1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4Overview of the Linux Virtual File System
5=========================================
6
7Original author: Richard Gooch <rgooch@atnf.csiro.au>
8
9- Copyright (C) 1999 Richard Gooch
10- Copyright (C) 2005 Pekka Enberg
11
12
13Introduction
14============
15
16The Virtual File System (also known as the Virtual Filesystem Switch) is
17the software layer in the kernel that provides the filesystem interface
18to userspace programs.  It also provides an abstraction within the
19kernel which allows different filesystem implementations to coexist.
20
21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22are called from a process context.  Filesystem locking is described in
23the document Documentation/filesystems/locking.rst.
24
25
26Directory Entry Cache (dcache)
27------------------------------
28
29The VFS implements the open(2), stat(2), chmod(2), and similar system
30calls.  The pathname argument that is passed to them is used by the VFS
31to search through the directory entry cache (also known as the dentry
32cache or dcache).  This provides a very fast look-up mechanism to
33translate a pathname (filename) into a specific dentry.  Dentries live
34in RAM and are never saved to disc: they exist only for performance.
35
36The dentry cache is meant to be a view into your entire filespace.  As
37most computers cannot fit all dentries in the RAM at the same time, some
38bits of the cache are missing.  In order to resolve your pathname into a
39dentry, the VFS may have to resort to creating dentries along the way,
40and then loading the inode.  This is done by looking up the inode.
41
42
43The Inode Object
44----------------
45
46An individual dentry usually has a pointer to an inode.  Inodes are
47filesystem objects such as regular files, directories, FIFOs and other
48beasts.  They live either on the disc (for block device filesystems) or
49in the memory (for pseudo filesystems).  Inodes that live on the disc
50are copied into the memory when required and changes to the inode are
51written back to disc.  A single inode can be pointed to by multiple
52dentries (hard links, for example, do this).
53
54To look up an inode requires that the VFS calls the lookup() method of
55the parent directory inode.  This method is installed by the specific
56filesystem implementation that the inode lives in.  Once the VFS has the
57required dentry (and hence the inode), we can do all those boring things
58like open(2) the file, or stat(2) it to peek at the inode data.  The
59stat(2) operation is fairly simple: once the VFS has the dentry, it
60peeks at the inode data and passes some of it back to userspace.
61
62
63The File Object
64---------------
65
66Opening a file requires another operation: allocation of a file
67structure (this is the kernel-side implementation of file descriptors).
68The freshly allocated file structure is initialized with a pointer to
69the dentry and a set of file operation member functions.  These are
70taken from the inode data.  The open() file method is then called so the
71specific filesystem implementation can do its work.  You can see that
72this is another switch performed by the VFS.  The file structure is
73placed into the file descriptor table for the process.
74
75Reading, writing and closing files (and other assorted VFS operations)
76is done by using the userspace file descriptor to grab the appropriate
77file structure, and then calling the required file structure method to
78do whatever is required.  For as long as the file is open, it keeps the
79dentry in use, which in turn means that the VFS inode is still in use.
80
81
82Registering and Mounting a Filesystem
83=====================================
84
85To register and unregister a filesystem, use the following API
86functions:
87
88.. code-block:: c
89
90	#include <linux/fs.h>
91
92	extern int register_filesystem(struct file_system_type *);
93	extern int unregister_filesystem(struct file_system_type *);
94
95The passed struct file_system_type describes your filesystem.  When a
96request is made to mount a filesystem onto a directory in your
97namespace, the VFS will call the appropriate mount() method for the
98specific filesystem.  New vfsmount referring to the tree returned by
99->mount() will be attached to the mountpoint, so that when pathname
100resolution reaches the mountpoint it will jump into the root of that
101vfsmount.
102
103You can see all filesystems that are registered to the kernel in the
104file /proc/filesystems.
105
106
107struct file_system_type
108-----------------------
109
110This describes the filesystem.  As of kernel 2.6.39, the following
111members are defined:
112
113.. code-block:: c
114
115	struct file_system_type {
116		const char *name;
117		int fs_flags;
118		struct dentry *(*mount) (struct file_system_type *, int,
119					 const char *, void *);
120		void (*kill_sb) (struct super_block *);
121		struct module *owner;
122		struct file_system_type * next;
123		struct list_head fs_supers;
124		struct lock_class_key s_lock_key;
125		struct lock_class_key s_umount_key;
126	};
127
128``name``
129	the name of the filesystem type, such as "ext2", "iso9660",
130	"msdos" and so on
131
132``fs_flags``
133	various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
134
135``mount``
136	the method to call when a new instance of this filesystem should
137	be mounted
138
139``kill_sb``
140	the method to call when an instance of this filesystem should be
141	shut down
142
143
144``owner``
145	for internal VFS use: you should initialize this to THIS_MODULE
146	in most cases.
147
148``next``
149	for internal VFS use: you should initialize this to NULL
150
151  s_lock_key, s_umount_key: lockdep-specific
152
153The mount() method has the following arguments:
154
155``struct file_system_type *fs_type``
156	describes the filesystem, partly initialized by the specific
157	filesystem code
158
159``int flags``
160	mount flags
161
162``const char *dev_name``
163	the device name we are mounting.
164
165``void *data``
166	arbitrary mount options, usually comes as an ASCII string (see
167	"Mount Options" section)
168
169The mount() method must return the root dentry of the tree requested by
170caller.  An active reference to its superblock must be grabbed and the
171superblock must be locked.  On failure it should return ERR_PTR(error).
172
173The arguments match those of mount(2) and their interpretation depends
174on filesystem type.  E.g. for block filesystems, dev_name is interpreted
175as block device name, that device is opened and if it contains a
176suitable filesystem image the method creates and initializes struct
177super_block accordingly, returning its root dentry to caller.
178
179->mount() may choose to return a subtree of existing filesystem - it
180doesn't have to create a new one.  The main result from the caller's
181point of view is a reference to dentry at the root of (sub)tree to be
182attached; creation of new superblock is a common side effect.
183
184The most interesting member of the superblock structure that the mount()
185method fills in is the "s_op" field.  This is a pointer to a "struct
186super_operations" which describes the next level of the filesystem
187implementation.
188
189Usually, a filesystem uses one of the generic mount() implementations
190and provides a fill_super() callback instead.  The generic variants are:
191
192``mount_bdev``
193	mount a filesystem residing on a block device
194
195``mount_nodev``
196	mount a filesystem that is not backed by a device
197
198``mount_single``
199	mount a filesystem which shares the instance between all mounts
200
201A fill_super() callback implementation has the following arguments:
202
203``struct super_block *sb``
204	the superblock structure.  The callback must initialize this
205	properly.
