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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

2		      Overview of the Linux Virtual File System
4		Original author: Richard Gooch <rgooch@atnf.csiro.au>
6			  Last updated on June 24, 2007.
8	  Copyright (C) 1999 Richard Gooch
9	  Copyright (C) 2005 Pekka Enberg
11	  This file is released under the GPLv2.
14	Introduction
15	============
17	The Virtual File System (also known as the Virtual Filesystem Switch)
18	is the software layer in the kernel that provides the filesystem
19	interface to userspace programs. It also provides an abstraction
20	within the kernel which allows different filesystem implementations to
21	coexist.
23	VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24	on are called from a process context. Filesystem locking is described
25	in the document Documentation/filesystems/Locking.
28	Directory Entry Cache (dcache)
29	------------------------------
31	The VFS implements the open(2), stat(2), chmod(2), and similar system
32	calls. The pathname argument that is passed to them is used by the VFS
33	to search through the directory entry cache (also known as the dentry
34	cache or dcache). This provides a very fast look-up mechanism to
35	translate a pathname (filename) into a specific dentry. Dentries live
36	in RAM and are never saved to disc: they exist only for performance.
38	The dentry cache is meant to be a view into your entire filespace. As
39	most computers cannot fit all dentries in the RAM at the same time,
40	some bits of the cache are missing. In order to resolve your pathname
41	into a dentry, the VFS may have to resort to creating dentries along
42	the way, and then loading the inode. This is done by looking up the
43	inode.
46	The Inode Object
47	----------------
49	An individual dentry usually has a pointer to an inode. Inodes are
50	filesystem objects such as regular files, directories, FIFOs and other
51	beasts.  They live either on the disc (for block device filesystems)
52	or in the memory (for pseudo filesystems). Inodes that live on the
53	disc are copied into the memory when required and changes to the inode
54	are written back to disc. A single inode can be pointed to by multiple
55	dentries (hard links, for example, do this).
57	To look up an inode requires that the VFS calls the lookup() method of
58	the parent directory inode. This method is installed by the specific
59	filesystem implementation that the inode lives in. Once the VFS has
60	the required dentry (and hence the inode), we can do all those boring
61	things like open(2) the file, or stat(2) it to peek at the inode
62	data. The stat(2) operation is fairly simple: once the VFS has the
63	dentry, it peeks at the inode data and passes some of it back to
64	userspace.
67	The File Object
68	---------------
70	Opening a file requires another operation: allocation of a file
71	structure (this is the kernel-side implementation of file
72	descriptors). The freshly allocated file structure is initialized with
73	a pointer to the dentry and a set of file operation member functions.
74	These are taken from the inode data. The open() file method is then
75	called so the specific filesystem implementation can do its work. You
76	can see that this is another switch performed by the VFS. The file
77	structure is placed into the file descriptor table for the process.
79	Reading, writing and closing files (and other assorted VFS operations)
80	is done by using the userspace file descriptor to grab the appropriate
81	file structure, and then calling the required file structure method to
82	do whatever is required. For as long as the file is open, it keeps the
83	dentry in use, which in turn means that the VFS inode is still in use.
86	Registering and Mounting a Filesystem
87	=====================================
89	To register and unregister a filesystem, use the following API
90	functions:
92	   #include <linux/fs.h>
94	   extern int register_filesystem(struct file_system_type *);
95	   extern int unregister_filesystem(struct file_system_type *);
97	The passed struct file_system_type describes your filesystem. When a
98	request is made to mount a filesystem onto a directory in your namespace,
99	the VFS will call the appropriate mount() method for the specific
100	filesystem.  New vfsmount referring to the tree returned by ->mount()
101	will be attached to the mountpoint, so that when pathname resolution
102	reaches the mountpoint it will jump into the root of that vfsmount.
104	You can see all filesystems that are registered to the kernel in the
105	file /proc/filesystems.
108	struct file_system_type
109	-----------------------
111	This describes the filesystem. As of kernel 2.6.39, the following
112	members are defined:
114	struct file_system_type {
115		const char *name;
116		int fs_flags;
117	        struct dentry *(*mount) (struct file_system_type *, int,
118	                       const char *, void *);
119	        void (*kill_sb) (struct super_block *);
120	        struct module *owner;
121	        struct file_system_type * next;
122	        struct list_head fs_supers;
123		struct lock_class_key s_lock_key;
124		struct lock_class_key s_umount_key;
125	};
127	  name: the name of the filesystem type, such as "ext2", "iso9660",
128		"msdos" and so on
130	  fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
132	  mount: the method to call when a new instance of this
133		filesystem should be mounted
135	  kill_sb: the method to call when an instance of this filesystem
136		should be shut down
138	  owner: for internal VFS use: you should initialize this to THIS_MODULE in
139	  	most cases.
