1.. SPDX-License-Identifier: GPL-2.0
2
3=========================================
4Overview of the Linux Virtual File System
5=========================================
6
7Original author: Richard Gooch <rgooch@atnf.csiro.au>
8
9- Copyright (C) 1999 Richard Gooch
10- Copyright (C) 2005 Pekka Enberg
11
12
13Introduction
14============
15
16The Virtual File System (also known as the Virtual Filesystem Switch) is
17the software layer in the kernel that provides the filesystem interface
18to userspace programs.  It also provides an abstraction within the
19kernel which allows different filesystem implementations to coexist.
20
21VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22are called from a process context.  Filesystem locking is described in
23the document Documentation/filesystems/locking.rst.
24
25
26Directory Entry Cache (dcache)
27------------------------------
28
29The VFS implements the open(2), stat(2), chmod(2), and similar system
30calls.  The pathname argument that is passed to them is used by the VFS
31to search through the directory entry cache (also known as the dentry
32cache or dcache).  This provides a very fast look-up mechanism to
33translate a pathname (filename) into a specific dentry.  Dentries live
34in RAM and are never saved to disc: they exist only for performance.
35
36The dentry cache is meant to be a view into your entire filespace.  As
37most computers cannot fit all dentries in the RAM at the same time, some
38bits of the cache are missing.  In order to resolve your pathname into a
39dentry, the VFS may have to resort to creating dentries along the way,
40and then loading the inode.  This is done by looking up the inode.
41
42
43The Inode Object
44----------------
45
46An individual dentry usually has a pointer to an inode.  Inodes are
47filesystem objects such as regular files, directories, FIFOs and other
48beasts.  They live either on the disc (for block device filesystems) or
49in the memory (for pseudo filesystems).  Inodes that live on the disc
50are copied into the memory when required and changes to the inode are
51written back to disc.  A single inode can be pointed to by multiple
52dentries (hard links, for example, do this).
53
54To look up an inode requires that the VFS calls the lookup() method of
55the parent directory inode.  This method is installed by the specific
56filesystem implementation that the inode lives in.  Once the VFS has the
57required dentry (and hence the inode), we can do all those boring things
58like open(2) the file, or stat(2) it to peek at the inode data.  The
59stat(2) operation is fairly simple: once the VFS has the dentry, it
60peeks at the inode data and passes some of it back to userspace.
61
62
63The File Object
64---------------
65
66Opening a file requires another operation: allocation of a file
67structure (this is the kernel-side implementation of file descriptors).
68The freshly allocated file structure is initialized with a pointer to
69the dentry and a set of file operation member functions.  These are
70taken from the inode data.  The open() file method is then called so the
71specific filesystem implementation can do its work.  You can see that
72this is another switch performed by the VFS.  The file structure is
73placed into the file descriptor table for the process.
74
75Reading, writing and closing files (and other assorted VFS operations)
76is done by using the userspace file descriptor to grab the appropriate
77file structure, and then calling the required file structure method to
78do whatever is required.  For as long as the file is open, it keeps the
79dentry in use, which in turn means that the VFS inode is still in use.
80
81
82Registering and Mounting a Filesystem
83=====================================
84
85To register and unregister a filesystem, use the following API
86functions:
87
88.. code-block:: c
89
90	#include <linux/fs.h>
91
92	extern int register_filesystem(struct file_system_type *);
93	extern int unregister_filesystem(struct file_system_type *);
94
95The passed struct file_system_type describes your filesystem.  When a
96request is made to mount a filesystem onto a directory in your
97namespace, the VFS will call the appropriate mount() method for the
98specific filesystem.  New vfsmount referring to the tree returned by
99->mount() will be attached to the mountpoint, so that when pathname
100resolution reaches the mountpoint it will jump into the root of that
101vfsmount.
102
103You can see all filesystems that are registered to the kernel in the
104file /proc/filesystems.
105
106
107struct file_system_type
108-----------------------
109
110This describes the filesystem.  As of kernel 2.6.39, the following
111members are defined:
112
113.. code-block:: c
114
115	struct file_system_type {
116		const char *name;
117		int fs_flags;
118		struct dentry *(*mount) (struct file_system_type *, int,
119					 const char *, void *);
120		void (*kill_sb) (struct super_block *);
121		struct module *owner;
122		struct file_system_type * next;
123		struct list_head fs_supers;
124		struct lock_class_key s_lock_key;
125		struct lock_class_key s_umount_key;
126	};
127
128``name``
129	the name of the filesystem type, such as "ext2", "iso9660",
130	"msdos" and so on
131
132``fs_flags``
133	various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
134
135``mount``
136	the method to call when a new instance of this filesystem should
137	be mounted
138
139``kill_sb``
140	the method to call when an instance of this filesystem should be
141	shut down
142
143
144``owner``
145	for internal VFS use: you should initialize this to THIS_MODULE
146	in most cases.
147
148``next``
149	for internal VFS use: you should initialize this to NULL
150
151  s_lock_key, s_umount_key: lockdep-specific
152
153The mount() method has the following arguments:
154
155``struct file_system_type *fs_type``
156	describes the filesystem, partly initialized by the specific
157	filesystem code
158
159``int flags``
160	mount flags
161
162``const char *dev_name``
163	the device name we are mounting.
164
165``void *data``
166	arbitrary mount options, usually comes as an ASCII string (see
167	"Mount Options" section)
168
169The mount() method must return the root dentry of the tree requested by
170caller.  An active reference to its superblock must be grabbed and the
171superblock must be locked.  On failure it should return ERR_PTR(error).
172
173The arguments match those of mount(2) and their interpretation depends
174on filesystem type.  E.g. for block filesystems, dev_name is interpreted
175as block device name, that device is opened and if it contains a
176suitable filesystem image the method creates and initializes struct
177super_block accordingly, returning its root dentry to caller.
178
179->mount() may choose to return a subtree of existing filesystem - it
180doesn't have to create a new one.  The main result from the caller's
181point of view is a reference to dentry at the root of (sub)tree to be
182attached; creation of new superblock is a common side effect.
183
184The most interesting member of the superblock structure that the mount()
185method fills in is the "s_op" field.  This is a pointer to a "struct
186super_operations" which describes the next level of the filesystem
187implementation.
188
189Usually, a filesystem uses one of the generic mount() implementations
190and provides a fill_super() callback instead.  The generic variants are:
191
192``mount_bdev``
193	mount a filesystem residing on a block device
194
195``mount_nodev``
196	mount a filesystem that is not backed by a device
197
198``mount_single``
199	mount a filesystem which shares the instance between all mounts
200
201A fill_super() callback implementation has the following arguments:
202
203``struct super_block *sb``
204	the superblock structure.  The callback must initialize this
205	properly.