206
207``void *data``
208	arbitrary mount options, usually comes as an ASCII string (see
209	"Mount Options" section)
210
211``int silent``
212	whether or not to be silent on error
213
214
215The Superblock Object
216=====================
217
218A superblock object represents a mounted filesystem.
219
220
221struct super_operations
222-----------------------
223
224This describes how the VFS can manipulate the superblock of your
225filesystem.  As of kernel 2.6.22, the following members are defined:
226
227.. code-block:: c
228
229	struct super_operations {
230		struct inode *(*alloc_inode)(struct super_block *sb);
231		void (*destroy_inode)(struct inode *);
232
233		void (*dirty_inode) (struct inode *, int flags);
234		int (*write_inode) (struct inode *, int);
235		void (*drop_inode) (struct inode *);
236		void (*delete_inode) (struct inode *);
237		void (*put_super) (struct super_block *);
238		int (*sync_fs)(struct super_block *sb, int wait);
239		int (*freeze_fs) (struct super_block *);
240		int (*unfreeze_fs) (struct super_block *);
241		int (*statfs) (struct dentry *, struct kstatfs *);
242		int (*remount_fs) (struct super_block *, int *, char *);
243		void (*clear_inode) (struct inode *);
244		void (*umount_begin) (struct super_block *);
245
246		int (*show_options)(struct seq_file *, struct dentry *);
247
248		ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
249		ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
250		int (*nr_cached_objects)(struct super_block *);
251		void (*free_cached_objects)(struct super_block *, int);
252	};
253
254All methods are called without any locks being held, unless otherwise
255noted.  This means that most methods can block safely.  All methods are
256only called from a process context (i.e. not from an interrupt handler
257or bottom half).
258
259``alloc_inode``
260	this method is called by alloc_inode() to allocate memory for
261	struct inode and initialize it.  If this function is not
262	defined, a simple 'struct inode' is allocated.  Normally
263	alloc_inode will be used to allocate a larger structure which
264	contains a 'struct inode' embedded within it.
265
266``destroy_inode``
267	this method is called by destroy_inode() to release resources
268	allocated for struct inode.  It is only required if
269	->alloc_inode was defined and simply undoes anything done by
270	->alloc_inode.
271
272``dirty_inode``
273	this method is called by the VFS when an inode is marked dirty.
274	This is specifically for the inode itself being marked dirty,
275	not its data.  If the update needs to be persisted by fdatasync(),
276	then I_DIRTY_DATASYNC will be set in the flags argument.
277
278``write_inode``
279	this method is called when the VFS needs to write an inode to
280	disc.  The second parameter indicates whether the write should
281	be synchronous or not, not all filesystems check this flag.
282
283``drop_inode``
284	called when the last access to the inode is dropped, with the
285	inode->i_lock spinlock held.
286
287	This method should be either NULL (normal UNIX filesystem
288	semantics) or "generic_delete_inode" (for filesystems that do
289	not want to cache inodes - causing "delete_inode" to always be
290	called regardless of the value of i_nlink)
291
292	The "generic_delete_inode()" behavior is equivalent to the old
293	practice of using "force_delete" in the put_inode() case, but
294	does not have the races that the "force_delete()" approach had.
295
296``delete_inode``
297	called when the VFS wants to delete an inode
298
299``put_super``
300	called when the VFS wishes to free the superblock
301	(i.e. unmount).  This is called with the superblock lock held
302
303``sync_fs``
304	called when VFS is writing out all dirty data associated with a
305	superblock.  The second parameter indicates whether the method
306	should wait until the write out has been completed.  Optional.
307
308``freeze_fs``
309	called when VFS is locking a filesystem and forcing it into a
310	consistent state.  This method is currently used by the Logical
311	Volume Manager (LVM).
312
313``unfreeze_fs``
314	called when VFS is unlocking a filesystem and making it writable
315	again.
316
317``statfs``
318	called when the VFS needs to get filesystem statistics.
319
320``remount_fs``
321	called when the filesystem is remounted.  This is called with
322	the kernel lock held
323
324``clear_inode``
325	called then the VFS clears the inode.  Optional
326
327``umount_begin``
328	called when the VFS is unmounting a filesystem.
329
330``show_options``
331	called by the VFS to show mount options for /proc/<pid>/mounts.
332	(see "Mount Options" section)
333
334``quota_read``
335	called by the VFS to read from filesystem quota file.
336
337``quota_write``
338	called by the VFS to write to filesystem quota file.
339
340``nr_cached_objects``
341	called by the sb cache shrinking function for the filesystem to
342	return the number of freeable cached objects it contains.
343	Optional.
344
345``free_cache_objects``
346	called by the sb cache shrinking function for the filesystem to
347	scan the number of objects indicated to try to free them.
348	Optional, but any filesystem implementing this method needs to
349	also implement ->nr_cached_objects for it to be called
350	correctly.
351
352	We can't do anything with any errors that the filesystem might
353	encountered, hence the void return type.  This will never be
354	called if the VM is trying to reclaim under GFP_NOFS conditions,
355	hence this method does not need to handle that situation itself.
356
357	Implementations must include conditional reschedule calls inside
358	any scanning loop that is done.  This allows the VFS to
359	determine appropriate scan batch sizes without having to worry
360	about whether implementations will cause holdoff problems due to
361	large scan batch sizes.
362
363Whoever sets up the inode is responsible for filling in the "i_op"
364field.  This is a pointer to a "struct inode_operations" which describes
365the methods that can be performed on individual inodes.
366
367
368struct xattr_handlers
369---------------------
370
371On filesystems that support extended attributes (xattrs), the s_xattr
372superblock field points to a NULL-terminated array of xattr handlers.
373Extended attributes are name:value pairs.
374
375``name``
376	Indicates that the handler matches attributes with the specified
377	name (such as "system.posix_acl_access"); the prefix field must
378	be NULL.
379
380``prefix``
381	Indicates that the handler matches all attributes with the
382	specified name prefix (such as "user."); the name field must be
383	NULL.
384
385``list``
386	Determine if attributes matching this xattr handler should be
387	listed for a particular dentry.  Used by some listxattr
388	implementations like generic_listxattr.
389
390``get``
391	Called by the VFS to get the value of a particular extended
392	attribute.  This method is called by the getxattr(2) system
393	call.
394
395``set``
396	Called by the VFS to set the value of a particular extended
397	attribute.  When the new value is NULL, called to remove a
398	particular extended attribute.  This method is called by the
399	setxattr(2) and removexattr(2) system calls.
