
1017 Called by the VM when reading a file through the pagecache when
1018 the underlying blocksize is smaller than the size of the folio.
1019 If the required block is up to date then the read can complete
1020 without needing I/O to bring the whole page up to date.
1022 ``is_dirty_writeback``
1023 Called by the VM when attempting to reclaim a folio. The VM uses
1024 dirty and writeback information to determine if it needs to
1025 stall to allow flushers a chance to complete some IO.
1026 Ordinarily it can use folio_test_dirty and folio_test_writeback but
1027 some filesystems have more complex state (unstable folios in NFS
1028 prevent reclaim) or do not set those flags due to locking
1029 problems. This callback allows a filesystem to indicate to the
1030 VM if a folio should be treated as dirty or writeback for the
1031 purposes of stalling.
1033 ``error_remove_page``
1034 normally set to generic_error_remove_page if truncation is ok
1035 for this address space. Used for memory failure handling.
1036 Setting this implies you deal with pages going away under you,
1037 unless you have them locked or reference counts increased.
1041 Called to prepare the given file for swap. It should perform
1042 any validation and preparation necessary to ensure that writes
1043 can be performed with minimal memory allocation. It should call
1044 add_swap_extent(), or the helper iomap_swapfile_activate(), and
1045 return the number of extents added. If IO should be submitted
1046 through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
1047 be submitted directly to the block device ``sis->bdev``.
1050 Called during swapoff on files where swap_activate was
1054 Called to read or write swap pages when SWP_FS_OPS is set.
1059 A file object represents a file opened by a process. This is also known
1060 as an "open file description" in POSIX parlance.
1063 struct file_operations
1064 ----------------------
1066 This describes how the VFS can manipulate an open file. As of kernel
1067 4.18, the following members are defined:
1071 struct file_operations {
1072 struct module *owner;
1073 loff_t (*llseek) (struct file *, loff_t, int);
1074 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1075 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1076 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1077 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1078 int (*iopoll)(struct kiocb *kiocb, bool spin);
1079 int (*iterate) (struct file *, struct dir_context *);
1080 int (*iterate_shared) (struct file *, struct dir_context *);
1081 __poll_t (*poll) (struct file *, struct poll_table_struct *);
1082 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1083 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1084 int (*mmap) (struct file *, struct vm_area_struct *);
1085 int (*open) (struct inode *, struct file *);
1086 int (*flush) (struct file *, fl_owner_t id);
1087 int (*release) (struct inode *, struct file *);
1088 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1089 int (*fasync) (int, struct file *, int);
1090 int (*lock) (struct file *, int, struct file_lock *);
1091 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1092 int (*check_flags)(int);
1093 int (*flock) (struct file *, int, struct file_lock *);
1094 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1095 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1096 int (*setlease)(struct file *, long, struct file_lock **, void **);
1097 long (*fallocate)(struct file *file, int mode, loff_t offset,
1099 void (*show_fdinfo)(struct seq_file *m, struct file *f);
1101 unsigned (*mmap_capabilities)(struct file *);
1103 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1104 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1105 struct file *file_out, loff_t pos_out,
1106 loff_t len, unsigned int remap_flags);
1107 int (*fadvise)(struct file *, loff_t, loff_t, int);
1110 Again, all methods are called without any locks being held, unless
1114 called when the VFS needs to move the file position index
1117 called by read(2) and related system calls
1120 possibly asynchronous read with iov_iter as destination
1123 called by write(2) and related system calls
1126 possibly asynchronous write with iov_iter as source
1129 called when aio wants to poll for completions on HIPRI iocbs
1132 called when the VFS needs to read the directory contents
1135 called when the VFS needs to read the directory contents when
1136 filesystem supports concurrent dir iterators
1139 called by the VFS when a process wants to check if there is
1140 activity on this file and (optionally) go to sleep until there
1141 is activity. Called by the select(2) and poll(2) system calls
1144 called by the ioctl(2) system call.
1147 called by the ioctl(2) system call when 32 bit system calls are
1148 used on 64 bit kernels.