141	  next: for internal VFS use: you should initialize this to NULL
143	  s_lock_key, s_umount_key: lockdep-specific
145	The mount() method has the following arguments:
147	  struct file_system_type *fs_type: describes the filesystem, partly initialized
148	  	by the specific filesystem code
150	  int flags: mount flags
152	  const char *dev_name: the device name we are mounting.
154	  void *data: arbitrary mount options, usually comes as an ASCII
155		string (see "Mount Options" section)
157	The mount() method must return the root dentry of the tree requested by
158	caller.  An active reference to its superblock must be grabbed and the
159	superblock must be locked.  On failure it should return ERR_PTR(error).
161	The arguments match those of mount(2) and their interpretation
162	depends on filesystem type.  E.g. for block filesystems, dev_name is
163	interpreted as block device name, that device is opened and if it
164	contains a suitable filesystem image the method creates and initializes
165	struct super_block accordingly, returning its root dentry to caller.
167	->mount() may choose to return a subtree of existing filesystem - it
168	doesn't have to create a new one.  The main result from the caller's
169	point of view is a reference to dentry at the root of (sub)tree to
170	be attached; creation of new superblock is a common side effect.
172	The most interesting member of the superblock structure that the
173	mount() method fills in is the "s_op" field. This is a pointer to
174	a "struct super_operations" which describes the next level of the
175	filesystem implementation.
177	Usually, a filesystem uses one of the generic mount() implementations
178	and provides a fill_super() callback instead. The generic variants are:
180	  mount_bdev: mount a filesystem residing on a block device
182	  mount_nodev: mount a filesystem that is not backed by a device
184	  mount_single: mount a filesystem which shares the instance between
185	  	all mounts
187	A fill_super() callback implementation has the following arguments:
189	  struct super_block *sb: the superblock structure. The callback
190	  	must initialize this properly.
192	  void *data: arbitrary mount options, usually comes as an ASCII
193		string (see "Mount Options" section)
195	  int silent: whether or not to be silent on error
198	The Superblock Object
199	=====================
201	A superblock object represents a mounted filesystem.
204	struct super_operations
205	-----------------------
207	This describes how the VFS can manipulate the superblock of your
208	filesystem. As of kernel 2.6.22, the following members are defined:
210	struct super_operations {
211	        struct inode *(*alloc_inode)(struct super_block *sb);
212	        void (*destroy_inode)(struct inode *);
214	        void (*dirty_inode) (struct inode *, int flags);
215	        int (*write_inode) (struct inode *, int);
216	        void (*drop_inode) (struct inode *);
217	        void (*delete_inode) (struct inode *);
218	        void (*put_super) (struct super_block *);
219	        int (*sync_fs)(struct super_block *sb, int wait);
220	        int (*freeze_fs) (struct super_block *);
221	        int (*unfreeze_fs) (struct super_block *);
222	        int (*statfs) (struct dentry *, struct kstatfs *);
223	        int (*remount_fs) (struct super_block *, int *, char *);
224	        void (*clear_inode) (struct inode *);
225	        void (*umount_begin) (struct super_block *);
227	        int (*show_options)(struct seq_file *, struct dentry *);
229	        ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230	        ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
231		int (*nr_cached_objects)(struct super_block *);
232		void (*free_cached_objects)(struct super_block *, int);
233	};
235	All methods are called without any locks being held, unless otherwise
236	noted. This means that most methods can block safely. All methods are
237	only called from a process context (i.e. not from an interrupt handler
238	or bottom half).
240	  alloc_inode: this method is called by alloc_inode() to allocate memory
241	 	for struct inode and initialize it.  If this function is not
242	 	defined, a simple 'struct inode' is allocated.  Normally
243	 	alloc_inode will be used to allocate a larger structure which
244	 	contains a 'struct inode' embedded within it.
246	  destroy_inode: this method is called by destroy_inode() to release
247	  	resources allocated for struct inode.  It is only required if
248	  	->alloc_inode was defined and simply undoes anything done by
249		->alloc_inode.
251	  dirty_inode: this method is called by the VFS to mark an inode dirty.
253	  write_inode: this method is called when the VFS needs to write an
254		inode to disc.  The second parameter indicates whether the write
255		should be synchronous or not, not all filesystems check this flag.
257	  drop_inode: called when the last access to the inode is dropped,
258		with the inode->i_lock spinlock held.
260		This method should be either NULL (normal UNIX filesystem
261		semantics) or "generic_delete_inode" (for filesystems that do not
262		want to cache inodes - causing "delete_inode" to always be
263		called regardless of the value of i_nlink)
265		The "generic_delete_inode()" behavior is equivalent to the
266		old practice of using "force_delete" in the put_inode() case,
267		but does not have the races that the "force_delete()" approach
268		had. 