400
401When none of the xattr handlers of a filesystem match the specified
402attribute name or when a filesystem doesn't support extended attributes,
403the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
404
405
406The Inode Object
407================
408
409An inode object represents an object within the filesystem.
410
411
412struct inode_operations
413-----------------------
414
415This describes how the VFS can manipulate an inode in your filesystem.
416As of kernel 2.6.22, the following members are defined:
417
418.. code-block:: c
419
420	struct inode_operations {
421		int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
422		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
423		int (*link) (struct dentry *,struct inode *,struct dentry *);
424		int (*unlink) (struct inode *,struct dentry *);
425		int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
426		int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
427		int (*rmdir) (struct inode *,struct dentry *);
428		int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
429		int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
430			       struct inode *, struct dentry *, unsigned int);
431		int (*readlink) (struct dentry *, char __user *,int);
432		const char *(*get_link) (struct dentry *, struct inode *,
433					 struct delayed_call *);
434		int (*permission) (struct user_namespace *, struct inode *, int);
435		struct posix_acl * (*get_acl)(struct inode *, int, bool);
436		int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
437		int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
438		ssize_t (*listxattr) (struct dentry *, char *, size_t);
439		void (*update_time)(struct inode *, struct timespec *, int);
440		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
441				   unsigned open_flag, umode_t create_mode);
442		int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t);
443	        int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
444		int (*fileattr_set)(struct user_namespace *mnt_userns,
445				    struct dentry *dentry, struct fileattr *fa);
446		int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
447	};
448
449Again, all methods are called without any locks being held, unless
450otherwise noted.
451
452``create``
453	called by the open(2) and creat(2) system calls.  Only required
454	if you want to support regular files.  The dentry you get should
455	not have an inode (i.e. it should be a negative dentry).  Here
456	you will probably call d_instantiate() with the dentry and the
457	newly created inode
458
459``lookup``
460	called when the VFS needs to look up an inode in a parent
461	directory.  The name to look for is found in the dentry.  This
462	method must call d_add() to insert the found inode into the
463	dentry.  The "i_count" field in the inode structure should be
464	incremented.  If the named inode does not exist a NULL inode
465	should be inserted into the dentry (this is called a negative
466	dentry).  Returning an error code from this routine must only be
467	done on a real error, otherwise creating inodes with system
468	calls like create(2), mknod(2), mkdir(2) and so on will fail.
469	If you wish to overload the dentry methods then you should
470	initialise the "d_dop" field in the dentry; this is a pointer to
471	a struct "dentry_operations".  This method is called with the
472	directory inode semaphore held
473
474``link``
475	called by the link(2) system call.  Only required if you want to
476	support hard links.  You will probably need to call
477	d_instantiate() just as you would in the create() method
478
479``unlink``
480	called by the unlink(2) system call.  Only required if you want
481	to support deleting inodes
482
483``symlink``
484	called by the symlink(2) system call.  Only required if you want
485	to support symlinks.  You will probably need to call
486	d_instantiate() just as you would in the create() method
487
488``mkdir``
489	called by the mkdir(2) system call.  Only required if you want
490	to support creating subdirectories.  You will probably need to
491	call d_instantiate() just as you would in the create() method
492
493``rmdir``
494	called by the rmdir(2) system call.  Only required if you want
495	to support deleting subdirectories
496
497``mknod``
498	called by the mknod(2) system call to create a device (char,
499	block) inode or a named pipe (FIFO) or socket.  Only required if
500	you want to support creating these types of inodes.  You will
501	probably need to call d_instantiate() just as you would in the
502	create() method
503
504``rename``
505	called by the rename(2) system call to rename the object to have
506	the parent and name given by the second inode and dentry.
507
508	The filesystem must return -EINVAL for any unsupported or
509	unknown flags.  Currently the following flags are implemented:
510	(1) RENAME_NOREPLACE: this flag indicates that if the target of
511	the rename exists the rename should fail with -EEXIST instead of
512	replacing the target.  The VFS already checks for existence, so
513	for local filesystems the RENAME_NOREPLACE implementation is
514	equivalent to plain rename.
515	(2) RENAME_EXCHANGE: exchange source and target.  Both must
516	exist; this is checked by the VFS.  Unlike plain rename, source
517	and target may be of different type.
518
519``get_link``
520	called by the VFS to follow a symbolic link to the inode it
521	points to.  Only required if you want to support symbolic links.
522	This method returns the symlink body to traverse (and possibly
523	resets the current position with nd_jump_link()).  If the body
524	won't go away until the inode is gone, nothing else is needed;
525	if it needs to be otherwise pinned, arrange for its release by
526	having get_link(..., ..., done) do set_delayed_call(done,
527	destructor, argument).  In that case destructor(argument) will
528	be called once VFS is done with the body you've returned.  May
529	be called in RCU mode; that is indicated by NULL dentry
530	argument.  If request can't be handled without leaving RCU mode,
531	have it return ERR_PTR(-ECHILD).
532
533	If the filesystem stores the symlink target in ->i_link, the
534	VFS may use it directly without calling ->get_link(); however,
535	->get_link() must still be provided.  ->i_link must not be
536	freed until after an RCU grace period.  Writing to ->i_link
537	post-iget() time requires a 'release' memory barrier.
538
539``readlink``
540	this is now just an override for use by readlink(2) for the
541	cases when ->get_link uses nd_jump_link() or object is not in
542	fact a symlink.  Normally filesystems should only implement
543	->get_link for symlinks and readlink(2) will automatically use
544	that.
545
546``permission``
547	called by the VFS to check for access rights on a POSIX-like
548	filesystem.
549
550	May be called in rcu-walk mode (mask & MAY_NOT_BLOCK).  If in
551	rcu-walk mode, the filesystem must check the permission without
552	blocking or storing to the inode.
553
554	If a situation is encountered that rcu-walk cannot handle,
555	return
556	-ECHILD and it will be called again in ref-walk mode.
557
558``setattr``
559	called by the VFS to set attributes for a file.  This method is
560	called by chmod(2) and related system calls.
561
562``getattr``
563	called by the VFS to get attributes of a file.  This method is
564	called by stat(2) and related system calls.
565
566``listxattr``
567	called by the VFS to list all extended attributes for a given
568	file.  This method is called by the listxattr(2) system call.
569
570``update_time``
571	called by the VFS to update a specific time or the i_version of
572	an inode.  If this is not defined the VFS will update the inode
573	itself and call mark_inode_dirty_sync.
