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 ->releasepage on clean pages with PagePrivate set.
624Clean pages without PagePrivate and with no external references will be
625released 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 'readpage'.  The write
660process is more complicated and uses write_begin/write_end or
661set_page_dirty 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 (*readpage)(struct file *, struct page *);
726		int (*writepages)(struct address_space *, struct writeback_control *);
727		int (*set_page_dirty)(struct page *page);
728		void (*readahead)(struct readahead_control *);
729		int (*readpages)(struct file *filp, struct address_space *mapping,
730				 struct list_head *pages, unsigned nr_pages);
731		int (*write_begin)(struct file *, struct address_space *mapping,
732				   loff_t pos, unsigned len, unsigned flags,
733				struct page **pagep, void **fsdata);
734		int (*write_end)(struct file *, struct address_space *mapping,
735				 loff_t pos, unsigned len, unsigned copied,
736				 struct page *page, void *fsdata);
737		sector_t (*bmap)(struct address_space *, sector_t);
738		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
739		int (*releasepage) (struct page *, int);
740		void (*freepage)(struct page *);
741		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
742		/* isolate a page for migration */
743		bool (*isolate_page) (struct page *, isolate_mode_t);
744		/* migrate the contents of a page to the specified target */
745		int (*migratepage) (struct page *, struct page *);
746		/* put migration-failed page back to right list */
747		void (*putback_page) (struct page *);
748		int (*launder_page) (struct page *);
749
750		int (*is_partially_uptodate) (struct page *, unsigned long,
751					      unsigned long);
752		void (*is_dirty_writeback) (struct page *, bool *, bool *);
753		int (*error_remove_page) (struct mapping *mapping, struct page *page);
754		int (*swap_activate)(struct file *);
755		int (*swap_deactivate)(struct file *);
756	};
757
758``writepage``
759	called by the VM to write a dirty page to backing store.  This
760	may happen for data integrity reasons (i.e. 'sync'), or to free
761	up memory (flush).  The difference can be seen in
762	wbc->sync_mode.  The PG_Dirty flag has been cleared and
763	PageLocked is true.  writepage should start writeout, should set
764	PG_Writeback, and should make sure the page is unlocked, either
765	synchronously or asynchronously when the write operation
766	completes.
767
768	If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
769	try too hard if there are problems, and may choose to write out
770	other pages from the mapping if that is easier (e.g. due to
771	internal dependencies).  If it chooses not to start writeout, it
772	should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
773	keep calling ->writepage on that page.
774
775	See the file "Locking" for more details.
776
777``readpage``
778	called by the VM to read a page from backing store.  The page
779	will be Locked when readpage is called, and should be unlocked
780	and marked uptodate once the read completes.  If ->readpage
781	discovers that it needs to unlock the page for some reason, it
782	can do so, and then return AOP_TRUNCATED_PAGE.  In this case,
783	the page will be relocated, relocked and if that all succeeds,
784	->readpage will be called again.
785
786``writepages``
787	called by the VM to write out pages associated with the
788	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
789	the writeback_control will specify a range of pages that must be
790	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
791	given and that many pages should be written if possible.  If no
792	->writepages is given, then mpage_writepages is used instead.
793	This will choose pages from the address space that are tagged as
794	DIRTY and will pass them to ->writepage.
795
796``set_page_dirty``
797	called by the VM to set a page dirty.  This is particularly
798	needed if an address space attaches private data to a page, and
799	that data needs to be updated when a page is dirtied.  This is
800	called, for example, when a memory mapped page gets modified.
801	If defined, it should set the PageDirty flag, and the
802	PAGECACHE_TAG_DIRTY tag in the radix tree.
803
804``readahead``
805	Called by the VM to read pages associated with the address_space
806	object.  The pages are consecutive in the page cache and are
807	locked.  The implementation should decrement the page refcount
808	after starting I/O on each page.  Usually the page will be
809	unlocked by the I/O completion handler.  If the filesystem decides
810	to stop attempting I/O before reaching the end of the readahead
811	window, it can simply return.  The caller will decrement the page
812	refcount and unlock the remaining pages for you.  Set PageUptodate
813	if the I/O completes successfully.  Setting PageError on any page
814	will be ignored; simply unlock the page if an I/O error occurs.