206
207``void *data``
208	arbitrary mount options, usually comes as an ASCII string (see
209	"Mount Options" section)
210
211``int silent``
212	whether or not to be silent on error
213
214
215The Superblock Object
216=====================
217
218A superblock object represents a mounted filesystem.
219
220
221struct super_operations
222-----------------------
223
224This describes how the VFS can manipulate the superblock of your
225filesystem.  As of kernel 2.6.22, the following members are defined:
226
227.. code-block:: c
228
229	struct super_operations {
230		struct inode *(*alloc_inode)(struct super_block *sb);
231		void (*destroy_inode)(struct inode *);
232
233		void (*dirty_inode) (struct inode *, int flags);
234		int (*write_inode) (struct inode *, int);
235		void (*drop_inode) (struct inode *);
236		void (*delete_inode) (struct inode *);
237		void (*put_super) (struct super_block *);
238		int (*sync_fs)(struct super_block *sb, int wait);
239		int (*freeze_fs) (struct super_block *);
240		int (*unfreeze_fs) (struct super_block *);
241		int (*statfs) (struct dentry *, struct kstatfs *);
242		int (*remount_fs) (struct super_block *, int *, char *);
243		void (*clear_inode) (struct inode *);
244		void (*umount_begin) (struct super_block *);
245
246		int (*show_options)(struct seq_file *, struct dentry *);
247
248		ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
249		ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
250		int (*nr_cached_objects)(struct super_block *);
251		void (*free_cached_objects)(struct super_block *, int);
252	};
253
254All methods are called without any locks being held, unless otherwise
255noted.  This means that most methods can block safely.  All methods are
256only called from a process context (i.e. not from an interrupt handler
257or bottom half).
258
259``alloc_inode``
260	this method is called by alloc_inode() to allocate memory for
261	struct inode and initialize it.  If this function is not
262	defined, a simple 'struct inode' is allocated.  Normally
263	alloc_inode will be used to allocate a larger structure which
264	contains a 'struct inode' embedded within it.
265
266``destroy_inode``
267	this method is called by destroy_inode() to release resources
268	allocated for struct inode.  It is only required if
269	->alloc_inode was defined and simply undoes anything done by
270	->alloc_inode.
271
272``dirty_inode``
273	this method is called by the VFS when an inode is marked dirty.
274	This is specifically for the inode itself being marked dirty,
275	not its data.  If the update needs to be persisted by fdatasync(),
276	then I_DIRTY_DATASYNC will be set in the flags argument.
277	I_DIRTY_TIME will be set in the flags in case lazytime is enabled
278	and struct inode has times updated since the last ->dirty_inode
279	call.
280
281``write_inode``
282	this method is called when the VFS needs to write an inode to
283	disc.  The second parameter indicates whether the write should
284	be synchronous or not, not all filesystems check this flag.
285
286``drop_inode``
287	called when the last access to the inode is dropped, with the
288	inode->i_lock spinlock held.
289
290	This method should be either NULL (normal UNIX filesystem
291	semantics) or "generic_delete_inode" (for filesystems that do
292	not want to cache inodes - causing "delete_inode" to always be
293	called regardless of the value of i_nlink)
294
295	The "generic_delete_inode()" behavior is equivalent to the old
296	practice of using "force_delete" in the put_inode() case, but
297	does not have the races that the "force_delete()" approach had.
298
299``delete_inode``
300	called when the VFS wants to delete an inode
301
302``put_super``
303	called when the VFS wishes to free the superblock
304	(i.e. unmount).  This is called with the superblock lock held
305
306``sync_fs``
307	called when VFS is writing out all dirty data associated with a
308	superblock.  The second parameter indicates whether the method
309	should wait until the write out has been completed.  Optional.
310
311``freeze_fs``
312	called when VFS is locking a filesystem and forcing it into a
313	consistent state.  This method is currently used by the Logical
314	Volume Manager (LVM).
315
316``unfreeze_fs``
317	called when VFS is unlocking a filesystem and making it writable
318	again.
319
320``statfs``
321	called when the VFS needs to get filesystem statistics.
322
323``remount_fs``
324	called when the filesystem is remounted.  This is called with
325	the kernel lock held
326
327``clear_inode``
328	called then the VFS clears the inode.  Optional
329
330``umount_begin``
331	called when the VFS is unmounting a filesystem.
332
333``show_options``
334	called by the VFS to show mount options for /proc/<pid>/mounts.
335	(see "Mount Options" section)
336
337``quota_read``
338	called by the VFS to read from filesystem quota file.
339
340``quota_write``
341	called by the VFS to write to filesystem quota file.
342
343``nr_cached_objects``
344	called by the sb cache shrinking function for the filesystem to
345	return the number of freeable cached objects it contains.
346	Optional.
347
348``free_cache_objects``
349	called by the sb cache shrinking function for the filesystem to
350	scan the number of objects indicated to try to free them.
351	Optional, but any filesystem implementing this method needs to
352	also implement ->nr_cached_objects for it to be called
353	correctly.
354
355	We can't do anything with any errors that the filesystem might
356	encountered, hence the void return type.  This will never be
357	called if the VM is trying to reclaim under GFP_NOFS conditions,
358	hence this method does not need to handle that situation itself.
359
360	Implementations must include conditional reschedule calls inside
361	any scanning loop that is done.  This allows the VFS to
362	determine appropriate scan batch sizes without having to worry
363	about whether implementations will cause holdoff problems due to
364	large scan batch sizes.
365
366Whoever sets up the inode is responsible for filling in the "i_op"
367field.  This is a pointer to a "struct inode_operations" which describes
368the methods that can be performed on individual inodes.
369
370
371struct xattr_handlers
372---------------------
373
374On filesystems that support extended attributes (xattrs), the s_xattr
375superblock field points to a NULL-terminated array of xattr handlers.
376Extended attributes are name:value pairs.
377
378``name``
379	Indicates that the handler matches attributes with the specified
380	name (such as "system.posix_acl_access"); the prefix field must
381	be NULL.
382
383``prefix``
384	Indicates that the handler matches all attributes with the
385	specified name prefix (such as "user."); the name field must be
386	NULL.
387
388``list``
389	Determine if attributes matching this xattr handler should be
390	listed for a particular dentry.  Used by some listxattr
391	implementations like generic_listxattr.
392
393``get``
394	Called by the VFS to get the value of a particular extended
395	attribute.  This method is called by the getxattr(2) system
396	call.
397
398``set``
399	Called by the VFS to set the value of a particular extended
400	attribute.  When the new value is NULL, called to remove a
401	particular extended attribute.  This method is called by the
402	setxattr(2) and removexattr(2) system calls.
403
404When none of the xattr handlers of a filesystem match the specified
405attribute name or when a filesystem doesn't support extended attributes,
406the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
407
408
409The Inode Object
410================
411
412An inode object represents an object within the filesystem.