270	  delete_inode: called when the VFS wants to delete an inode
272	  put_super: called when the VFS wishes to free the superblock
273		(i.e. unmount). This is called with the superblock lock held
275	  sync_fs: called when VFS is writing out all dirty data associated with
276	  	a superblock. The second parameter indicates whether the method
277		should wait until the write out has been completed. Optional.
279	  freeze_fs: called when VFS is locking a filesystem and
280	  	forcing it into a consistent state.  This method is currently
281	  	used by the Logical Volume Manager (LVM).
283	  unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284	  	again.
286	  statfs: called when the VFS needs to get filesystem statistics.
288	  remount_fs: called when the filesystem is remounted. This is called
289		with the kernel lock held
291	  clear_inode: called then the VFS clears the inode. Optional
293	  umount_begin: called when the VFS is unmounting a filesystem.
295	  show_options: called by the VFS to show mount options for
296		/proc/<pid>/mounts.  (see "Mount Options" section)
298	  quota_read: called by the VFS to read from filesystem quota file.
300	  quota_write: called by the VFS to write to filesystem quota file.
302	  nr_cached_objects: called by the sb cache shrinking function for the
303		filesystem to return the number of freeable cached objects it contains.
304		Optional.
306	  free_cache_objects: called by the sb cache shrinking function for the
307		filesystem to scan the number of objects indicated to try to free them.
308		Optional, but any filesystem implementing this method needs to also
309		implement ->nr_cached_objects for it to be called correctly.
311		We can't do anything with any errors that the filesystem might
312		encountered, hence the void return type. This will never be called if
313		the VM is trying to reclaim under GFP_NOFS conditions, hence this
314		method does not need to handle that situation itself.
316		Implementations must include conditional reschedule calls inside any
317		scanning loop that is done. This allows the VFS to determine
318		appropriate scan batch sizes without having to worry about whether
319		implementations will cause holdoff problems due to large scan batch
320		sizes.
322	Whoever sets up the inode is responsible for filling in the "i_op" field. This
323	is a pointer to a "struct inode_operations" which describes the methods that
324	can be performed on individual inodes.
326	struct xattr_handlers
327	---------------------
329	On filesystems that support extended attributes (xattrs), the s_xattr
330	superblock field points to a NULL-terminated array of xattr handlers.  Extended
331	attributes are name:value pairs.
333	  name: Indicates that the handler matches attributes with the specified name
334		(such as "system.posix_acl_access"); the prefix field must be NULL.
336	  prefix: Indicates that the handler matches all attributes with the specified
337		name prefix (such as "user."); the name field must be NULL.
339	  list: Determine if attributes matching this xattr handler should be listed
340		for a particular dentry.  Used by some listxattr implementations like
341		generic_listxattr.
343	  get: Called by the VFS to get the value of a particular extended attribute.
344		This method is called by the getxattr(2) system call.
346	  set: Called by the VFS to set the value of a particular extended attribute.
347		When the new value is NULL, called to remove a particular extended
348		attribute.  This method is called by the the setxattr(2) and
349		removexattr(2) system calls.
351	When none of the xattr handlers of a filesystem match the specified attribute
352	name or when a filesystem doesn't support extended attributes, the various
353	*xattr(2) system calls return -EOPNOTSUPP.
356	The Inode Object
357	================
359	An inode object represents an object within the filesystem.
362	struct inode_operations
363	-----------------------
365	This describes how the VFS can manipulate an inode in your
366	filesystem. As of kernel 2.6.22, the following members are defined:
368	struct inode_operations {
369		int (*create) (struct inode *,struct dentry *, umode_t, bool);
370		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
371		int (*link) (struct dentry *,struct inode *,struct dentry *);
372		int (*unlink) (struct inode *,struct dentry *);
373		int (*symlink) (struct inode *,struct dentry *,const char *);
374		int (*mkdir) (struct inode *,struct dentry *,umode_t);
375		int (*rmdir) (struct inode *,struct dentry *);
376		int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
377		int (*rename) (struct inode *, struct dentry *,
378				struct inode *, struct dentry *, unsigned int);
379		int (*readlink) (struct dentry *, char __user *,int);
380		const char *(*get_link) (struct dentry *, struct inode *,
381					 struct delayed_call *);
382		int (*permission) (struct inode *, int);
383		int (*get_acl)(struct inode *, int);
384		int (*setattr) (struct dentry *, struct iattr *);
385		int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
386		ssize_t (*listxattr) (struct dentry *, char *, size_t);
387		void (*update_time)(struct inode *, struct timespec *, int);
388		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
389				unsigned open_flag, umode_t create_mode, int *opened);
390		int (*tmpfile) (struct inode *, struct dentry *, umode_t);
391	};
393	Again, all methods are called without any locks being held, unless
394	otherwise noted.