815
816``readpages``
817	called by the VM to read pages associated with the address_space
818	object.  This is essentially just a vector version of readpage.
819	Instead of just one page, several pages are requested.
820	readpages is only used for read-ahead, so read errors are
821	ignored.  If anything goes wrong, feel free to give up.
822	This interface is deprecated and will be removed by the end of
823	2020; implement readahead instead.
824
825``write_begin``
826	Called by the generic buffered write code to ask the filesystem
827	to prepare to write len bytes at the given offset in the file.
828	The address_space should check that the write will be able to
829	complete, by allocating space if necessary and doing any other
830	internal housekeeping.  If the write will update parts of any
831	basic-blocks on storage, then those blocks should be pre-read
832	(if they haven't been read already) so that the updated blocks
833	can be written out properly.
834
835	The filesystem must return the locked pagecache page for the
836	specified offset, in ``*pagep``, for the caller to write into.
837
838	It must be able to cope with short writes (where the length
839	passed to write_begin is greater than the number of bytes copied
840	into the page).
841
842	flags is a field for AOP_FLAG_xxx flags, described in
843	include/linux/fs.h.
844
845	A void * may be returned in fsdata, which then gets passed into
846	write_end.
847
848	Returns 0 on success; < 0 on failure (which is the error code),
849	in which case write_end is not called.
850
851``write_end``
852	After a successful write_begin, and data copy, write_end must be
853	called.  len is the original len passed to write_begin, and
854	copied is the amount that was able to be copied.
855
856	The filesystem must take care of unlocking the page and
857	releasing it refcount, and updating i_size.
858
859	Returns < 0 on failure, otherwise the number of bytes (<=
860	'copied') that were able to be copied into pagecache.
861
862``bmap``
863	called by the VFS to map a logical block offset within object to
864	physical block number.  This method is used by the FIBMAP ioctl
865	and for working with swap-files.  To be able to swap to a file,
866	the file must have a stable mapping to a block device.  The swap
867	system does not go through the filesystem but instead uses bmap
868	to find out where the blocks in the file are and uses those
869	addresses directly.
870
871``invalidatepage``
872	If a page has PagePrivate set, then invalidatepage will be
873	called when part or all of the page is to be removed from the
874	address space.  This generally corresponds to either a
875	truncation, punch hole or a complete invalidation of the address
876	space (in the latter case 'offset' will always be 0 and 'length'
877	will be PAGE_SIZE).  Any private data associated with the page
878	should be updated to reflect this truncation.  If offset is 0
879	and length is PAGE_SIZE, then the private data should be
880	released, because the page must be able to be completely
881	discarded.  This may be done by calling the ->releasepage
882	function, but in this case the release MUST succeed.
883
884``releasepage``
885	releasepage is called on PagePrivate pages to indicate that the
886	page should be freed if possible.  ->releasepage should remove
887	any private data from the page and clear the PagePrivate flag.
888	If releasepage() fails for some reason, it must indicate failure
889	with a 0 return value.  releasepage() is used in two distinct
890	though related cases.  The first is when the VM finds a clean
891	page with no active users and wants to make it a free page.  If
892	->releasepage succeeds, the page will be removed from the
893	address_space and become free.
894
895	The second case is when a request has been made to invalidate
896	some or all pages in an address_space.  This can happen through
897	the fadvise(POSIX_FADV_DONTNEED) system call or by the
898	filesystem explicitly requesting it as nfs and 9fs do (when they
899	believe the cache may be out of date with storage) by calling
900	invalidate_inode_pages2().  If the filesystem makes such a call,
901	and needs to be certain that all pages are invalidated, then its
902	releasepage will need to ensure this.  Possibly it can clear the
903	PageUptodate bit if it cannot free private data yet.
904
905``freepage``
906	freepage is called once the page is no longer visible in the
907	page cache in order to allow the cleanup of any private data.