574
575``atomic_open``
576	called on the last component of an open.  Using this optional
577	method the filesystem can look up, possibly create and open the
578	file in one atomic operation.  If it wants to leave actual
579	opening to the caller (e.g. if the file turned out to be a
580	symlink, device, or just something filesystem won't do atomic
581	open for), it may signal this by returning finish_no_open(file,
582	dentry).  This method is only called if the last component is
583	negative or needs lookup.  Cached positive dentries are still
584	handled by f_op->open().  If the file was created, FMODE_CREATED
585	flag should be set in file->f_mode.  In case of O_EXCL the
586	method must only succeed if the file didn't exist and hence
587	FMODE_CREATED shall always be set on success.
588
589``tmpfile``
590	called in the end of O_TMPFILE open().  Optional, equivalent to
591	atomically creating, opening and unlinking a file in given
592	directory.
593
594``fileattr_get``
595	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
596	retrieve miscellaneous file flags and attributes.  Also called
597	before the relevant SET operation to check what is being changed
598	(in this case with i_rwsem locked exclusive).  If unset, then
599	fall back to f_op->ioctl().
600
601``fileattr_set``
602	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
603	change miscellaneous file flags and attributes.  Callers hold
604	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
605
606
607The Address Space Object
608========================
609
610The address space object is used to group and manage pages in the page
611cache.  It can be used to keep track of the pages in a file (or anything
612else) and also track the mapping of sections of the file into process
613address spaces.
614
615There are a number of distinct yet related services that an
616address-space can provide.  These include communicating memory pressure,
617page lookup by address, and keeping track of pages tagged as Dirty or
618Writeback.
619
620The first can be used independently to the others.  The VM can try to
621either write dirty pages in order to clean them, or release clean pages
622in order to reuse them.  To do this it can call the ->writepage method
623on dirty pages, and ->release_folio on clean folios with the private
624flag set.  Clean pages without PagePrivate and with no external references
625will be released without notice being given to the address_space.
626
627To achieve this functionality, pages need to be placed on an LRU with
628lru_cache_add and mark_page_active needs to be called whenever the page
629is used.
630
631Pages are normally kept in a radix tree index by ->index.  This tree
632maintains information about the PG_Dirty and PG_Writeback status of each
633page, so that pages with either of these flags can be found quickly.
634
635The Dirty tag is primarily used by mpage_writepages - the default
636->writepages method.  It uses the tag to find dirty pages to call
637->writepage on.  If mpage_writepages is not used (i.e. the address
638provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
639unused.  write_inode_now and sync_inode do use it (through
640__sync_single_inode) to check if ->writepages has been successful in
641writing out the whole address_space.
642
643The Writeback tag is used by filemap*wait* and sync_page* functions, via
644filemap_fdatawait_range, to wait for all writeback to complete.
645
646An address_space handler may attach extra information to a page,
647typically using the 'private' field in the 'struct page'.  If such
648information is attached, the PG_Private flag should be set.  This will
649cause various VM routines to make extra calls into the address_space
650handler to deal with that data.
651
652An address space acts as an intermediate between storage and
653application.  Data is read into the address space a whole page at a
654time, and provided to the application either by copying of the page, or
655by memory-mapping the page.  Data is written into the address space by
656the application, and then written-back to storage typically in whole
657pages, however the address_space has finer control of write sizes.
658
659The read process essentially only requires 'read_folio'.  The write
660process is more complicated and uses write_begin/write_end or
661dirty_folio to write data into the address_space, and writepage and
662writepages to writeback data to storage.
663
664Adding and removing pages to/from an address_space is protected by the
665inode's i_mutex.
666
667When data is written to a page, the PG_Dirty flag should be set.  It
668typically remains set until writepage asks for it to be written.  This
669should clear PG_Dirty and set PG_Writeback.  It can be actually written
670at any point after PG_Dirty is clear.  Once it is known to be safe,
671PG_Writeback is cleared.
672
673Writeback makes use of a writeback_control structure to direct the
674operations.  This gives the writepage and writepages operations some
675information about the nature of and reason for the writeback request,
676and the constraints under which it is being done.  It is also used to
677return information back to the caller about the result of a writepage or
678writepages request.
679
680
681Handling errors during writeback
682--------------------------------
683
684Most applications that do buffered I/O will periodically call a file
685synchronization call (fsync, fdatasync, msync or sync_file_range) to
686ensure that data written has made it to the backing store.  When there
687is an error during writeback, they expect that error to be reported when
688a file sync request is made.  After an error has been reported on one
689request, subsequent requests on the same file descriptor should return
6900, unless further writeback errors have occurred since the previous file
691syncronization.
692
693Ideally, the kernel would report errors only on file descriptions on
694which writes were done that subsequently failed to be written back.  The
695generic pagecache infrastructure does not track the file descriptions
696that have dirtied each individual page however, so determining which
697file descriptors should get back an error is not possible.
698
699Instead, the generic writeback error tracking infrastructure in the
700kernel settles for reporting errors to fsync on all file descriptions
701that were open at the time that the error occurred.  In a situation with
702multiple writers, all of them will get back an error on a subsequent
703fsync, even if all of the writes done through that particular file
704descriptor succeeded (or even if there were no writes on that file
705descriptor at all).
706
707Filesystems that wish to use this infrastructure should call
708mapping_set_error to record the error in the address_space when it
709occurs.  Then, after writing back data from the pagecache in their
710file->fsync operation, they should call file_check_and_advance_wb_err to
711ensure that the struct file's error cursor has advanced to the correct
712point in the stream of errors emitted by the backing device(s).
713
714
715struct address_space_operations
716-------------------------------
717
718This describes how the VFS can manipulate mapping of a file to page
719cache in your filesystem.  The following members are defined:
720
721.. code-block:: c
722
723	struct address_space_operations {
724		int (*writepage)(struct page *page, struct writeback_control *wbc);
725		int (*read_folio)(struct file *, struct folio *);
726		int (*writepages)(struct address_space *, struct writeback_control *);
727		bool (*dirty_folio)(struct address_space *, struct folio *);
728		void (*readahead)(struct readahead_control *);
729		int (*write_begin)(struct file *, struct address_space *mapping,
730				   loff_t pos, unsigned len,
731				struct page **pagep, void **fsdata);
732		int (*write_end)(struct file *, struct address_space *mapping,
733				 loff_t pos, unsigned len, unsigned copied,
734				 struct page *page, void *fsdata);
735		sector_t (*bmap)(struct address_space *, sector_t);
736		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
737		bool (*release_folio)(struct folio *, gfp_t);
738		void (*free_folio)(struct folio *);
739		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
740		/* isolate a page for migration */
741		bool (*isolate_page) (struct page *, isolate_mode_t);
742		/* migrate the contents of a page to the specified target */
743		int (*migratepage) (struct page *, struct page *);
744		/* put migration-failed page back to right list */
745		void (*putback_page) (struct page *);
746		int (*launder_folio) (struct folio *);
747
748		bool (*is_partially_uptodate) (struct folio *, size_t from,
749					       size_t count);
750		void (*is_dirty_writeback)(struct folio *, bool *, bool *);
751		int (*error_remove_page) (struct mapping *mapping, struct page *page);
752		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
753		int (*swap_deactivate)(struct file *);
754		int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
755	};
756
757``writepage``
758	called by the VM to write a dirty page to backing store.  This
759	may happen for data integrity reasons (i.e. 'sync'), or to free
760	up memory (flush).  The difference can be seen in
761	wbc->sync_mode.  The PG_Dirty flag has been cleared and
762	PageLocked is true.  writepage should start writeout, should set
763	PG_Writeback, and should make sure the page is unlocked, either
764	synchronously or asynchronously when the write operation
765	completes.