413
414
415struct inode_operations
416-----------------------
417
418This describes how the VFS can manipulate an inode in your filesystem.
419As of kernel 2.6.22, the following members are defined:
420
421.. code-block:: c
422
423	struct inode_operations {
424		int (*create) (struct mnt_idmap *, struct inode *,struct dentry *, umode_t, bool);
425		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
426		int (*link) (struct dentry *,struct inode *,struct dentry *);
427		int (*unlink) (struct inode *,struct dentry *);
428		int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
429		int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
430		int (*rmdir) (struct inode *,struct dentry *);
431		int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
432		int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
433			       struct inode *, struct dentry *, unsigned int);
434		int (*readlink) (struct dentry *, char __user *,int);
435		const char *(*get_link) (struct dentry *, struct inode *,
436					 struct delayed_call *);
437		int (*permission) (struct mnt_idmap *, struct inode *, int);
438		struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
439		int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
440		int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
441		ssize_t (*listxattr) (struct dentry *, char *, size_t);
442		void (*update_time)(struct inode *, struct timespec *, int);
443		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
444				   unsigned open_flag, umode_t create_mode);
445		int (*tmpfile) (struct mnt_idmap *, struct inode *, struct file *, umode_t);
446		struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
447	        int (*set_acl)(struct mnt_idmap *, struct dentry *, struct posix_acl *, int);
448		int (*fileattr_set)(struct mnt_idmap *idmap,
449				    struct dentry *dentry, struct fileattr *fa);
450		int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
451	};
452
453Again, all methods are called without any locks being held, unless
454otherwise noted.
455
456``create``
457	called by the open(2) and creat(2) system calls.  Only required
458	if you want to support regular files.  The dentry you get should
459	not have an inode (i.e. it should be a negative dentry).  Here
460	you will probably call d_instantiate() with the dentry and the
461	newly created inode
462
463``lookup``
464	called when the VFS needs to look up an inode in a parent
465	directory.  The name to look for is found in the dentry.  This
466	method must call d_add() to insert the found inode into the
467	dentry.  The "i_count" field in the inode structure should be
468	incremented.  If the named inode does not exist a NULL inode
469	should be inserted into the dentry (this is called a negative
470	dentry).  Returning an error code from this routine must only be
471	done on a real error, otherwise creating inodes with system
472	calls like create(2), mknod(2), mkdir(2) and so on will fail.
473	If you wish to overload the dentry methods then you should
474	initialise the "d_dop" field in the dentry; this is a pointer to
475	a struct "dentry_operations".  This method is called with the
476	directory inode semaphore held
477
478``link``
479	called by the link(2) system call.  Only required if you want to
480	support hard links.  You will probably need to call
481	d_instantiate() just as you would in the create() method
482
483``unlink``
484	called by the unlink(2) system call.  Only required if you want
485	to support deleting inodes
486
487``symlink``
488	called by the symlink(2) system call.  Only required if you want
489	to support symlinks.  You will probably need to call
490	d_instantiate() just as you would in the create() method
491
492``mkdir``
493	called by the mkdir(2) system call.  Only required if you want
494	to support creating subdirectories.  You will probably need to
495	call d_instantiate() just as you would in the create() method
496
497``rmdir``
498	called by the rmdir(2) system call.  Only required if you want
499	to support deleting subdirectories
500
501``mknod``
502	called by the mknod(2) system call to create a device (char,
503	block) inode or a named pipe (FIFO) or socket.  Only required if
504	you want to support creating these types of inodes.  You will
505	probably need to call d_instantiate() just as you would in the
506	create() method
507
508``rename``
509	called by the rename(2) system call to rename the object to have
510	the parent and name given by the second inode and dentry.
511
512	The filesystem must return -EINVAL for any unsupported or
513	unknown flags.  Currently the following flags are implemented:
514	(1) RENAME_NOREPLACE: this flag indicates that if the target of
515	the rename exists the rename should fail with -EEXIST instead of
516	replacing the target.  The VFS already checks for existence, so
517	for local filesystems the RENAME_NOREPLACE implementation is
518	equivalent to plain rename.
519	(2) RENAME_EXCHANGE: exchange source and target.  Both must
520	exist; this is checked by the VFS.  Unlike plain rename, source
521	and target may be of different type.
522
523``get_link``
524	called by the VFS to follow a symbolic link to the inode it
525	points to.  Only required if you want to support symbolic links.
526	This method returns the symlink body to traverse (and possibly
527	resets the current position with nd_jump_link()).  If the body
528	won't go away until the inode is gone, nothing else is needed;
529	if it needs to be otherwise pinned, arrange for its release by
530	having get_link(..., ..., done) do set_delayed_call(done,
531	destructor, argument).  In that case destructor(argument) will
532	be called once VFS is done with the body you've returned.  May
533	be called in RCU mode; that is indicated by NULL dentry
534	argument.  If request can't be handled without leaving RCU mode,
535	have it return ERR_PTR(-ECHILD).
536
537	If the filesystem stores the symlink target in ->i_link, the
538	VFS may use it directly without calling ->get_link(); however,
539	->get_link() must still be provided.  ->i_link must not be
540	freed until after an RCU grace period.  Writing to ->i_link
541	post-iget() time requires a 'release' memory barrier.
542
543``readlink``
544	this is now just an override for use by readlink(2) for the
545	cases when ->get_link uses nd_jump_link() or object is not in
546	fact a symlink.  Normally filesystems should only implement
547	->get_link for symlinks and readlink(2) will automatically use
548	that.
549
550``permission``
551	called by the VFS to check for access rights on a POSIX-like
552	filesystem.
553
554	May be called in rcu-walk mode (mask & MAY_NOT_BLOCK).  If in
555	rcu-walk mode, the filesystem must check the permission without
556	blocking or storing to the inode.
557
558	If a situation is encountered that rcu-walk cannot handle,
559	return
560	-ECHILD and it will be called again in ref-walk mode.
561
562``setattr``
563	called by the VFS to set attributes for a file.  This method is
564	called by chmod(2) and related system calls.
565
566``getattr``
567	called by the VFS to get attributes of a file.  This method is
568	called by stat(2) and related system calls.
569
570``listxattr``
571	called by the VFS to list all extended attributes for a given
572	file.  This method is called by the listxattr(2) system call.
573
574``update_time``
575	called by the VFS to update a specific time or the i_version of
576	an inode.  If this is not defined the VFS will update the inode
577	itself and call mark_inode_dirty_sync.