908	Since it may be called by the memory reclaimer, it should not
909	assume that the original address_space mapping still exists, and
910	it should not block.
911
912``direct_IO``
913	called by the generic read/write routines to perform direct_IO -
914	that is IO requests which bypass the page cache and transfer
915	data directly between the storage and the application's address
916	space.
917
918``isolate_page``
919	Called by the VM when isolating a movable non-lru page.  If page
920	is successfully isolated, VM marks the page as PG_isolated via
921	__SetPageIsolated.
922
923``migrate_page``
924	This is used to compact the physical memory usage.  If the VM
925	wants to relocate a page (maybe off a memory card that is
926	signalling imminent failure) it will pass a new page and an old
927	page to this function.  migrate_page should transfer any private
928	data across and update any references that it has to the page.
929
930``putback_page``
931	Called by the VM when isolated page's migration fails.
932
933``launder_page``
934	Called before freeing a page - it writes back the dirty page.
935	To prevent redirtying the page, it is kept locked during the
936	whole operation.
937
938``is_partially_uptodate``
939	Called by the VM when reading a file through the pagecache when
940	the underlying blocksize != pagesize.  If the required block is
941	up to date then the read can complete without needing the IO to
942	bring the whole page up to date.
943
944``is_dirty_writeback``
945	Called by the VM when attempting to reclaim a page.  The VM uses
946	dirty and writeback information to determine if it needs to
947	stall to allow flushers a chance to complete some IO.
948	Ordinarily it can use PageDirty and PageWriteback but some
949	filesystems have more complex state (unstable pages in NFS
950	prevent reclaim) or do not set those flags due to locking
951	problems.  This callback allows a filesystem to indicate to the
952	VM if a page should be treated as dirty or writeback for the
953	purposes of stalling.
954
955``error_remove_page``
956	normally set to generic_error_remove_page if truncation is ok
957	for this address space.  Used for memory failure handling.
958	Setting this implies you deal with pages going away under you,
959	unless you have them locked or reference counts increased.
960
961``swap_activate``
962	Called when swapon is used on a file to allocate space if
963	necessary and pin the block lookup information in memory.  A
964	return value of zero indicates success, in which case this file
965	can be used to back swapspace.
966
967``swap_deactivate``
968	Called during swapoff on files where swap_activate was
969	successful.
970
971
972The File Object
973===============
974
975A file object represents a file opened by a process.  This is also known
976as an "open file description" in POSIX parlance.
977
978
979struct file_operations
980----------------------
981
982This describes how the VFS can manipulate an open file.  As of kernel
9834.18, the following members are defined:
984
985.. code-block:: c
986
987	struct file_operations {
988		struct module *owner;
989		loff_t (*llseek) (struct file *, loff_t, int);
990		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
991		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
992		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
993		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
994		int (*iopoll)(struct kiocb *kiocb, bool spin);
995		int (*iterate) (struct file *, struct dir_context *);
996		int (*iterate_shared) (struct file *, struct dir_context *);
997		__poll_t (*poll) (struct file *, struct poll_table_struct *);
998		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
999		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1000		int (*mmap) (struct file *, struct vm_area_struct *);
1001		int (*open) (struct inode *, struct file *);
1002		int (*flush) (struct file *, fl_owner_t id);
1003		int (*release) (struct inode *, struct file *);
1004		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1005		int (*fasync) (int, struct file *, int);
1006		int (*lock) (struct file *, int, struct file_lock *);
1007		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1008		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1009		int (*check_flags)(int);
1010		int (*flock) (struct file *, int, struct file_lock *);
1011		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1012		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1013		int (*setlease)(struct file *, long, struct file_lock **, void **);
1014		long (*fallocate)(struct file *file, int mode, loff_t offset,
1015				  loff_t len);
1016		void (*show_fdinfo)(struct seq_file *m, struct file *f);
1017	#ifndef CONFIG_MMU
1018		unsigned (*mmap_capabilities)(struct file *);
1019	#endif
1020		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1021		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1022					   struct file *file_out, loff_t pos_out,
1023					   loff_t len, unsigned int remap_flags);
1024		int (*fadvise)(struct file *, loff_t, loff_t, int);
1025	};
1026
1027Again, all methods are called without any locks being held, unless
1028otherwise noted.