766
767	If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
768	try too hard if there are problems, and may choose to write out
769	other pages from the mapping if that is easier (e.g. due to
770	internal dependencies).  If it chooses not to start writeout, it
771	should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
772	keep calling ->writepage on that page.
773
774	See the file "Locking" for more details.
775
776``read_folio``
777	called by the VM to read a folio from backing store.  The folio
778	will be locked when read_folio is called, and should be unlocked
779	and marked uptodate once the read completes.  If ->read_folio
780	discovers that it cannot perform the I/O at this time, it can
781        unlock the folio and return AOP_TRUNCATED_PAGE.  In this case,
782	the folio will be looked up again, relocked and if that all succeeds,
783	->read_folio will be called again.
784
785``writepages``
786	called by the VM to write out pages associated with the
787	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
788	the writeback_control will specify a range of pages that must be
789	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
790	given and that many pages should be written if possible.  If no
791	->writepages is given, then mpage_writepages is used instead.
792	This will choose pages from the address space that are tagged as
793	DIRTY and will pass them to ->writepage.
794
795``dirty_folio``
796	called by the VM to mark a folio as dirty.  This is particularly
797	needed if an address space attaches private data to a folio, and
798	that data needs to be updated when a folio is dirtied.  This is
799	called, for example, when a memory mapped page gets modified.
800	If defined, it should set the folio dirty flag, and the
801	PAGECACHE_TAG_DIRTY search mark in i_pages.
802
803``readahead``
804	Called by the VM to read pages associated with the address_space
805	object.  The pages are consecutive in the page cache and are
806	locked.  The implementation should decrement the page refcount
807	after starting I/O on each page.  Usually the page will be
808	unlocked by the I/O completion handler.  The set of pages are
809	divided into some sync pages followed by some async pages,
810	rac->ra->async_size gives the number of async pages.  The
811	filesystem should attempt to read all sync pages but may decide
812	to stop once it reaches the async pages.  If it does decide to
813	stop attempting I/O, it can simply return.  The caller will
814	remove the remaining pages from the address space, unlock them
815	and decrement the page refcount.  Set PageUptodate if the I/O
816	completes successfully.  Setting PageError on any page will be
817	ignored; simply unlock the page if an I/O error occurs.
818
819``write_begin``
820	Called by the generic buffered write code to ask the filesystem
821	to prepare to write len bytes at the given offset in the file.
822	The address_space should check that the write will be able to
823	complete, by allocating space if necessary and doing any other
824	internal housekeeping.  If the write will update parts of any
825	basic-blocks on storage, then those blocks should be pre-read
826	(if they haven't been read already) so that the updated blocks
827	can be written out properly.
828
829	The filesystem must return the locked pagecache page for the
830	specified offset, in ``*pagep``, for the caller to write into.
831
832	It must be able to cope with short writes (where the length
833	passed to write_begin is greater than the number of bytes copied
834	into the page).
835
836	A void * may be returned in fsdata, which then gets passed into
837	write_end.
838
839	Returns 0 on success; < 0 on failure (which is the error code),
840	in which case write_end is not called.
841
842``write_end``
843	After a successful write_begin, and data copy, write_end must be
844	called.  len is the original len passed to write_begin, and
845	copied is the amount that was able to be copied.
846
847	The filesystem must take care of unlocking the page and
848	releasing it refcount, and updating i_size.
849
850	Returns < 0 on failure, otherwise the number of bytes (<=
851	'copied') that were able to be copied into pagecache.
852
853``bmap``
854	called by the VFS to map a logical block offset within object to
855	physical block number.  This method is used by the FIBMAP ioctl
856	and for working with swap-files.  To be able to swap to a file,
857	the file must have a stable mapping to a block device.  The swap
858	system does not go through the filesystem but instead uses bmap
859	to find out where the blocks in the file are and uses those
860	addresses directly.
861
862``invalidate_folio``
863	If a folio has private data, then invalidate_folio will be
864	called when part or all of the folio is to be removed from the
865	address space.  This generally corresponds to either a
866	truncation, punch hole or a complete invalidation of the address
867	space (in the latter case 'offset' will always be 0 and 'length'
868	will be folio_size()).  Any private data associated with the folio
869	should be updated to reflect this truncation.  If offset is 0
870	and length is folio_size(), then the private data should be
871	released, because the folio must be able to be completely
872	discarded.  This may be done by calling the ->release_folio
873	function, but in this case the release MUST succeed.
874
875``release_folio``
876	release_folio is called on folios with private data to tell the
877	filesystem that the folio is about to be freed.  ->release_folio
878	should remove any private data from the folio and clear the
879	private flag.  If release_folio() fails, it should return false.
880	release_folio() is used in two distinct though related cases.
881	The first is when the VM wants to free a clean folio with no
882	active users.  If ->release_folio succeeds, the folio will be
883	removed from the address_space and be freed.
884
885	The second case is when a request has been made to invalidate
886	some or all folios in an address_space.  This can happen
887	through the fadvise(POSIX_FADV_DONTNEED) system call or by the
888	filesystem explicitly requesting it as nfs and 9p do (when they
889	believe the cache may be out of date with storage) by calling
890	invalidate_inode_pages2().  If the filesystem makes such a call,
891	and needs to be certain that all folios are invalidated, then
892	its release_folio will need to ensure this.  Possibly it can
893	clear the uptodate flag if it cannot free private data yet.
894
895``free_folio``
896	free_folio is called once the folio is no longer visible in the
897	page cache in order to allow the cleanup of any private data.
898	Since it may be called by the memory reclaimer, it should not
899	assume that the original address_space mapping still exists, and
900	it should not block.