1151 called by the mmap(2) system call
1154 called by the VFS when an inode should be opened. When the VFS
1155 opens a file, it creates a new "struct file". It then calls the
1156 open method for the newly allocated file structure. You might
1157 think that the open method really belongs in "struct
1158 inode_operations", and you may be right. I think it's done the
1159 way it is because it makes filesystems simpler to implement.
1160 The open() method is a good place to initialize the
1161 "private_data" member in the file structure if you want to point
1162 to a device structure
1165 called by the close(2) system call to flush a file
1168 called when the last reference to an open file is closed
1171 called by the fsync(2) system call. Also see the section above
1172 entitled "Handling errors during writeback".
1175 called by the fcntl(2) system call when asynchronous
1176 (non-blocking) mode is enabled for a file
1179 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1182 ``get_unmapped_area``
1183 called by the mmap(2) system call
1186 called by the fcntl(2) system call for F_SETFL command
1189 called by the flock(2) system call
1192 called by the VFS to splice data from a pipe to a file. This
1193 method is used by the splice(2) system call
1196 called by the VFS to splice data from file to a pipe. This
1197 method is used by the splice(2) system call
1200 called by the VFS to set or release a file lock lease. setlease
1201 implementations should call generic_setlease to record or remove
1202 the lease in the inode after setting it.
1205 called by the VFS to preallocate blocks or punch a hole.
1208 called by the copy_file_range(2) system call.
1210 ``remap_file_range``
1211 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1212 and FIDEDUPERANGE commands to remap file ranges. An
1213 implementation should remap len bytes at pos_in of the source
1214 file into the dest file at pos_out. Implementations must handle
1215 callers passing in len == 0; this means "remap to the end of the
1216 source file". The return value should the number of bytes
1217 remapped, or the usual negative error code if errors occurred
1218 before any bytes were remapped. The remap_flags parameter
1219 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1220 implementation must only remap if the requested file ranges have
1221 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
1222 ok with the implementation shortening the request length to
1223 satisfy alignment or EOF requirements (or any other reason).
1226 possibly called by the fadvise64() system call.
1228 Note that the file operations are implemented by the specific
1229 filesystem in which the inode resides. When opening a device node
1230 (character or block special) most filesystems will call special
1231 support routines in the VFS which will locate the required device
1232 driver information. These support routines replace the filesystem file
1233 operations with those for the device driver, and then proceed to call
1234 the new open() method for the file. This is how opening a device file
1235 in the filesystem eventually ends up calling the device driver open()
1239 Directory Entry Cache (dcache)
1240 ==============================
1243 struct dentry_operations
1244 ------------------------
1246 This describes how a filesystem can overload the standard dentry
1247 operations. Dentries and the dcache are the domain of the VFS and the
1248 individual filesystem implementations. Device drivers have no business
1249 here. These methods may be set to NULL, as they are either optional or
1250 the VFS uses a default. As of kernel 2.6.22, the following members are
1255 struct dentry_operations {
1256 int (*d_revalidate)(struct dentry *, unsigned int);
1257 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1258 int (*d_hash)(const struct dentry *, struct qstr *);
1259 int (*d_compare)(const struct dentry *,
1260 unsigned int, const char *, const struct qstr *);
1261 int (*d_delete)(const struct dentry *);
1262 int (*d_init)(struct dentry *);
1263 void (*d_release)(struct dentry *);
1264 void (*d_iput)(struct dentry *, struct inode *);
1265 char *(*d_dname)(struct dentry *, char *, int);
1266 struct vfsmount *(*d_automount)(struct path *);
1267 int (*d_manage)(const struct path *, bool);
1268 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1272 called when the VFS needs to revalidate a dentry. This is
1273 called whenever a name look-up finds a dentry in the dcache.
1274 Most local filesystems leave this as NULL, because all their
1275 dentries in the dcache are valid. Network filesystems are
1276 different since things can change on the server without the
1277 client necessarily being aware of it.
1279 This function should return a positive value if the dentry is
1280 still valid, and zero or a negative error code if it isn't.