1029
1030``llseek``
1031	called when the VFS needs to move the file position index
1032
1033``read``
1034	called by read(2) and related system calls
1035
1036``read_iter``
1037	possibly asynchronous read with iov_iter as destination
1038
1039``write``
1040	called by write(2) and related system calls
1041
1042``write_iter``
1043	possibly asynchronous write with iov_iter as source
1044
1045``iopoll``
1046	called when aio wants to poll for completions on HIPRI iocbs
1047
1048``iterate``
1049	called when the VFS needs to read the directory contents
1050
1051``iterate_shared``
1052	called when the VFS needs to read the directory contents when
1053	filesystem supports concurrent dir iterators
1054
1055``poll``
1056	called by the VFS when a process wants to check if there is
1057	activity on this file and (optionally) go to sleep until there
1058	is activity.  Called by the select(2) and poll(2) system calls
1059
1060``unlocked_ioctl``
1061	called by the ioctl(2) system call.
1062
1063``compat_ioctl``
1064	called by the ioctl(2) system call when 32 bit system calls are
1065	 used on 64 bit kernels.
1066
1067``mmap``
1068	called by the mmap(2) system call
1069
1070``open``
1071	called by the VFS when an inode should be opened.  When the VFS
1072	opens a file, it creates a new "struct file".  It then calls the
1073	open method for the newly allocated file structure.  You might
1074	think that the open method really belongs in "struct
1075	inode_operations", and you may be right.  I think it's done the
1076	way it is because it makes filesystems simpler to implement.
1077	The open() method is a good place to initialize the
1078	"private_data" member in the file structure if you want to point
1079	to a device structure
1080
1081``flush``
1082	called by the close(2) system call to flush a file
1083
1084``release``
1085	called when the last reference to an open file is closed
1086
1087``fsync``
1088	called by the fsync(2) system call.  Also see the section above
1089	entitled "Handling errors during writeback".
1090
1091``fasync``
1092	called by the fcntl(2) system call when asynchronous
1093	(non-blocking) mode is enabled for a file
1094
1095``lock``
1096	called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1097	F_SETLKW commands
1098
1099``get_unmapped_area``
1100	called by the mmap(2) system call
1101
1102``check_flags``
1103	called by the fcntl(2) system call for F_SETFL command
1104
1105``flock``
1106	called by the flock(2) system call
1107
1108``splice_write``
1109	called by the VFS to splice data from a pipe to a file.  This
1110	method is used by the splice(2) system call
1111
1112``splice_read``
1113	called by the VFS to splice data from file to a pipe.  This
1114	method is used by the splice(2) system call
1115
1116``setlease``
1117	called by the VFS to set or release a file lock lease.  setlease
1118	implementations should call generic_setlease to record or remove
1119	the lease in the inode after setting it.
1120
1121``fallocate``
1122	called by the VFS to preallocate blocks or punch a hole.
1123
1124``copy_file_range``
1125	called by the copy_file_range(2) system call.
1126
1127``remap_file_range``
1128	called by the ioctl(2) system call for FICLONERANGE and FICLONE
1129	and FIDEDUPERANGE commands to remap file ranges.  An
1130	implementation should remap len bytes at pos_in of the source
1131	file into the dest file at pos_out.  Implementations must handle
1132	callers passing in len == 0; this means "remap to the end of the
1133	source file".  The return value should the number of bytes
1134	remapped, or the usual negative error code if errors occurred
1135	before any bytes were remapped.  The remap_flags parameter
1136	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
1137	implementation must only remap if the requested file ranges have
1138	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
1139	ok with the implementation shortening the request length to
1140	satisfy alignment or EOF requirements (or any other reason).