396	  create: called by the open(2) and creat(2) system calls. Only
397		required if you want to support regular files. The dentry you
398		get should not have an inode (i.e. it should be a negative
399		dentry). Here you will probably call d_instantiate() with the
400		dentry and the newly created inode
402	  lookup: called when the VFS needs to look up an inode in a parent
403		directory. The name to look for is found in the dentry. This
404		method must call d_add() to insert the found inode into the
405		dentry. The "i_count" field in the inode structure should be
406		incremented. If the named inode does not exist a NULL inode
407		should be inserted into the dentry (this is called a negative
408		dentry). Returning an error code from this routine must only
409		be done on a real error, otherwise creating inodes with system
410		calls like create(2), mknod(2), mkdir(2) and so on will fail.
411		If you wish to overload the dentry methods then you should
412		initialise the "d_dop" field in the dentry; this is a pointer
413		to a struct "dentry_operations".
414		This method is called with the directory inode semaphore held
416	  link: called by the link(2) system call. Only required if you want
417		to support hard links. You will probably need to call
418		d_instantiate() just as you would in the create() method
420	  unlink: called by the unlink(2) system call. Only required if you
421		want to support deleting inodes
423	  symlink: called by the symlink(2) system call. Only required if you
424		want to support symlinks. You will probably need to call
425		d_instantiate() just as you would in the create() method
427	  mkdir: called by the mkdir(2) system call. Only required if you want
428		to support creating subdirectories. You will probably need to
429		call d_instantiate() just as you would in the create() method
431	  rmdir: called by the rmdir(2) system call. Only required if you want
432		to support deleting subdirectories
434	  mknod: called by the mknod(2) system call to create a device (char,
435		block) inode or a named pipe (FIFO) or socket. Only required
436		if you want to support creating these types of inodes. You
437		will probably need to call d_instantiate() just as you would
438		in the create() method
440	  rename: called by the rename(2) system call to rename the object to
441		have the parent and name given by the second inode and dentry.
443		The filesystem must return -EINVAL for any unsupported or
444		unknown	flags.  Currently the following flags are implemented:
445		(1) RENAME_NOREPLACE: this flag indicates that if the target
446		of the rename exists the rename should fail with -EEXIST
447		instead of replacing the target.  The VFS already checks for
448		existence, so for local filesystems the RENAME_NOREPLACE
449		implementation is equivalent to plain rename.
450		(2) RENAME_EXCHANGE: exchange source and target.  Both must
451		exist; this is checked by the VFS.  Unlike plain rename,
452		source and target may be of different type.
454	  readlink: called by the readlink(2) system call. Only required if
455		you want to support reading symbolic links
457	  get_link: called by the VFS to follow a symbolic link to the
458		inode it points to.  Only required if you want to support
459		symbolic links.  This method returns the symlink body
460		to traverse (and possibly resets the current position with
461		nd_jump_link()).  If the body won't go away until the inode
462		is gone, nothing else is needed; if it needs to be otherwise
463		pinned, arrange for its release by having get_link(..., ..., done)
464		do set_delayed_call(done, destructor, argument).
465		In that case destructor(argument) will be called once VFS is
466		done with the body you've returned.
467		May be called in RCU mode; that is indicated by NULL dentry
468		argument.  If request can't be handled without leaving RCU mode,
469		have it return ERR_PTR(-ECHILD).
471	  permission: called by the VFS to check for access rights on a POSIX-like
472	  	filesystem.
474		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
475	        mode, the filesystem must check the permission without blocking or
476		storing to the inode.
478		If a situation is encountered that rcu-walk cannot handle, return
479		-ECHILD and it will be called again in ref-walk mode.
481	  setattr: called by the VFS to set attributes for a file. This method
482	  	is called by chmod(2) and related system calls.
484	  getattr: called by the VFS to get attributes of a file. This method
485	  	is called by stat(2) and related system calls.
487	  listxattr: called by the VFS to list all extended attributes for a
488		given file. This method is called by the listxattr(2) system call.
490	  update_time: called by the VFS to update a specific time or the i_version of
491	  	an inode.  If this is not defined the VFS will update the inode itself
492	  	and call mark_inode_dirty_sync.
494	  atomic_open: called on the last component of an open.  Using this optional
495	  	method the filesystem can look up, possibly create and open the file in
496	  	one atomic operation.  If it cannot perform this (e.g. the file type
497	  	turned out to be wrong) it may signal this by returning 1 instead of
498		usual 0 or -ve .  This method is only called if the last component is
499		negative or needs lookup.  Cached positive dentries are still handled by
500		f_op->open().  If the file was created, the FILE_CREATED flag should be
501		set in "opened".  In case of O_EXCL the method must only succeed if the
502		file didn't exist and hence FILE_CREATED shall always be set on success.
504	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
505		atomically creating, opening and unlinking a file in given directory.