1282 d_revalidate may be called in rcu-walk mode (flags &
1283 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1284 revalidate the dentry without blocking or storing to the dentry,
1285 d_parent and d_inode should not be used without care (because
1286 they can change and, in d_inode case, even become NULL under
1289 If a situation is encountered that rcu-walk cannot handle,
1291 -ECHILD and it will be called again in ref-walk mode.
1293 ``d_weak_revalidate``
1294 called when the VFS needs to revalidate a "jumped" dentry. This
1295 is called when a path-walk ends at dentry that was not acquired
1296 by doing a lookup in the parent directory. This includes "/",
1297 "." and "..", as well as procfs-style symlinks and mountpoint
1300 In this case, we are less concerned with whether the dentry is
1301 still fully correct, but rather that the inode is still valid.
1302 As with d_revalidate, most local filesystems will set this to
1303 NULL since their dcache entries are always valid.
1305 This function has the same return code semantics as
1308 d_weak_revalidate is only called after leaving rcu-walk mode.
1311 called when the VFS adds a dentry to the hash table. The first
1312 dentry passed to d_hash is the parent directory that the name is
1315 Same locking and synchronisation rules as d_compare regarding
1316 what is safe to dereference etc.
1319 called to compare a dentry name with a given name. The first
1320 dentry is the parent of the dentry to be compared, the second is
1321 the child dentry. len and name string are properties of the
1322 dentry to be compared. qstr is the name to compare it with.
1324 Must be constant and idempotent, and should not take locks if
1325 possible, and should not or store into the dentry. Should not
1326 dereference pointers outside the dentry without lots of care
1327 (eg. d_parent, d_inode, d_name should not be used).
1329 However, our vfsmount is pinned, and RCU held, so the dentries
1330 and inodes won't disappear, neither will our sb or filesystem
1331 module. ->d_sb may be used.
1333 It is a tricky calling convention because it needs to be called
1334 under "rcu-walk", ie. without any locks or references on things.
1337 called when the last reference to a dentry is dropped and the
1338 dcache is deciding whether or not to cache it. Return 1 to
1339 delete immediately, or 0 to cache the dentry. Default is NULL
1340 which means to always cache a reachable dentry. d_delete must
1341 be constant and idempotent.
1344 called when a dentry is allocated
1347 called when a dentry is really deallocated
1350 called when a dentry loses its inode (just prior to its being
1351 deallocated). The default when this is NULL is that the VFS
1352 calls iput(). If you define this method, you must call iput()
1356 called when the pathname of a dentry should be generated.
1357 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1358 delay pathname generation. (Instead of doing it when dentry is
1359 created, it's done only when the path is needed.). Real
1360 filesystems probably dont want to use it, because their dentries
1361 are present in global dcache hash, so their hash should be an
1362 invariant. As no lock is held, d_dname() should not try to
1363 modify the dentry itself, unless appropriate SMP safety is used.
1364 CAUTION : d_path() logic is quite tricky. The correct way to
1365 return for example "Hello" is to put it at the end of the
1366 buffer, and returns a pointer to the first char.
1367 dynamic_dname() helper function is provided to take care of
1374 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1376 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1377 dentry->d_inode->i_ino);
1381 called when an automount dentry is to be traversed (optional).
1382 This should create a new VFS mount record and return the record
1383 to the caller. The caller is supplied with a path parameter
1384 giving the automount directory to describe the automount target
1385 and the parent VFS mount record to provide inheritable mount
1386 parameters. NULL should be returned if someone else managed to
1387 make the automount first. If the vfsmount creation failed, then
1388 an error code should be returned. If -EISDIR is returned, then
1389 the directory will be treated as an ordinary directory and
1390 returned to pathwalk to continue walking.
1392 If a vfsmount is returned, the caller will attempt to mount it
1393 on the mountpoint and will remove the vfsmount from its
1394 expiration list in the case of failure. The vfsmount should be
1395 returned with 2 refs on it to prevent automatic expiration - the
1396 caller will clean up the additional ref.
1398 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1399 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1400 set on the inode being added.