578
579``atomic_open``
580	called on the last component of an open.  Using this optional
581	method the filesystem can look up, possibly create and open the
582	file in one atomic operation.  If it wants to leave actual
583	opening to the caller (e.g. if the file turned out to be a
584	symlink, device, or just something filesystem won't do atomic
585	open for), it may signal this by returning finish_no_open(file,
586	dentry).  This method is only called if the last component is
587	negative or needs lookup.  Cached positive dentries are still
588	handled by f_op->open().  If the file was created, FMODE_CREATED
589	flag should be set in file->f_mode.  In case of O_EXCL the
590	method must only succeed if the file didn't exist and hence
591	FMODE_CREATED shall always be set on success.
592
593``tmpfile``
594	called in the end of O_TMPFILE open().  Optional, equivalent to
595	atomically creating, opening and unlinking a file in given
596	directory.  On success needs to return with the file already
597	open; this can be done by calling finish_open_simple() right at
598	the end.
599
600``fileattr_get``
601	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
602	retrieve miscellaneous file flags and attributes.  Also called
603	before the relevant SET operation to check what is being changed
604	(in this case with i_rwsem locked exclusive).  If unset, then
605	fall back to f_op->ioctl().
606
607``fileattr_set``
608	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
609	change miscellaneous file flags and attributes.  Callers hold
610	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
611
612
613The Address Space Object
614========================
615
616The address space object is used to group and manage pages in the page
617cache.  It can be used to keep track of the pages in a file (or anything
618else) and also track the mapping of sections of the file into process
619address spaces.
620
621There are a number of distinct yet related services that an
622address-space can provide.  These include communicating memory pressure,
623page lookup by address, and keeping track of pages tagged as Dirty or
624Writeback.
625
626The first can be used independently to the others.  The VM can try to
627either write dirty pages in order to clean them, or release clean pages
628in order to reuse them.  To do this it can call the ->writepage method
629on dirty pages, and ->release_folio on clean folios with the private
630flag set.  Clean pages without PagePrivate and with no external references
631will be released without notice being given to the address_space.
632
633To achieve this functionality, pages need to be placed on an LRU with
634lru_cache_add and mark_page_active needs to be called whenever the page
635is used.
636
637Pages are normally kept in a radix tree index by ->index.  This tree
638maintains information about the PG_Dirty and PG_Writeback status of each
639page, so that pages with either of these flags can be found quickly.
640
641The Dirty tag is primarily used by mpage_writepages - the default
642->writepages method.  It uses the tag to find dirty pages to call
643->writepage on.  If mpage_writepages is not used (i.e. the address
644provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
645unused.  write_inode_now and sync_inode do use it (through
646__sync_single_inode) to check if ->writepages has been successful in
647writing out the whole address_space.
648
649The Writeback tag is used by filemap*wait* and sync_page* functions, via
650filemap_fdatawait_range, to wait for all writeback to complete.
651
652An address_space handler may attach extra information to a page,
653typically using the 'private' field in the 'struct page'.  If such
654information is attached, the PG_Private flag should be set.  This will
655cause various VM routines to make extra calls into the address_space
656handler to deal with that data.
657
658An address space acts as an intermediate between storage and
659application.  Data is read into the address space a whole page at a
660time, and provided to the application either by copying of the page, or
661by memory-mapping the page.  Data is written into the address space by
662the application, and then written-back to storage typically in whole
663pages, however the address_space has finer control of write sizes.
664
665The read process essentially only requires 'read_folio'.  The write
666process is more complicated and uses write_begin/write_end or
667dirty_folio to write data into the address_space, and writepage and
668writepages to writeback data to storage.
669
670Adding and removing pages to/from an address_space is protected by the
671inode's i_mutex.
672
673When data is written to a page, the PG_Dirty flag should be set.  It
674typically remains set until writepage asks for it to be written.  This
675should clear PG_Dirty and set PG_Writeback.  It can be actually written
676at any point after PG_Dirty is clear.  Once it is known to be safe,
677PG_Writeback is cleared.
678
679Writeback makes use of a writeback_control structure to direct the
680operations.  This gives the writepage and writepages operations some
681information about the nature of and reason for the writeback request,
682and the constraints under which it is being done.  It is also used to
683return information back to the caller about the result of a writepage or
684writepages request.
685
686
687Handling errors during writeback
688--------------------------------
689
690Most applications that do buffered I/O will periodically call a file
691synchronization call (fsync, fdatasync, msync or sync_file_range) to
692ensure that data written has made it to the backing store.  When there
693is an error during writeback, they expect that error to be reported when
694a file sync request is made.  After an error has been reported on one
695request, subsequent requests on the same file descriptor should return
6960, unless further writeback errors have occurred since the previous file
697syncronization.
698
699Ideally, the kernel would report errors only on file descriptions on
700which writes were done that subsequently failed to be written back.  The
701generic pagecache infrastructure does not track the file descriptions
702that have dirtied each individual page however, so determining which
703file descriptors should get back an error is not possible.
704
705Instead, the generic writeback error tracking infrastructure in the
706kernel settles for reporting errors to fsync on all file descriptions
707that were open at the time that the error occurred.  In a situation with
708multiple writers, all of them will get back an error on a subsequent
709fsync, even if all of the writes done through that particular file
710descriptor succeeded (or even if there were no writes on that file
711descriptor at all).
712
713Filesystems that wish to use this infrastructure should call
714mapping_set_error to record the error in the address_space when it
715occurs.  Then, after writing back data from the pagecache in their
716file->fsync operation, they should call file_check_and_advance_wb_err to
717ensure that the struct file's error cursor has advanced to the correct
718point in the stream of errors emitted by the backing device(s).
719
720
721struct address_space_operations
722-------------------------------
723
724This describes how the VFS can manipulate mapping of a file to page
725cache in your filesystem.  The following members are defined:
726
727.. code-block:: c
728
729	struct address_space_operations {
730		int (*writepage)(struct page *page, struct writeback_control *wbc);
731		int (*read_folio)(struct file *, struct folio *);
732		int (*writepages)(struct address_space *, struct writeback_control *);
733		bool (*dirty_folio)(struct address_space *, struct folio *);
734		void (*readahead)(struct readahead_control *);
735		int (*write_begin)(struct file *, struct address_space *mapping,
736				   loff_t pos, unsigned len,
737				struct page **pagep, void **fsdata);
738		int (*write_end)(struct file *, struct address_space *mapping,
739				 loff_t pos, unsigned len, unsigned copied,
740				 struct page *page, void *fsdata);
741		sector_t (*bmap)(struct address_space *, sector_t);
742		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
743		bool (*release_folio)(struct folio *, gfp_t);
744		void (*free_folio)(struct folio *);
745		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
746		int (*migrate_folio)(struct mapping *, struct folio *dst,
747				struct folio *src, enum migrate_mode);
748		int (*launder_folio) (struct folio *);
749
750		bool (*is_partially_uptodate) (struct folio *, size_t from,
751					       size_t count);
752		void (*is_dirty_writeback)(struct folio *, bool *, bool *);
753		int (*error_remove_page) (struct mapping *mapping, struct page *page);
754		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
755		int (*swap_deactivate)(struct file *);
756		int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
757	};
758
759``writepage``
760	called by the VM to write a dirty page to backing store.  This
761	may happen for data integrity reasons (i.e. 'sync'), or to free
762	up memory (flush).  The difference can be seen in
763	wbc->sync_mode.  The PG_Dirty flag has been cleared and
764	PageLocked is true.  writepage should start writeout, should set
765	PG_Writeback, and should make sure the page is unlocked, either
766	synchronously or asynchronously when the write operation
767	completes.