507	The Address Space Object
508	========================
510	The address space object is used to group and manage pages in the page
511	cache.  It can be used to keep track of the pages in a file (or
512	anything else) and also track the mapping of sections of the file into
513	process address spaces.
515	There are a number of distinct yet related services that an
516	address-space can provide.  These include communicating memory
517	pressure, page lookup by address, and keeping track of pages tagged as
518	Dirty or Writeback.
520	The first can be used independently to the others.  The VM can try to
521	either write dirty pages in order to clean them, or release clean
522	pages in order to reuse them.  To do this it can call the ->writepage
523	method on dirty pages, and ->releasepage on clean pages with
524	PagePrivate set. Clean pages without PagePrivate and with no external
525	references will be released without notice being given to the
526	address_space.
528	To achieve this functionality, pages need to be placed on an LRU with
529	lru_cache_add and mark_page_active needs to be called whenever the
530	page is used.
532	Pages are normally kept in a radix tree index by ->index. This tree
533	maintains information about the PG_Dirty and PG_Writeback status of
534	each page, so that pages with either of these flags can be found
535	quickly.
537	The Dirty tag is primarily used by mpage_writepages - the default
538	->writepages method.  It uses the tag to find dirty pages to call
539	->writepage on.  If mpage_writepages is not used (i.e. the address
540	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
541	almost unused.  write_inode_now and sync_inode do use it (through
542	__sync_single_inode) to check if ->writepages has been successful in
543	writing out the whole address_space.
545	The Writeback tag is used by filemap*wait* and sync_page* functions,
546	via filemap_fdatawait_range, to wait for all writeback to complete.
548	An address_space handler may attach extra information to a page,
549	typically using the 'private' field in the 'struct page'.  If such
550	information is attached, the PG_Private flag should be set.  This will
551	cause various VM routines to make extra calls into the address_space
552	handler to deal with that data.
554	An address space acts as an intermediate between storage and
555	application.  Data is read into the address space a whole page at a
556	time, and provided to the application either by copying of the page,
557	or by memory-mapping the page.
558	Data is written into the address space by the application, and then
559	written-back to storage typically in whole pages, however the
560	address_space has finer control of write sizes.
562	The read process essentially only requires 'readpage'.  The write
563	process is more complicated and uses write_begin/write_end or
564	set_page_dirty to write data into the address_space, and writepage
565	and writepages to writeback data to storage.
567	Adding and removing pages to/from an address_space is protected by the
568	inode's i_mutex.
570	When data is written to a page, the PG_Dirty flag should be set.  It
571	typically remains set until writepage asks for it to be written.  This
572	should clear PG_Dirty and set PG_Writeback.  It can be actually
573	written at any point after PG_Dirty is clear.  Once it is known to be
574	safe, PG_Writeback is cleared.
576	Writeback makes use of a writeback_control structure...
578	struct address_space_operations
579	-------------------------------
581	This describes how the VFS can manipulate mapping of a file to page cache in
582	your filesystem. The following members are defined:
584	struct address_space_operations {
585		int (*writepage)(struct page *page, struct writeback_control *wbc);
586		int (*readpage)(struct file *, struct page *);
587		int (*writepages)(struct address_space *, struct writeback_control *);
588		int (*set_page_dirty)(struct page *page);
589		int (*readpages)(struct file *filp, struct address_space *mapping,
590				struct list_head *pages, unsigned nr_pages);
591		int (*write_begin)(struct file *, struct address_space *mapping,
592					loff_t pos, unsigned len, unsigned flags,
593					struct page **pagep, void **fsdata);
594		int (*write_end)(struct file *, struct address_space *mapping,
595					loff_t pos, unsigned len, unsigned copied,
596					struct page *page, void *fsdata);
597		sector_t (*bmap)(struct address_space *, sector_t);
598		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
599		int (*releasepage) (struct page *, int);
600		void (*freepage)(struct page *);
601		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
602		/* isolate a page for migration */
603		bool (*isolate_page) (struct page *, isolate_mode_t);
604		/* migrate the contents of a page to the specified target */
605		int (*migratepage) (struct page *, struct page *);
606		/* put migration-failed page back to right list */
607		void (*putback_page) (struct page *);
608		int (*launder_page) (struct page *);
610		int (*is_partially_uptodate) (struct page *, unsigned long,
611						unsigned long);
612		void (*is_dirty_writeback) (struct page *, bool *, bool *);
613		int (*error_remove_page) (struct mapping *mapping, struct page *page);
614		int (*swap_activate)(struct file *);
615		int (*swap_deactivate)(struct file *);
616	};
618	  writepage: called by the VM to write a dirty page to backing store.
619	      This may happen for data integrity reasons (i.e. 'sync'), or
620	      to free up memory (flush).  The difference can be seen in
621	      wbc->sync_mode.