901
902``direct_IO``
903	called by the generic read/write routines to perform direct_IO -
904	that is IO requests which bypass the page cache and transfer
905	data directly between the storage and the application's address
906	space.
907
908``isolate_page``
909	Called by the VM when isolating a movable non-lru page.  If page
910	is successfully isolated, VM marks the page as PG_isolated via
911	__SetPageIsolated.
912
913``migrate_page``
914	This is used to compact the physical memory usage.  If the VM
915	wants to relocate a page (maybe off a memory card that is
916	signalling imminent failure) it will pass a new page and an old
917	page to this function.  migrate_page should transfer any private
918	data across and update any references that it has to the page.
919
920``putback_page``
921	Called by the VM when isolated page's migration fails.
922
923``launder_folio``
924	Called before freeing a folio - it writes back the dirty folio.
925	To prevent redirtying the folio, it is kept locked during the
926	whole operation.
927
928``is_partially_uptodate``
929	Called by the VM when reading a file through the pagecache when
930	the underlying blocksize is smaller than the size of the folio.
931	If the required block is up to date then the read can complete
932	without needing I/O to bring the whole page up to date.
933
934``is_dirty_writeback``
935	Called by the VM when attempting to reclaim a folio.  The VM uses
936	dirty and writeback information to determine if it needs to
937	stall to allow flushers a chance to complete some IO.
938	Ordinarily it can use folio_test_dirty and folio_test_writeback but
939	some filesystems have more complex state (unstable folios in NFS
940	prevent reclaim) or do not set those flags due to locking
941	problems.  This callback allows a filesystem to indicate to the
942	VM if a folio should be treated as dirty or writeback for the
943	purposes of stalling.
944
945``error_remove_page``
946	normally set to generic_error_remove_page if truncation is ok
947	for this address space.  Used for memory failure handling.
948	Setting this implies you deal with pages going away under you,
949	unless you have them locked or reference counts increased.
950
951``swap_activate``
952
953	Called to prepare the given file for swap.  It should perform
954	any validation and preparation necessary to ensure that writes
955	can be performed with minimal memory allocation.  It should call
956	add_swap_extent(), or the helper iomap_swapfile_activate(), and
957	return the number of extents added.  If IO should be submitted
958	through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
959	be submitted directly to the block device ``sis->bdev``.
960
961``swap_deactivate``
962	Called during swapoff on files where swap_activate was
963	successful.
964
965``swap_rw``
966	Called to read or write swap pages when SWP_FS_OPS is set.
967
968The File Object
969===============
970
971A file object represents a file opened by a process.  This is also known
972as an "open file description" in POSIX parlance.
973
974
975struct file_operations
976----------------------
977
978This describes how the VFS can manipulate an open file.  As of kernel
9794.18, the following members are defined:
980
981.. code-block:: c
982
983	struct file_operations {
984		struct module *owner;
985		loff_t (*llseek) (struct file *, loff_t, int);
986		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
987		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
988		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
989		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
990		int (*iopoll)(struct kiocb *kiocb, bool spin);
991		int (*iterate) (struct file *, struct dir_context *);
992		int (*iterate_shared) (struct file *, struct dir_context *);
993		__poll_t (*poll) (struct file *, struct poll_table_struct *);
994		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
995		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
996		int (*mmap) (struct file *, struct vm_area_struct *);
997		int (*open) (struct inode *, struct file *);
998		int (*flush) (struct file *, fl_owner_t id);
999		int (*release) (struct inode *, struct file *);
1000		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1001		int (*fasync) (int, struct file *, int);
1002		int (*lock) (struct file *, int, struct file_lock *);
1003		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1004		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1005		int (*check_flags)(int);
1006		int (*flock) (struct file *, int, struct file_lock *);
1007		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1008		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1009		int (*setlease)(struct file *, long, struct file_lock **, void **);
1010		long (*fallocate)(struct file *file, int mode, loff_t offset,
1011				  loff_t len);
1012		void (*show_fdinfo)(struct seq_file *m, struct file *f);
1013	#ifndef CONFIG_MMU
1014		unsigned (*mmap_capabilities)(struct file *);
1015	#endif
1016		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1017		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1018					   struct file *file_out, loff_t pos_out,
1019					   loff_t len, unsigned int remap_flags);
1020		int (*fadvise)(struct file *, loff_t, loff_t, int);
1021	};
1022
1023Again, all methods are called without any locks being held, unless
1024otherwise noted.
1025
1026``llseek``
1027	called when the VFS needs to move the file position index
1028
1029``read``
1030	called by read(2) and related system calls
1031
1032``read_iter``
1033	possibly asynchronous read with iov_iter as destination
1034
1035``write``
1036	called by write(2) and related system calls
1037
1038``write_iter``
1039	possibly asynchronous write with iov_iter as source
1040
1041``iopoll``
1042	called when aio wants to poll for completions on HIPRI iocbs
1043
1044``iterate``
1045	called when the VFS needs to read the directory contents
1046
1047``iterate_shared``
1048	called when the VFS needs to read the directory contents when
1049	filesystem supports concurrent dir iterators
1050
1051``poll``
1052	called by the VFS when a process wants to check if there is
1053	activity on this file and (optionally) go to sleep until there
1054	is activity.  Called by the select(2) and poll(2) system calls
1055
1056``unlocked_ioctl``
1057	called by the ioctl(2) system call.
1058
1059``compat_ioctl``
1060	called by the ioctl(2) system call when 32 bit system calls are
1061	 used on 64 bit kernels.
1062
1063``mmap``
1064	called by the mmap(2) system call
1065
1066``open``
1067	called by the VFS when an inode should be opened.  When the VFS
1068	opens a file, it creates a new "struct file".  It then calls the
1069	open method for the newly allocated file structure.  You might
1070	think that the open method really belongs in "struct
1071	inode_operations", and you may be right.  I think it's done the
1072	way it is because it makes filesystems simpler to implement.
1073	The open() method is a good place to initialize the
1074	"private_data" member in the file structure if you want to point
1075	to a device structure
1076
1077``flush``
1078	called by the close(2) system call to flush a file
1079
1080``release``
1081	called when the last reference to an open file is closed
1082
1083``fsync``
1084	called by the fsync(2) system call.  Also see the section above
1085	entitled "Handling errors during writeback".