768
769	If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
770	try too hard if there are problems, and may choose to write out
771	other pages from the mapping if that is easier (e.g. due to
772	internal dependencies).  If it chooses not to start writeout, it
773	should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
774	keep calling ->writepage on that page.
775
776	See the file "Locking" for more details.
777
778``read_folio``
779	Called by the page cache to read a folio from the backing store.
780	The 'file' argument supplies authentication information to network
781	filesystems, and is generally not used by block based filesystems.
782	It may be NULL if the caller does not have an open file (eg if
783	the kernel is performing a read for itself rather than on behalf
784	of a userspace process with an open file).
785
786	If the mapping does not support large folios, the folio will
787	contain a single page.	The folio will be locked when read_folio
788	is called.  If the read completes successfully, the folio should
789	be marked uptodate.  The filesystem should unlock the folio
790	once the read has completed, whether it was successful or not.
791	The filesystem does not need to modify the refcount on the folio;
792	the page cache holds a reference count and that will not be
793	released until the folio is unlocked.
794
795	Filesystems may implement ->read_folio() synchronously.
796	In normal operation, folios are read through the ->readahead()
797	method.  Only if this fails, or if the caller needs to wait for
798	the read to complete will the page cache call ->read_folio().
799	Filesystems should not attempt to perform their own readahead
800	in the ->read_folio() operation.
801
802	If the filesystem cannot perform the read at this time, it can
803	unlock the folio, do whatever action it needs to ensure that the
804	read will succeed in the future and return AOP_TRUNCATED_PAGE.
805	In this case, the caller should look up the folio, lock it,
806	and call ->read_folio again.
807
808	Callers may invoke the ->read_folio() method directly, but using
809	read_mapping_folio() will take care of locking, waiting for the
810	read to complete and handle cases such as AOP_TRUNCATED_PAGE.
811
812``writepages``
813	called by the VM to write out pages associated with the
814	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
815	the writeback_control will specify a range of pages that must be
816	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
817	given and that many pages should be written if possible.  If no
818	->writepages is given, then mpage_writepages is used instead.
819	This will choose pages from the address space that are tagged as
820	DIRTY and will pass them to ->writepage.
821
822``dirty_folio``
823	called by the VM to mark a folio as dirty.  This is particularly
824	needed if an address space attaches private data to a folio, and
825	that data needs to be updated when a folio is dirtied.  This is
826	called, for example, when a memory mapped page gets modified.
827	If defined, it should set the folio dirty flag, and the
828	PAGECACHE_TAG_DIRTY search mark in i_pages.
829
830``readahead``
831	Called by the VM to read pages associated with the address_space
832	object.  The pages are consecutive in the page cache and are
833	locked.  The implementation should decrement the page refcount
834	after starting I/O on each page.  Usually the page will be
835	unlocked by the I/O completion handler.  The set of pages are
836	divided into some sync pages followed by some async pages,
837	rac->ra->async_size gives the number of async pages.  The
838	filesystem should attempt to read all sync pages but may decide
839	to stop once it reaches the async pages.  If it does decide to
840	stop attempting I/O, it can simply return.  The caller will
841	remove the remaining pages from the address space, unlock them
842	and decrement the page refcount.  Set PageUptodate if the I/O
843	completes successfully.  Setting PageError on any page will be
844	ignored; simply unlock the page if an I/O error occurs.
845
846``write_begin``
847	Called by the generic buffered write code to ask the filesystem
848	to prepare to write len bytes at the given offset in the file.
849	The address_space should check that the write will be able to
850	complete, by allocating space if necessary and doing any other
851	internal housekeeping.  If the write will update parts of any
852	basic-blocks on storage, then those blocks should be pre-read
853	(if they haven't been read already) so that the updated blocks
854	can be written out properly.
855
856	The filesystem must return the locked pagecache page for the
857	specified offset, in ``*pagep``, for the caller to write into.
858
859	It must be able to cope with short writes (where the length
860	passed to write_begin is greater than the number of bytes copied
861	into the page).
862
863	A void * may be returned in fsdata, which then gets passed into
864	write_end.
865
866	Returns 0 on success; < 0 on failure (which is the error code),
867	in which case write_end is not called.
868
869``write_end``
870	After a successful write_begin, and data copy, write_end must be
871	called.  len is the original len passed to write_begin, and
872	copied is the amount that was able to be copied.
873
874	The filesystem must take care of unlocking the page and
875	releasing it refcount, and updating i_size.
876
877	Returns < 0 on failure, otherwise the number of bytes (<=
878	'copied') that were able to be copied into pagecache.
879
880``bmap``
881	called by the VFS to map a logical block offset within object to
882	physical block number.  This method is used by the FIBMAP ioctl
883	and for working with swap-files.  To be able to swap to a file,
884	the file must have a stable mapping to a block device.  The swap
885	system does not go through the filesystem but instead uses bmap
886	to find out where the blocks in the file are and uses those
887	addresses directly.
888
889``invalidate_folio``
890	If a folio has private data, then invalidate_folio will be
891	called when part or all of the folio is to be removed from the
892	address space.  This generally corresponds to either a
893	truncation, punch hole or a complete invalidation of the address
894	space (in the latter case 'offset' will always be 0 and 'length'
895	will be folio_size()).  Any private data associated with the folio
896	should be updated to reflect this truncation.  If offset is 0
897	and length is folio_size(), then the private data should be
898	released, because the folio must be able to be completely
899	discarded.  This may be done by calling the ->release_folio
900	function, but in this case the release MUST succeed.
901
902``release_folio``
903	release_folio is called on folios with private data to tell the
904	filesystem that the folio is about to be freed.  ->release_folio
905	should remove any private data from the folio and clear the
906	private flag.  If release_folio() fails, it should return false.
907	release_folio() is used in two distinct though related cases.
908	The first is when the VM wants to free a clean folio with no
909	active users.  If ->release_folio succeeds, the folio will be
910	removed from the address_space and be freed.