622	      The PG_Dirty flag has been cleared and PageLocked is true.
623	      writepage should start writeout, should set PG_Writeback,
624	      and should make sure the page is unlocked, either synchronously
625	      or asynchronously when the write operation completes.
627	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
628	      try too hard if there are problems, and may choose to write out
629	      other pages from the mapping if that is easier (e.g. due to
630	      internal dependencies).  If it chooses not to start writeout, it
631	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
632	      calling ->writepage on that page.
634	      See the file "Locking" for more details.
636	  readpage: called by the VM to read a page from backing store.
637	       The page will be Locked when readpage is called, and should be
638	       unlocked and marked uptodate once the read completes.
639	       If ->readpage discovers that it needs to unlock the page for
640	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
641	       In this case, the page will be relocated, relocked and if
642	       that all succeeds, ->readpage will be called again.
644	  writepages: called by the VM to write out pages associated with the
645	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
646	  	the writeback_control will specify a range of pages that must be
647	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
648		and that many pages should be written if possible.
649		If no ->writepages is given, then mpage_writepages is used
650	  	instead.  This will choose pages from the address space that are
651	  	tagged as DIRTY and will pass them to ->writepage.
653	  set_page_dirty: called by the VM to set a page dirty.
654	        This is particularly needed if an address space attaches
655	        private data to a page, and that data needs to be updated when
656	        a page is dirtied.  This is called, for example, when a memory
657		mapped page gets modified.
658		If defined, it should set the PageDirty flag, and the
659	        PAGECACHE_TAG_DIRTY tag in the radix tree.
661	  readpages: called by the VM to read pages associated with the address_space
662	  	object. This is essentially just a vector version of
663	  	readpage.  Instead of just one page, several pages are
664	  	requested.
665		readpages is only used for read-ahead, so read errors are
666	  	ignored.  If anything goes wrong, feel free to give up.
668	  write_begin:
669		Called by the generic buffered write code to ask the filesystem to
670		prepare to write len bytes at the given offset in the file. The
671		address_space should check that the write will be able to complete,
672		by allocating space if necessary and doing any other internal
673		housekeeping.  If the write will update parts of any basic-blocks on
674		storage, then those blocks should be pre-read (if they haven't been
675		read already) so that the updated blocks can be written out properly.
677	        The filesystem must return the locked pagecache page for the specified
678		offset, in *pagep, for the caller to write into.
680		It must be able to cope with short writes (where the length passed to
681		write_begin is greater than the number of bytes copied into the page).
683		flags is a field for AOP_FLAG_xxx flags, described in
684		include/linux/fs.h.
686	        A void * may be returned in fsdata, which then gets passed into
687	        write_end.
689	        Returns 0 on success; < 0 on failure (which is the error code), in
690		which case write_end is not called.
692	  write_end: After a successful write_begin, and data copy, write_end must
693	        be called. len is the original len passed to write_begin, and copied
694	        is the amount that was able to be copied (copied == len is always true
695		if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
697	        The filesystem must take care of unlocking the page and releasing it
698	        refcount, and updating i_size.
700	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
701	        that were able to be copied into pagecache.
703	  bmap: called by the VFS to map a logical block offset within object to
704	  	physical block number. This method is used by the FIBMAP
705	  	ioctl and for working with swap-files.  To be able to swap to
706	  	a file, the file must have a stable mapping to a block
707	  	device.  The swap system does not go through the filesystem
708	  	but instead uses bmap to find out where the blocks in the file
709	  	are and uses those addresses directly.
711	  invalidatepage: If a page has PagePrivate set, then invalidatepage
712	        will be called when part or all of the page is to be removed
713		from the address space.  This generally corresponds to either a
714		truncation, punch hole  or a complete invalidation of the address
715		space (in the latter case 'offset' will always be 0 and 'length'
716		will be PAGE_SIZE). Any private data associated with the page
717		should be updated to reflect this truncation.  If offset is 0 and
718		length is PAGE_SIZE, then the private data should be released,
719		because the page must be able to be completely discarded.  This may
720		be done by calling the ->releasepage function, but in this case the
721		release MUST succeed.
723	  releasepage: releasepage is called on PagePrivate pages to indicate
724	        that the page should be freed if possible.  ->releasepage
725	        should remove any private data from the page and clear the
726	        PagePrivate flag. If releasepage() fails for some reason, it must
727		indicate failure with a 0 return value.
728		releasepage() is used in two distinct though related cases.  The
729		first is when the VM finds a clean page with no active users and
730	        wants to make it a free page.  If ->releasepage succeeds, the
731	        page will be removed from the address_space and become free.
733		The second case is when a request has been made to invalidate
734	        some or all pages in an address_space.  This can happen
735	        through the fadvise(POSIX_FADV_DONTNEED) system call or by the
736	        filesystem explicitly requesting it as nfs and 9fs do (when
737	        they believe the cache may be out of date with storage) by
738	        calling invalidate_inode_pages2().