1086
1087``fasync``
1088	called by the fcntl(2) system call when asynchronous
1089	(non-blocking) mode is enabled for a file
1090
1091``lock``
1092	called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1093	F_SETLKW commands
1094
1095``get_unmapped_area``
1096	called by the mmap(2) system call
1097
1098``check_flags``
1099	called by the fcntl(2) system call for F_SETFL command
1100
1101``flock``
1102	called by the flock(2) system call
1103
1104``splice_write``
1105	called by the VFS to splice data from a pipe to a file.  This
1106	method is used by the splice(2) system call
1107
1108``splice_read``
1109	called by the VFS to splice data from file to a pipe.  This
1110	method is used by the splice(2) system call
1111
1112``setlease``
1113	called by the VFS to set or release a file lock lease.  setlease
1114	implementations should call generic_setlease to record or remove
1115	the lease in the inode after setting it.
1116
1117``fallocate``
1118	called by the VFS to preallocate blocks or punch a hole.
1119
1120``copy_file_range``
1121	called by the copy_file_range(2) system call.
1122
1123``remap_file_range``
1124	called by the ioctl(2) system call for FICLONERANGE and FICLONE
1125	and FIDEDUPERANGE commands to remap file ranges.  An
1126	implementation should remap len bytes at pos_in of the source
1127	file into the dest file at pos_out.  Implementations must handle
1128	callers passing in len == 0; this means "remap to the end of the
1129	source file".  The return value should the number of bytes
1130	remapped, or the usual negative error code if errors occurred
1131	before any bytes were remapped.  The remap_flags parameter
1132	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
1133	implementation must only remap if the requested file ranges have
1134	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
1135	ok with the implementation shortening the request length to
1136	satisfy alignment or EOF requirements (or any other reason).
1137
1138``fadvise``
1139	possibly called by the fadvise64() system call.
1140
1141Note that the file operations are implemented by the specific
1142filesystem in which the inode resides.  When opening a device node
1143(character or block special) most filesystems will call special
1144support routines in the VFS which will locate the required device
1145driver information.  These support routines replace the filesystem file
1146operations with those for the device driver, and then proceed to call
1147the new open() method for the file.  This is how opening a device file
1148in the filesystem eventually ends up calling the device driver open()
1149method.
1150
1151
1152Directory Entry Cache (dcache)
1153==============================
1154
1155
1156struct dentry_operations
1157------------------------
1158
1159This describes how a filesystem can overload the standard dentry
1160operations.  Dentries and the dcache are the domain of the VFS and the
1161individual filesystem implementations.  Device drivers have no business
1162here.  These methods may be set to NULL, as they are either optional or
1163the VFS uses a default.  As of kernel 2.6.22, the following members are
1164defined:
1165
1166.. code-block:: c
1167
1168	struct dentry_operations {
1169		int (*d_revalidate)(struct dentry *, unsigned int);
1170		int (*d_weak_revalidate)(struct dentry *, unsigned int);
1171		int (*d_hash)(const struct dentry *, struct qstr *);
1172		int (*d_compare)(const struct dentry *,
1173				 unsigned int, const char *, const struct qstr *);
1174		int (*d_delete)(const struct dentry *);
1175		int (*d_init)(struct dentry *);
1176		void (*d_release)(struct dentry *);
1177		void (*d_iput)(struct dentry *, struct inode *);
1178		char *(*d_dname)(struct dentry *, char *, int);
1179		struct vfsmount *(*d_automount)(struct path *);
1180		int (*d_manage)(const struct path *, bool);
1181		struct dentry *(*d_real)(struct dentry *, const struct inode *);
1182	};
1183
1184``d_revalidate``
1185	called when the VFS needs to revalidate a dentry.  This is
1186	called whenever a name look-up finds a dentry in the dcache.
1187	Most local filesystems leave this as NULL, because all their
1188	dentries in the dcache are valid.  Network filesystems are
1189	different since things can change on the server without the
1190	client necessarily being aware of it.
1191
1192	This function should return a positive value if the dentry is
1193	still valid, and zero or a negative error code if it isn't.
1194
1195	d_revalidate may be called in rcu-walk mode (flags &
1196	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
1197	revalidate the dentry without blocking or storing to the dentry,
1198	d_parent and d_inode should not be used without care (because
1199	they can change and, in d_inode case, even become NULL under
1200	us).
1201
1202	If a situation is encountered that rcu-walk cannot handle,
1203	return
1204	-ECHILD and it will be called again in ref-walk mode.
1205
1206``_weak_revalidate``
1207	called when the VFS needs to revalidate a "jumped" dentry.  This
1208	is called when a path-walk ends at dentry that was not acquired
1209	by doing a lookup in the parent directory.  This includes "/",
1210	"." and "..", as well as procfs-style symlinks and mountpoint
1211	traversal.
1212
1213	In this case, we are less concerned with whether the dentry is
1214	still fully correct, but rather that the inode is still valid.
1215	As with d_revalidate, most local filesystems will set this to
1216	NULL since their dcache entries are always valid.
1217
1218	This function has the same return code semantics as
1219	d_revalidate.
1220
1221	d_weak_revalidate is only called after leaving rcu-walk mode.
1222
1223``d_hash``
1224	called when the VFS adds a dentry to the hash table.  The first
1225	dentry passed to d_hash is the parent directory that the name is
1226	to be hashed into.
1227
1228	Same locking and synchronisation rules as d_compare regarding
1229	what is safe to dereference etc.
1230
1231``d_compare``
1232	called to compare a dentry name with a given name.  The first
1233	dentry is the parent of the dentry to be compared, the second is
1234	the child dentry.  len and name string are properties of the
1235	dentry to be compared.  qstr is the name to compare it with.
1236
1237	Must be constant and idempotent, and should not take locks if
1238	possible, and should not or store into the dentry.  Should not
1239	dereference pointers outside the dentry without lots of care
1240	(eg.  d_parent, d_inode, d_name should not be used).
1241
1242	However, our vfsmount is pinned, and RCU held, so the dentries
1243	and inodes won't disappear, neither will our sb or filesystem
1244	module.  ->d_sb may be used.
1245
1246	It is a tricky calling convention because it needs to be called
1247	under "rcu-walk", ie. without any locks or references on things.
1248
1249``d_delete``
1250	called when the last reference to a dentry is dropped and the
1251	dcache is deciding whether or not to cache it.  Return 1 to
1252	delete immediately, or 0 to cache the dentry.  Default is NULL
1253	which means to always cache a reachable dentry.  d_delete must
1254	be constant and idempotent.
1255
1256``d_init``
1257	called when a dentry is allocated
1258
1259``d_release``
1260	called when a dentry is really deallocated
1261
1262``d_iput``
1263	called when a dentry loses its inode (just prior to its being
1264	deallocated).  The default when this is NULL is that the VFS
1265	calls iput().  If you define this method, you must call iput()
1266	yourself
1267
1268``d_dname``
1269	called when the pathname of a dentry should be generated.