1141
1142``fadvise``
1143	possibly called by the fadvise64() system call.
1144
1145Note that the file operations are implemented by the specific
1146filesystem in which the inode resides.  When opening a device node
1147(character or block special) most filesystems will call special
1148support routines in the VFS which will locate the required device
1149driver information.  These support routines replace the filesystem file
1150operations with those for the device driver, and then proceed to call
1151the new open() method for the file.  This is how opening a device file
1152in the filesystem eventually ends up calling the device driver open()
1153method.
1154
1155
1156Directory Entry Cache (dcache)
1157==============================
1158
1159
1160struct dentry_operations
1161------------------------
1162
1163This describes how a filesystem can overload the standard dentry
1164operations.  Dentries and the dcache are the domain of the VFS and the
1165individual filesystem implementations.  Device drivers have no business
1166here.  These methods may be set to NULL, as they are either optional or
1167the VFS uses a default.  As of kernel 2.6.22, the following members are
1168defined:
1169
1170.. code-block:: c
1171
1172	struct dentry_operations {
1173		int (*d_revalidate)(struct dentry *, unsigned int);
1174		int (*d_weak_revalidate)(struct dentry *, unsigned int);
1175		int (*d_hash)(const struct dentry *, struct qstr *);
1176		int (*d_compare)(const struct dentry *,
1177				 unsigned int, const char *, const struct qstr *);
1178		int (*d_delete)(const struct dentry *);
1179		int (*d_init)(struct dentry *);
1180		void (*d_release)(struct dentry *);
1181		void (*d_iput)(struct dentry *, struct inode *);
1182		char *(*d_dname)(struct dentry *, char *, int);
1183		struct vfsmount *(*d_automount)(struct path *);
1184		int (*d_manage)(const struct path *, bool);
1185		struct dentry *(*d_real)(struct dentry *, const struct inode *);
1186	};
1187
1188``d_revalidate``
1189	called when the VFS needs to revalidate a dentry.  This is
1190	called whenever a name look-up finds a dentry in the dcache.
1191	Most local filesystems leave this as NULL, because all their
1192	dentries in the dcache are valid.  Network filesystems are
1193	different since things can change on the server without the
1194	client necessarily being aware of it.
1195
1196	This function should return a positive value if the dentry is
1197	still valid, and zero or a negative error code if it isn't.
1198
1199	d_revalidate may be called in rcu-walk mode (flags &
1200	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
1201	revalidate the dentry without blocking or storing to the dentry,
1202	d_parent and d_inode should not be used without care (because
1203	they can change and, in d_inode case, even become NULL under
1204	us).
1205
1206	If a situation is encountered that rcu-walk cannot handle,
1207	return
1208	-ECHILD and it will be called again in ref-walk mode.
1209
1210``_weak_revalidate``
1211	called when the VFS needs to revalidate a "jumped" dentry.  This
1212	is called when a path-walk ends at dentry that was not acquired
1213	by doing a lookup in the parent directory.  This includes "/",
1214	"." and "..", as well as procfs-style symlinks and mountpoint
1215	traversal.
1216
1217	In this case, we are less concerned with whether the dentry is
1218	still fully correct, but rather that the inode is still valid.
1219	As with d_revalidate, most local filesystems will set this to
1220	NULL since their dcache entries are always valid.
1221
1222	This function has the same return code semantics as
1223	d_revalidate.
1224
1225	d_weak_revalidate is only called after leaving rcu-walk mode.
1226
1227``d_hash``
1228	called when the VFS adds a dentry to the hash table.  The first
1229	dentry passed to d_hash is the parent directory that the name is
1230	to be hashed into.
1231
1232	Same locking and synchronisation rules as d_compare regarding
1233	what is safe to dereference etc.
1234
1235``d_compare``
1236	called to compare a dentry name with a given name.  The first
1237	dentry is the parent of the dentry to be compared, the second is
1238	the child dentry.  len and name string are properties of the
1239	dentry to be compared.  qstr is the name to compare it with.