739		If the filesystem makes such a call, and needs to be certain
740	        that all pages are invalidated, then its releasepage will
741	        need to ensure this.  Possibly it can clear the PageUptodate
742	        bit if it cannot free private data yet.
744	  freepage: freepage is called once the page is no longer visible in
745	        the page cache in order to allow the cleanup of any private
746		data. Since it may be called by the memory reclaimer, it
747		should not assume that the original address_space mapping still
748		exists, and it should not block.
750	  direct_IO: called by the generic read/write routines to perform
751	        direct_IO - that is IO requests which bypass the page cache
752	        and transfer data directly between the storage and the
753	        application's address space.
755	  isolate_page: Called by the VM when isolating a movable non-lru page.
756		If page is successfully isolated, VM marks the page as PG_isolated
757		via __SetPageIsolated.
759	  migrate_page:  This is used to compact the physical memory usage.
760	        If the VM wants to relocate a page (maybe off a memory card
761	        that is signalling imminent failure) it will pass a new page
762		and an old page to this function.  migrate_page should
763		transfer any private data across and update any references
764	        that it has to the page.
766	  putback_page: Called by the VM when isolated page's migration fails.
768	  launder_page: Called before freeing a page - it writes back the dirty page. To
769	  	prevent redirtying the page, it is kept locked during the whole
770		operation.
772	  is_partially_uptodate: Called by the VM when reading a file through the
773		pagecache when the underlying blocksize != pagesize. If the required
774		block is up to date then the read can complete without needing the IO
775		to bring the whole page up to date.
777	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
778		The VM uses dirty and writeback information to determine if it needs
779		to stall to allow flushers a chance to complete some IO. Ordinarily
780		it can use PageDirty and PageWriteback but some filesystems have
781		more complex state (unstable pages in NFS prevent reclaim) or
782		do not set those flags due to locking problems. This callback
783		allows a filesystem to indicate to the VM if a page should be
784		treated as dirty or writeback for the purposes of stalling.
786	  error_remove_page: normally set to generic_error_remove_page if truncation
787		is ok for this address space. Used for memory failure handling.
788		Setting this implies you deal with pages going away under you,
789		unless you have them locked or reference counts increased.
791	  swap_activate: Called when swapon is used on a file to allocate
792		space if necessary and pin the block lookup information in
793		memory. A return value of zero indicates success,
794		in which case this file can be used to back swapspace. The
795		swapspace operations will be proxied to this address space's
796		->swap_{out,in} methods.
798	  swap_deactivate: Called during swapoff on files where swap_activate
799		was successful.
802	The File Object
803	===============
805	A file object represents a file opened by a process.
808	struct file_operations
809	----------------------
811	This describes how the VFS can manipulate an open file. As of kernel
812	4.1, the following members are defined:
814	struct file_operations {
815		struct module *owner;
816		loff_t (*llseek) (struct file *, loff_t, int);
817		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
818		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
819		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
820		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
821		int (*iterate) (struct file *, struct dir_context *);
822		unsigned int (*poll) (struct file *, struct poll_table_struct *);
823		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
824		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
825		int (*mmap) (struct file *, struct vm_area_struct *);
826		int (*mremap)(struct file *, struct vm_area_struct *);
827		int (*open) (struct inode *, struct file *);
828		int (*flush) (struct file *, fl_owner_t id);
829		int (*release) (struct inode *, struct file *);
830		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
831		int (*fasync) (int, struct file *, int);
832		int (*lock) (struct file *, int, struct file_lock *);
833		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
834		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
835		int (*check_flags)(int);
836		int (*flock) (struct file *, int, struct file_lock *);
837		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
838		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
839		int (*setlease)(struct file *, long, struct file_lock **, void **);
840		long (*fallocate)(struct file *file, int mode, loff_t offset,
841				  loff_t len);
842		void (*show_fdinfo)(struct seq_file *m, struct file *f);
843	#ifndef CONFIG_MMU
844		unsigned (*mmap_capabilities)(struct file *);
845	#endif
846	};
848	Again, all methods are called without any locks being held, unless
849	otherwise noted.
851	  llseek: called when the VFS needs to move the file position index
853	  read: called by read(2) and related system calls
855	  read_iter: possibly asynchronous read with iov_iter as destination
857	  write: called by write(2) and related system calls
859	  write_iter: possibly asynchronous write with iov_iter as source
861	  iterate: called when the VFS needs to read the directory contents
863	  poll: called by the VFS when a process wants to check if there is
864		activity on this file and (optionally) go to sleep until there
865		is activity. Called by the select(2) and poll(2) system calls
867	  unlocked_ioctl: called by the ioctl(2) system call.
869	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
870	 	 are used on 64 bit kernels.