911
912	The second case is when a request has been made to invalidate
913	some or all folios in an address_space.  This can happen
914	through the fadvise(POSIX_FADV_DONTNEED) system call or by the
915	filesystem explicitly requesting it as nfs and 9p do (when they
916	believe the cache may be out of date with storage) by calling
917	invalidate_inode_pages2().  If the filesystem makes such a call,
918	and needs to be certain that all folios are invalidated, then
919	its release_folio will need to ensure this.  Possibly it can
920	clear the uptodate flag if it cannot free private data yet.
921
922``free_folio``
923	free_folio is called once the folio is no longer visible in the
924	page cache in order to allow the cleanup of any private data.
925	Since it may be called by the memory reclaimer, it should not
926	assume that the original address_space mapping still exists, and
927	it should not block.
928
929``direct_IO``
930	called by the generic read/write routines to perform direct_IO -
931	that is IO requests which bypass the page cache and transfer
932	data directly between the storage and the application's address
933	space.
934
935``migrate_folio``
936	This is used to compact the physical memory usage.  If the VM
937	wants to relocate a folio (maybe from a memory device that is
938	signalling imminent failure) it will pass a new folio and an old
939	folio to this function.  migrate_folio should transfer any private
940	data across and update any references that it has to the folio.
941
942``launder_folio``
943	Called before freeing a folio - it writes back the dirty folio.
944	To prevent redirtying the folio, it is kept locked during the
945	whole operation.
946
947``is_partially_uptodate``
948	Called by the VM when reading a file through the pagecache when
949	the underlying blocksize is smaller than the size of the folio.
950	If the required block is up to date then the read can complete
951	without needing I/O to bring the whole page up to date.
952
953``is_dirty_writeback``
954	Called by the VM when attempting to reclaim a folio.  The VM uses
955	dirty and writeback information to determine if it needs to
956	stall to allow flushers a chance to complete some IO.
957	Ordinarily it can use folio_test_dirty and folio_test_writeback but
958	some filesystems have more complex state (unstable folios in NFS
959	prevent reclaim) or do not set those flags due to locking
960	problems.  This callback allows a filesystem to indicate to the
961	VM if a folio should be treated as dirty or writeback for the
962	purposes of stalling.
963
964``error_remove_page``
965	normally set to generic_error_remove_page if truncation is ok
966	for this address space.  Used for memory failure handling.
967	Setting this implies you deal with pages going away under you,
968	unless you have them locked or reference counts increased.
969
970``swap_activate``
971
972	Called to prepare the given file for swap.  It should perform
973	any validation and preparation necessary to ensure that writes
974	can be performed with minimal memory allocation.  It should call
975	add_swap_extent(), or the helper iomap_swapfile_activate(), and
976	return the number of extents added.  If IO should be submitted
977	through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
978	be submitted directly to the block device ``sis->bdev``.
979
980``swap_deactivate``
981	Called during swapoff on files where swap_activate was
982	successful.
983
984``swap_rw``
985	Called to read or write swap pages when SWP_FS_OPS is set.
986
987The File Object
988===============
989
990A file object represents a file opened by a process.  This is also known
991as an "open file description" in POSIX parlance.
992
993
994struct file_operations
995----------------------
996
997This describes how the VFS can manipulate an open file.  As of kernel
9984.18, the following members are defined:
999
1000.. code-block:: c
1001
1002	struct file_operations {
1003		struct module *owner;
1004		loff_t (*llseek) (struct file *, loff_t, int);
1005		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1006		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1007		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1008		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1009		int (*iopoll)(struct kiocb *kiocb, bool spin);
1010		int (*iterate) (struct file *, struct dir_context *);
1011		int (*iterate_shared) (struct file *, struct dir_context *);
1012		__poll_t (*poll) (struct file *, struct poll_table_struct *);
1013		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1014		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1015		int (*mmap) (struct file *, struct vm_area_struct *);
1016		int (*open) (struct inode *, struct file *);
1017		int (*flush) (struct file *, fl_owner_t id);
1018		int (*release) (struct inode *, struct file *);
1019		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1020		int (*fasync) (int, struct file *, int);
1021		int (*lock) (struct file *, int, struct file_lock *);
1022		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1023		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1024		int (*check_flags)(int);
1025		int (*flock) (struct file *, int, struct file_lock *);
1026		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1027		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1028		int (*setlease)(struct file *, long, struct file_lock **, void **);
1029		long (*fallocate)(struct file *file, int mode, loff_t offset,
1030				  loff_t len);
1031		void (*show_fdinfo)(struct seq_file *m, struct file *f);
1032	#ifndef CONFIG_MMU
1033		unsigned (*mmap_capabilities)(struct file *);
1034	#endif
1035		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1036		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1037					   struct file *file_out, loff_t pos_out,
1038					   loff_t len, unsigned int remap_flags);
1039		int (*fadvise)(struct file *, loff_t, loff_t, int);
1040	};
1041
1042Again, all methods are called without any locks being held, unless
1043otherwise noted.
1044
1045``llseek``
1046	called when the VFS needs to move the file position index
1047
1048``read``
1049	called by read(2) and related system calls
1050
1051``read_iter``
1052	possibly asynchronous read with iov_iter as destination
1053
1054``write``
1055	called by write(2) and related system calls
1056
1057``write_iter``
1058	possibly asynchronous write with iov_iter as source
1059
1060``iopoll``
1061	called when aio wants to poll for completions on HIPRI iocbs
1062
1063``iterate``
1064	called when the VFS needs to read the directory contents
1065
1066``iterate_shared``
1067	called when the VFS needs to read the directory contents when
1068	filesystem supports concurrent dir iterators
1069
1070``poll``
1071	called by the VFS when a process wants to check if there is
1072	activity on this file and (optionally) go to sleep until there
1073	is activity.  Called by the select(2) and poll(2) system calls
1074
1075``unlocked_ioctl``
1076	called by the ioctl(2) system call.
1077
1078``compat_ioctl``
1079	called by the ioctl(2) system call when 32 bit system calls are
1080	 used on 64 bit kernels.
1081
1082``mmap``
1083	called by the mmap(2) system call
1084
1085``open``
1086	called by the VFS when an inode should be opened.  When the VFS
1087	opens a file, it creates a new "struct file".  It then calls the
1088	open method for the newly allocated file structure.  You might
1089	think that the open method really belongs in "struct
1090	inode_operations", and you may be right.  I think it's done the
1091	way it is because it makes filesystems simpler to implement.
1092	The open() method is a good place to initialize the
1093	"private_data" member in the file structure if you want to point
1094	to a device structure
1095
1096``flush``
1097	called by the close(2) system call to flush a file
1098
1099``release``
1100	called when the last reference to an open file is closed
1101
1102``fsync``
1103	called by the fsync(2) system call.  Also see the section above
1104	entitled "Handling errors during writeback".