1240
1241	Must be constant and idempotent, and should not take locks if
1242	possible, and should not or store into the dentry.  Should not
1243	dereference pointers outside the dentry without lots of care
1244	(eg.  d_parent, d_inode, d_name should not be used).
1245
1246	However, our vfsmount is pinned, and RCU held, so the dentries
1247	and inodes won't disappear, neither will our sb or filesystem
1248	module.  ->d_sb may be used.
1249
1250	It is a tricky calling convention because it needs to be called
1251	under "rcu-walk", ie. without any locks or references on things.
1252
1253``d_delete``
1254	called when the last reference to a dentry is dropped and the
1255	dcache is deciding whether or not to cache it.  Return 1 to
1256	delete immediately, or 0 to cache the dentry.  Default is NULL
1257	which means to always cache a reachable dentry.  d_delete must
1258	be constant and idempotent.
1259
1260``d_init``
1261	called when a dentry is allocated
1262
1263``d_release``
1264	called when a dentry is really deallocated
1265
1266``d_iput``
1267	called when a dentry loses its inode (just prior to its being
1268	deallocated).  The default when this is NULL is that the VFS
1269	calls iput().  If you define this method, you must call iput()
1270	yourself
1271
1272``d_dname``
1273	called when the pathname of a dentry should be generated.
1274	Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1275	delay pathname generation.  (Instead of doing it when dentry is
1276	created, it's done only when the path is needed.).  Real
1277	filesystems probably dont want to use it, because their dentries
1278	are present in global dcache hash, so their hash should be an
1279	invariant.  As no lock is held, d_dname() should not try to
1280	modify the dentry itself, unless appropriate SMP safety is used.
1281	CAUTION : d_path() logic is quite tricky.  The correct way to
1282	return for example "Hello" is to put it at the end of the
1283	buffer, and returns a pointer to the first char.
1284	dynamic_dname() helper function is provided to take care of
1285	this.
1286
1287	Example :
1288
1289.. code-block:: c
1290
1291	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1292	{
1293		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1294				dentry->d_inode->i_ino);
1295	}
1296
1297``d_automount``
1298	called when an automount dentry is to be traversed (optional).
1299	This should create a new VFS mount record and return the record
1300	to the caller.  The caller is supplied with a path parameter
1301	giving the automount directory to describe the automount target
1302	and the parent VFS mount record to provide inheritable mount
1303	parameters.  NULL should be returned if someone else managed to
1304	make the automount first.  If the vfsmount creation failed, then
1305	an error code should be returned.  If -EISDIR is returned, then
1306	the directory will be treated as an ordinary directory and
1307	returned to pathwalk to continue walking.
1308
1309	If a vfsmount is returned, the caller will attempt to mount it
1310	on the mountpoint and will remove the vfsmount from its
1311	expiration list in the case of failure.  The vfsmount should be
1312	returned with 2 refs on it to prevent automatic expiration - the
1313	caller will clean up the additional ref.
1314
1315	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1316	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is
1317	set on the inode being added.
1318
1319``d_manage``
1320	called to allow the filesystem to manage the transition from a
1321	dentry (optional).  This allows autofs, for example, to hold up
1322	clients waiting to explore behind a 'mountpoint' while letting
1323	the daemon go past and construct the subtree there.  0 should be
1324	returned to let the calling process continue.  -EISDIR can be
1325	returned to tell pathwalk to use this directory as an ordinary
1326	directory and to ignore anything mounted on it and not to check
1327	the automount flag.  Any other error code will abort pathwalk
1328	completely.
1329
1330	If the 'rcu_walk' parameter is true, then the caller is doing a
1331	pathwalk in RCU-walk mode.  Sleeping is not permitted in this
1332	mode, and the caller can be asked to leave it and call again by
1333	returning -ECHILD.  -EISDIR may also be returned to tell
1334	pathwalk to ignore d_automount or any mounts.
1335
1336	This function is only used if DCACHE_MANAGE_TRANSIT is set on
1337	the dentry being transited from.