1403 called to allow the filesystem to manage the transition from a
1404 dentry (optional). This allows autofs, for example, to hold up
1405 clients waiting to explore behind a 'mountpoint' while letting
1406 the daemon go past and construct the subtree there. 0 should be
1407 returned to let the calling process continue. -EISDIR can be
1408 returned to tell pathwalk to use this directory as an ordinary
1409 directory and to ignore anything mounted on it and not to check
1410 the automount flag. Any other error code will abort pathwalk
1413 If the 'rcu_walk' parameter is true, then the caller is doing a
1414 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1415 mode, and the caller can be asked to leave it and call again by
1416 returning -ECHILD. -EISDIR may also be returned to tell
1417 pathwalk to ignore d_automount or any mounts.
1419 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1420 the dentry being transited from.
1423 overlay/union type filesystems implement this method to return
1424 one of the underlying dentries hidden by the overlay. It is
1425 used in two different modes:
1427 Called from file_dentry() it returns the real dentry matching
1428 the inode argument. The real dentry may be from a lower layer
1429 already copied up, but still referenced from the file. This
1430 mode is selected with a non-NULL inode argument.
1432 With NULL inode the topmost real underlying dentry is returned.
1434 Each dentry has a pointer to its parent dentry, as well as a hash list
1435 of child dentries. Child dentries are basically like files in a
1439 Directory Entry Cache API
1440 --------------------------
1442 There are a number of functions defined which permit a filesystem to
1443 manipulate dentries:
1446 open a new handle for an existing dentry (this just increments
1450 close a handle for a dentry (decrements the usage count). If
1451 the usage count drops to 0, and the dentry is still in its
1452 parent's hash, the "d_delete" method is called to check whether
1453 it should be cached. If it should not be cached, or if the
1454 dentry is not hashed, it is deleted. Otherwise cached dentries
1455 are put into an LRU list to be reclaimed on memory shortage.
1458 this unhashes a dentry from its parents hash list. A subsequent
1459 call to dput() will deallocate the dentry if its usage count
1463 delete a dentry. If there are no other open references to the
1464 dentry then the dentry is turned into a negative dentry (the
1465 d_iput() method is called). If there are other references, then
1466 d_drop() is called instead
1469 add a dentry to its parents hash list and then calls
1473 add a dentry to the alias hash list for the inode and updates
1474 the "d_inode" member. The "i_count" member in the inode
1475 structure should be set/incremented. If the inode pointer is
1476 NULL, the dentry is called a "negative dentry". This function
1477 is commonly called when an inode is created for an existing
1481 look up a dentry given its parent and path name component It
1482 looks up the child of that given name from the dcache hash
1483 table. If it is found, the reference count is incremented and
1484 the dentry is returned. The caller must use dput() to free the
1485 dentry when it finishes using it.
1495 On mount and remount the filesystem is passed a string containing a
1496 comma separated list of mount options. The options can have either of
1502 The <linux/parser.h> header defines an API that helps parse these
1503 options. There are plenty of examples on how to use it in existing
1510 If a filesystem accepts mount options, it must define show_options() to
1511 show all the currently active options. The rules are:
1513 - options MUST be shown which are not default or their values differ
1516 - options MAY be shown which are enabled by default or have their
1519 Options used only internally between a mount helper and the kernel (such
1520 as file descriptors), or which only have an effect during the mounting
1521 (such as ones controlling the creation of a journal) are exempt from the
1524 The underlying reason for the above rules is to make sure, that a mount
1525 can be accurately replicated (e.g. umounting and mounting again) based
1526 on the information found in /proc/mounts.
1532 (Note some of these resources are not up-to-date with the latest kernel
1535 Creating Linux virtual filesystems. 2002
1536 <https://lwn.net/Articles/13325/>
1538 The Linux Virtual File-system Layer by Neil Brown. 1999
1539 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1541 A tour of the Linux VFS by Michael K. Johnson. 1996
1542 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1544 A small trail through the Linux kernel by Andries Brouwer. 2001
1545 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>