1270	Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1271	delay pathname generation.  (Instead of doing it when dentry is
1272	created, it's done only when the path is needed.).  Real
1273	filesystems probably dont want to use it, because their dentries
1274	are present in global dcache hash, so their hash should be an
1275	invariant.  As no lock is held, d_dname() should not try to
1276	modify the dentry itself, unless appropriate SMP safety is used.
1277	CAUTION : d_path() logic is quite tricky.  The correct way to
1278	return for example "Hello" is to put it at the end of the
1279	buffer, and returns a pointer to the first char.
1280	dynamic_dname() helper function is provided to take care of
1281	this.
1282
1283	Example :
1284
1285.. code-block:: c
1286
1287	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1288	{
1289		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1290				dentry->d_inode->i_ino);
1291	}
1292
1293``d_automount``
1294	called when an automount dentry is to be traversed (optional).
1295	This should create a new VFS mount record and return the record
1296	to the caller.  The caller is supplied with a path parameter
1297	giving the automount directory to describe the automount target
1298	and the parent VFS mount record to provide inheritable mount
1299	parameters.  NULL should be returned if someone else managed to
1300	make the automount first.  If the vfsmount creation failed, then
1301	an error code should be returned.  If -EISDIR is returned, then
1302	the directory will be treated as an ordinary directory and
1303	returned to pathwalk to continue walking.
1304
1305	If a vfsmount is returned, the caller will attempt to mount it
1306	on the mountpoint and will remove the vfsmount from its
1307	expiration list in the case of failure.  The vfsmount should be
1308	returned with 2 refs on it to prevent automatic expiration - the
1309	caller will clean up the additional ref.
1310
1311	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1312	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is
1313	set on the inode being added.
1314
1315``d_manage``
1316	called to allow the filesystem to manage the transition from a
1317	dentry (optional).  This allows autofs, for example, to hold up
1318	clients waiting to explore behind a 'mountpoint' while letting
1319	the daemon go past and construct the subtree there.  0 should be
1320	returned to let the calling process continue.  -EISDIR can be
1321	returned to tell pathwalk to use this directory as an ordinary
1322	directory and to ignore anything mounted on it and not to check
1323	the automount flag.  Any other error code will abort pathwalk
1324	completely.
1325
1326	If the 'rcu_walk' parameter is true, then the caller is doing a
1327	pathwalk in RCU-walk mode.  Sleeping is not permitted in this
1328	mode, and the caller can be asked to leave it and call again by
1329	returning -ECHILD.  -EISDIR may also be returned to tell
1330	pathwalk to ignore d_automount or any mounts.
1331
1332	This function is only used if DCACHE_MANAGE_TRANSIT is set on
1333	the dentry being transited from.
1334
1335``d_real``
1336	overlay/union type filesystems implement this method to return
1337	one of the underlying dentries hidden by the overlay.  It is
1338	used in two different modes:
1339
1340	Called from file_dentry() it returns the real dentry matching
1341	the inode argument.  The real dentry may be from a lower layer
1342	already copied up, but still referenced from the file.  This
1343	mode is selected with a non-NULL inode argument.
1344
1345	With NULL inode the topmost real underlying dentry is returned.
1346
1347Each dentry has a pointer to its parent dentry, as well as a hash list
1348of child dentries.  Child dentries are basically like files in a
1349directory.
1350
1351
1352Directory Entry Cache API
1353--------------------------
1354
1355There are a number of functions defined which permit a filesystem to
1356manipulate dentries:
1357
1358``dget``
1359	open a new handle for an existing dentry (this just increments
1360	the usage count)
1361
1362``dput``
1363	close a handle for a dentry (decrements the usage count).  If
1364	the usage count drops to 0, and the dentry is still in its
1365	parent's hash, the "d_delete" method is called to check whether
1366	it should be cached.  If it should not be cached, or if the
1367	dentry is not hashed, it is deleted.  Otherwise cached dentries
1368	are put into an LRU list to be reclaimed on memory shortage.
1369
1370``d_drop``
1371	this unhashes a dentry from its parents hash list.  A subsequent
1372	call to dput() will deallocate the dentry if its usage count
1373	drops to 0
1374
1375``d_delete``
1376	delete a dentry.  If there are no other open references to the
1377	dentry then the dentry is turned into a negative dentry (the
1378	d_iput() method is called).  If there are other references, then
1379	d_drop() is called instead
1380
1381``d_add``
1382	add a dentry to its parents hash list and then calls
1383	d_instantiate()
1384
1385``d_instantiate``
1386	add a dentry to the alias hash list for the inode and updates
1387	the "d_inode" member.  The "i_count" member in the inode
1388	structure should be set/incremented.  If the inode pointer is
1389	NULL, the dentry is called a "negative dentry".  This function
1390	is commonly called when an inode is created for an existing
1391	negative dentry
1392
1393``d_lookup``
1394	look up a dentry given its parent and path name component It
1395	looks up the child of that given name from the dcache hash
1396	table.  If it is found, the reference count is incremented and
1397	the dentry is returned.  The caller must use dput() to free the
1398	dentry when it finishes using it.
1399
1400
1401Mount Options
1402=============
1403
1404
1405Parsing options
1406---------------
1407
1408On mount and remount the filesystem is passed a string containing a
1409comma separated list of mount options.  The options can have either of
1410these forms:
1411
1412  option
1413  option=value
1414
1415The <linux/parser.h> header defines an API that helps parse these
1416options.  There are plenty of examples on how to use it in existing
1417filesystems.
1418
1419
1420Showing options
1421---------------
1422
1423If a filesystem accepts mount options, it must define show_options() to
1424show all the currently active options.  The rules are:
1425
1426  - options MUST be shown which are not default or their values differ
1427    from the default
1428
1429  - options MAY be shown which are enabled by default or have their
1430    default value
1431
1432Options used only internally between a mount helper and the kernel (such
1433as file descriptors), or which only have an effect during the mounting
1434(such as ones controlling the creation of a journal) are exempt from the
1435above rules.
1436
1437The underlying reason for the above rules is to make sure, that a mount
1438can be accurately replicated (e.g. umounting and mounting again) based
1439on the information found in /proc/mounts.
1440
1441
1442Resources
1443=========
1444
1445(Note some of these resources are not up-to-date with the latest kernel
1446 version.)
1447
1448Creating Linux virtual filesystems. 2002
1449    <https://lwn.net/Articles/13325/>
1450
1451The Linux Virtual File-system Layer by Neil Brown. 1999
1452    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1453
1454A tour of the Linux VFS by Michael K. Johnson. 1996
1455    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1456
1457A small trail through the Linux kernel by Andries Brouwer. 2001
1458    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>
1459