1338
1339``d_real``
1340	overlay/union type filesystems implement this method to return
1341	one of the underlying dentries hidden by the overlay.  It is
1342	used in two different modes:
1343
1344	Called from file_dentry() it returns the real dentry matching
1345	the inode argument.  The real dentry may be from a lower layer
1346	already copied up, but still referenced from the file.  This
1347	mode is selected with a non-NULL inode argument.
1348
1349	With NULL inode the topmost real underlying dentry is returned.
1350
1351Each dentry has a pointer to its parent dentry, as well as a hash list
1352of child dentries.  Child dentries are basically like files in a
1353directory.
1354
1355
1356Directory Entry Cache API
1357--------------------------
1358
1359There are a number of functions defined which permit a filesystem to
1360manipulate dentries:
1361
1362``dget``
1363	open a new handle for an existing dentry (this just increments
1364	the usage count)
1365
1366``dput``
1367	close a handle for a dentry (decrements the usage count).  If
1368	the usage count drops to 0, and the dentry is still in its
1369	parent's hash, the "d_delete" method is called to check whether
1370	it should be cached.  If it should not be cached, or if the
1371	dentry is not hashed, it is deleted.  Otherwise cached dentries
1372	are put into an LRU list to be reclaimed on memory shortage.
1373
1374``d_drop``
1375	this unhashes a dentry from its parents hash list.  A subsequent
1376	call to dput() will deallocate the dentry if its usage count
1377	drops to 0
1378
1379``d_delete``
1380	delete a dentry.  If there are no other open references to the
1381	dentry then the dentry is turned into a negative dentry (the
1382	d_iput() method is called).  If there are other references, then
1383	d_drop() is called instead
1384
1385``d_add``
1386	add a dentry to its parents hash list and then calls
1387	d_instantiate()
1388
1389``d_instantiate``
1390	add a dentry to the alias hash list for the inode and updates
1391	the "d_inode" member.  The "i_count" member in the inode
1392	structure should be set/incremented.  If the inode pointer is
1393	NULL, the dentry is called a "negative dentry".  This function
1394	is commonly called when an inode is created for an existing
1395	negative dentry
1396
1397``d_lookup``
1398	look up a dentry given its parent and path name component It
1399	looks up the child of that given name from the dcache hash
1400	table.  If it is found, the reference count is incremented and
1401	the dentry is returned.  The caller must use dput() to free the
1402	dentry when it finishes using it.
1403
1404
1405Mount Options
1406=============
1407
1408
1409Parsing options
1410---------------
1411
1412On mount and remount the filesystem is passed a string containing a
1413comma separated list of mount options.  The options can have either of
1414these forms:
1415
1416  option
1417  option=value
1418
1419The <linux/parser.h> header defines an API that helps parse these
1420options.  There are plenty of examples on how to use it in existing
1421filesystems.
1422
1423
1424Showing options
1425---------------
1426
1427If a filesystem accepts mount options, it must define show_options() to
1428show all the currently active options.  The rules are:
1429
1430  - options MUST be shown which are not default or their values differ
1431    from the default
1432
1433  - options MAY be shown which are enabled by default or have their
1434    default value
1435
1436Options used only internally between a mount helper and the kernel (such
1437as file descriptors), or which only have an effect during the mounting
1438(such as ones controlling the creation of a journal) are exempt from the
1439above rules.
1440
1441The underlying reason for the above rules is to make sure, that a mount
1442can be accurately replicated (e.g. umounting and mounting again) based
1443on the information found in /proc/mounts.
1444
1445
1446Resources
1447=========
1448
1449(Note some of these resources are not up-to-date with the latest kernel
1450 version.)
1451
1452Creating Linux virtual filesystems. 2002
1453    <https://lwn.net/Articles/13325/>
1454
1455The Linux Virtual File-system Layer by Neil Brown. 1999
1456    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1457
1458A tour of the Linux VFS by Michael K. Johnson. 1996
1459    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1460
1461A small trail through the Linux kernel by Andries Brouwer. 2001
1462    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>
1463