1105
1106``fasync``
1107	called by the fcntl(2) system call when asynchronous
1108	(non-blocking) mode is enabled for a file
1109
1110``lock``
1111	called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1112	F_SETLKW commands
1113
1114``get_unmapped_area``
1115	called by the mmap(2) system call
1116
1117``check_flags``
1118	called by the fcntl(2) system call for F_SETFL command
1119
1120``flock``
1121	called by the flock(2) system call
1122
1123``splice_write``
1124	called by the VFS to splice data from a pipe to a file.  This
1125	method is used by the splice(2) system call
1126
1127``splice_read``
1128	called by the VFS to splice data from file to a pipe.  This
1129	method is used by the splice(2) system call
1130
1131``setlease``
1132	called by the VFS to set or release a file lock lease.  setlease
1133	implementations should call generic_setlease to record or remove
1134	the lease in the inode after setting it.
1135
1136``fallocate``
1137	called by the VFS to preallocate blocks or punch a hole.
1138
1139``copy_file_range``
1140	called by the copy_file_range(2) system call.
1141
1142``remap_file_range``
1143	called by the ioctl(2) system call for FICLONERANGE and FICLONE
1144	and FIDEDUPERANGE commands to remap file ranges.  An
1145	implementation should remap len bytes at pos_in of the source
1146	file into the dest file at pos_out.  Implementations must handle
1147	callers passing in len == 0; this means "remap to the end of the
1148	source file".  The return value should the number of bytes
1149	remapped, or the usual negative error code if errors occurred
1150	before any bytes were remapped.  The remap_flags parameter
1151	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
1152	implementation must only remap if the requested file ranges have
1153	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
1154	ok with the implementation shortening the request length to
1155	satisfy alignment or EOF requirements (or any other reason).
1156
1157``fadvise``
1158	possibly called by the fadvise64() system call.
1159
1160Note that the file operations are implemented by the specific
1161filesystem in which the inode resides.  When opening a device node
1162(character or block special) most filesystems will call special
1163support routines in the VFS which will locate the required device
1164driver information.  These support routines replace the filesystem file
1165operations with those for the device driver, and then proceed to call
1166the new open() method for the file.  This is how opening a device file
1167in the filesystem eventually ends up calling the device driver open()
1168method.
1169
1170
1171Directory Entry Cache (dcache)
1172==============================
1173
1174
1175struct dentry_operations
1176------------------------
1177
1178This describes how a filesystem can overload the standard dentry
1179operations.  Dentries and the dcache are the domain of the VFS and the
1180individual filesystem implementations.  Device drivers have no business
1181here.  These methods may be set to NULL, as they are either optional or
1182the VFS uses a default.  As of kernel 2.6.22, the following members are
1183defined:
1184
1185.. code-block:: c
1186
1187	struct dentry_operations {
1188		int (*d_revalidate)(struct dentry *, unsigned int);
1189		int (*d_weak_revalidate)(struct dentry *, unsigned int);
1190		int (*d_hash)(const struct dentry *, struct qstr *);
1191		int (*d_compare)(const struct dentry *,
1192				 unsigned int, const char *, const struct qstr *);
1193		int (*d_delete)(const struct dentry *);
1194		int (*d_init)(struct dentry *);
1195		void (*d_release)(struct dentry *);
1196		void (*d_iput)(struct dentry *, struct inode *);
1197		char *(*d_dname)(struct dentry *, char *, int);
1198		struct vfsmount *(*d_automount)(struct path *);
1199		int (*d_manage)(const struct path *, bool);
1200		struct dentry *(*d_real)(struct dentry *, const struct inode *);
1201	};
1202
1203``d_revalidate``
1204	called when the VFS needs to revalidate a dentry.  This is
1205	called whenever a name look-up finds a dentry in the dcache.
1206	Most local filesystems leave this as NULL, because all their
1207	dentries in the dcache are valid.  Network filesystems are
1208	different since things can change on the server without the
1209	client necessarily being aware of it.
1210
1211	This function should return a positive value if the dentry is
1212	still valid, and zero or a negative error code if it isn't.
1213
1214	d_revalidate may be called in rcu-walk mode (flags &
1215	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
1216	revalidate the dentry without blocking or storing to the dentry,
1217	d_parent and d_inode should not be used without care (because
1218	they can change and, in d_inode case, even become NULL under
1219	us).
1220
1221	If a situation is encountered that rcu-walk cannot handle,
1222	return
1223	-ECHILD and it will be called again in ref-walk mode.
1224
1225``_weak_revalidate``
1226	called when the VFS needs to revalidate a "jumped" dentry.  This
1227	is called when a path-walk ends at dentry that was not acquired
1228	by doing a lookup in the parent directory.  This includes "/",
1229	"." and "..", as well as procfs-style symlinks and mountpoint
1230	traversal.
1231
1232	In this case, we are less concerned with whether the dentry is
1233	still fully correct, but rather that the inode is still valid.
1234	As with d_revalidate, most local filesystems will set this to
1235	NULL since their dcache entries are always valid.
1236
1237	This function has the same return code semantics as
1238	d_revalidate.
1239
1240	d_weak_revalidate is only called after leaving rcu-walk mode.
1241
1242``d_hash``
1243	called when the VFS adds a dentry to the hash table.  The first
1244	dentry passed to d_hash is the parent directory that the name is
1245	to be hashed into.
1246
1247	Same locking and synchronisation rules as d_compare regarding
1248	what is safe to dereference etc.
1249
1250``d_compare``
1251	called to compare a dentry name with a given name.  The first
1252	dentry is the parent of the dentry to be compared, the second is
1253	the child dentry.  len and name string are properties of the
1254	dentry to be compared.  qstr is the name to compare it with.
1255
1256	Must be constant and idempotent, and should not take locks if
1257	possible, and should not or store into the dentry.  Should not
1258	dereference pointers outside the dentry without lots of care
1259	(eg.  d_parent, d_inode, d_name should not be used).
1260
1261	However, our vfsmount is pinned, and RCU held, so the dentries
1262	and inodes won't disappear, neither will our sb or filesystem
1263	module.  ->d_sb may be used.
1264
1265	It is a tricky calling convention because it needs to be called
1266	under "rcu-walk", ie. without any locks or references on things.
1267
1268``d_delete``
1269	called when the last reference to a dentry is dropped and the
1270	dcache is deciding whether or not to cache it.  Return 1 to
1271	delete immediately, or 0 to cache the dentry.  Default is NULL
1272	which means to always cache a reachable dentry.  d_delete must
1273	be constant and idempotent.
1274
1275``d_init``
1276	called when a dentry is allocated
1277
1278``d_release``
1279	called when a dentry is really deallocated
1280
1281``d_iput``
1282	called when a dentry loses its inode (just prior to its being
1283	deallocated).  The default when this is NULL is that the VFS
1284	calls iput().  If you define this method, you must call iput()
1285	yourself
1286
1287``d_dname``
1288	called when the pathname of a dentry should be generated.
1289	Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1290	delay pathname generation.  (Instead of doing it when dentry is
1291	created, it's done only when the path is needed.).  Real
1292	filesystems probably dont want to use it, because their dentries
1293	are present in global dcache hash, so their hash should be an
1294	invariant.  As no lock is held, d_dname() should not try to
1295	modify the dentry itself, unless appropriate SMP safety is used.
1296	CAUTION : d_path() logic is quite tricky.  The correct way to
1297	return for example "Hello" is to put it at the end of the
1298	buffer, and returns a pointer to the first char.
1299	dynamic_dname() helper function is provided to take care of
1300	this.
1301
1302	Example :
1303
1304.. code-block:: c
1305
1306	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1307	{
1308		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1309				dentry->d_inode->i_ino);
1310	}
1311
1312``d_automount``
1313	called when an automount dentry is to be traversed (optional).
1314	This should create a new VFS mount record and return the record
1315	to the caller.  The caller is supplied with a path parameter
1316	giving the automount directory to describe the automount target
1317	and the parent VFS mount record to provide inheritable mount
1318	parameters.  NULL should be returned if someone else managed to
1319	make the automount first.  If the vfsmount creation failed, then
1320	an error code should be returned.  If -EISDIR is returned, then
1321	the directory will be treated as an ordinary directory and
1322	returned to pathwalk to continue walking.
1323
1324	If a vfsmount is returned, the caller will attempt to mount it
1325	on the mountpoint and will remove the vfsmount from its
1326	expiration list in the case of failure.  The vfsmount should be
1327	returned with 2 refs on it to prevent automatic expiration - the
1328	caller will clean up the additional ref.
1329
1330	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1331	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is
1332	set on the inode being added.
1333
1334``d_manage``
1335	called to allow the filesystem to manage the transition from a
1336	dentry (optional).  This allows autofs, for example, to hold up
1337	clients waiting to explore behind a 'mountpoint' while letting
1338	the daemon go past and construct the subtree there.  0 should be
1339	returned to let the calling process continue.  -EISDIR can be
1340	returned to tell pathwalk to use this directory as an ordinary
1341	directory and to ignore anything mounted on it and not to check
1342	the automount flag.  Any other error code will abort pathwalk
1343	completely.
1344
1345	If the 'rcu_walk' parameter is true, then the caller is doing a
1346	pathwalk in RCU-walk mode.  Sleeping is not permitted in this
1347	mode, and the caller can be asked to leave it and call again by
1348	returning -ECHILD.  -EISDIR may also be returned to tell
1349	pathwalk to ignore d_automount or any mounts.
1350
1351	This function is only used if DCACHE_MANAGE_TRANSIT is set on
1352	the dentry being transited from.
1353
1354``d_real``
1355	overlay/union type filesystems implement this method to return
1356	one of the underlying dentries hidden by the overlay.  It is
1357	used in two different modes:
1358
1359	Called from file_dentry() it returns the real dentry matching
1360	the inode argument.  The real dentry may be from a lower layer
1361	already copied up, but still referenced from the file.  This
1362	mode is selected with a non-NULL inode argument.
1363
1364	With NULL inode the topmost real underlying dentry is returned.
1365
1366Each dentry has a pointer to its parent dentry, as well as a hash list
1367of child dentries.  Child dentries are basically like files in a
1368directory.
1369
1370
1371Directory Entry Cache API
1372--------------------------
1373
1374There are a number of functions defined which permit a filesystem to
1375manipulate dentries:
1376
1377``dget``
1378	open a new handle for an existing dentry (this just increments
1379	the usage count)
1380
1381``dput``
1382	close a handle for a dentry (decrements the usage count).  If
1383	the usage count drops to 0, and the dentry is still in its
1384	parent's hash, the "d_delete" method is called to check whether
1385	it should be cached.  If it should not be cached, or if the
1386	dentry is not hashed, it is deleted.  Otherwise cached dentries
1387	are put into an LRU list to be reclaimed on memory shortage.
1388
1389``d_drop``
1390	this unhashes a dentry from its parents hash list.  A subsequent
1391	call to dput() will deallocate the dentry if its usage count
1392	drops to 0
1393
1394``d_delete``
1395	delete a dentry.  If there are no other open references to the
1396	dentry then the dentry is turned into a negative dentry (the
1397	d_iput() method is called).  If there are other references, then
1398	d_drop() is called instead
1399
1400``d_add``
1401	add a dentry to its parents hash list and then calls
1402	d_instantiate()
1403
1404``d_instantiate``
1405	add a dentry to the alias hash list for the inode and updates
1406	the "d_inode" member.  The "i_count" member in the inode
1407	structure should be set/incremented.  If the inode pointer is
1408	NULL, the dentry is called a "negative dentry".  This function
1409	is commonly called when an inode is created for an existing
1410	negative dentry
1411
1412``d_lookup``
1413	look up a dentry given its parent and path name component It
1414	looks up the child of that given name from the dcache hash
1415	table.  If it is found, the reference count is incremented and
1416	the dentry is returned.  The caller must use dput() to free the
1417	dentry when it finishes using it.
1418
1419
1420Mount Options
1421=============
1422
1423
1424Parsing options
1425---------------
1426
1427On mount and remount the filesystem is passed a string containing a
1428comma separated list of mount options.  The options can have either of
1429these forms:
1430
1431  option
1432  option=value
1433
1434The <linux/parser.h> header defines an API that helps parse these
1435options.  There are plenty of examples on how to use it in existing
1436filesystems.
1437
1438
1439Showing options
1440---------------
1441
1442If a filesystem accepts mount options, it must define show_options() to
1443show all the currently active options.  The rules are:
1444
1445  - options MUST be shown which are not default or their values differ
1446    from the default
1447
1448  - options MAY be shown which are enabled by default or have their
1449    default value
1450
1451Options used only internally between a mount helper and the kernel (such
1452as file descriptors), or which only have an effect during the mounting
1453(such as ones controlling the creation of a journal) are exempt from the
1454above rules.
1455
1456The underlying reason for the above rules is to make sure, that a mount
1457can be accurately replicated (e.g. umounting and mounting again) based
1458on the information found in /proc/mounts.
1459
1460
1461Resources
1462=========
1463
1464(Note some of these resources are not up-to-date with the latest kernel
1465 version.)
1466
1467Creating Linux virtual filesystems. 2002
1468    <https://lwn.net/Articles/13325/>
1469
1470The Linux Virtual File-system Layer by Neil Brown. 1999
1471    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1472
1473A tour of the Linux VFS by Michael K. Johnson. 1996
1474    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1475
1476A small trail through the Linux kernel by Andries Brouwer. 2001
1477    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>
1478