872	  mmap: called by the mmap(2) system call
874	  open: called by the VFS when an inode should be opened. When the VFS
875		opens a file, it creates a new "struct file". It then calls the
876		open method for the newly allocated file structure. You might
877		think that the open method really belongs in
878		"struct inode_operations", and you may be right. I think it's
879		done the way it is because it makes filesystems simpler to
880		implement. The open() method is a good place to initialize the
881		"private_data" member in the file structure if you want to point
882		to a device structure
884	  flush: called by the close(2) system call to flush a file
886	  release: called when the last reference to an open file is closed
888	  fsync: called by the fsync(2) system call
890	  fasync: called by the fcntl(2) system call when asynchronous
891		(non-blocking) mode is enabled for a file
893	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
894	  	commands
896	  get_unmapped_area: called by the mmap(2) system call
898	  check_flags: called by the fcntl(2) system call for F_SETFL command
900	  flock: called by the flock(2) system call
902	  splice_write: called by the VFS to splice data from a pipe to a file. This
903			method is used by the splice(2) system call
905	  splice_read: called by the VFS to splice data from file to a pipe. This
906		       method is used by the splice(2) system call
908	  setlease: called by the VFS to set or release a file lock lease. setlease
909		    implementations should call generic_setlease to record or remove
910		    the lease in the inode after setting it.
912	  fallocate: called by the VFS to preallocate blocks or punch a hole.
914	Note that the file operations are implemented by the specific
915	filesystem in which the inode resides. When opening a device node
916	(character or block special) most filesystems will call special
917	support routines in the VFS which will locate the required device
918	driver information. These support routines replace the filesystem file
919	operations with those for the device driver, and then proceed to call
920	the new open() method for the file. This is how opening a device file
921	in the filesystem eventually ends up calling the device driver open()
922	method.
925	Directory Entry Cache (dcache)
926	==============================
929	struct dentry_operations
930	------------------------
932	This describes how a filesystem can overload the standard dentry
933	operations. Dentries and the dcache are the domain of the VFS and the
934	individual filesystem implementations. Device drivers have no business
935	here. These methods may be set to NULL, as they are either optional or
936	the VFS uses a default. As of kernel 2.6.22, the following members are
937	defined:
939	struct dentry_operations {
940		int (*d_revalidate)(struct dentry *, unsigned int);
941		int (*d_weak_revalidate)(struct dentry *, unsigned int);
942		int (*d_hash)(const struct dentry *, struct qstr *);
943		int (*d_compare)(const struct dentry *,
944				unsigned int, const char *, const struct qstr *);
945		int (*d_delete)(const struct dentry *);
946		int (*d_init)(struct dentry *);
947		void (*d_release)(struct dentry *);
948		void (*d_iput)(struct dentry *, struct inode *);
949		char *(*d_dname)(struct dentry *, char *, int);
950		struct vfsmount *(*d_automount)(struct path *);
951		int (*d_manage)(struct dentry *, bool);
952		struct dentry *(*d_real)(struct dentry *, const struct inode *,
953					 unsigned int);
954	};
956	  d_revalidate: called when the VFS needs to revalidate a dentry. This
957		is called whenever a name look-up finds a dentry in the
958		dcache. Most local filesystems leave this as NULL, because all their
959		dentries in the dcache are valid. Network filesystems are different
960		since things can change on the server without the client necessarily
961		being aware of it.
963		This function should return a positive value if the dentry is still
964		valid, and zero or a negative error code if it isn't.
966		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
967		If in rcu-walk mode, the filesystem must revalidate the dentry without
968		blocking or storing to the dentry, d_parent and d_inode should not be
969		used without care (because they can change and, in d_inode case, even
970		become NULL under us).
972		If a situation is encountered that rcu-walk cannot handle, return
973		-ECHILD and it will be called again in ref-walk mode.
975	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
976		This is called when a path-walk ends at dentry that was not acquired by
977		doing a lookup in the parent directory. This includes "/", "." and "..",
978		as well as procfs-style symlinks and mountpoint traversal.
980		In this case, we are less concerned with whether the dentry is still
981		fully correct, but rather that the inode is still valid. As with
982		d_revalidate, most local filesystems will set this to NULL since their
983		dcache entries are always valid.
985		This function has the same return code semantics as d_revalidate.
987		d_weak_revalidate is only called after leaving rcu-walk mode.
989	  d_hash: called when the VFS adds a dentry to the hash table. The first
990		dentry passed to d_hash is the parent directory that the name is
991		to be hashed into.
993		Same locking and synchronisation rules as d_compare regarding
994		what is safe to dereference etc.
996	  d_compare: called to compare a dentry name with a given name. The first
997		dentry is the parent of the dentry to be compared, the second is
998		the child dentry. len and name string are properties of the dentry
999		to be compared. qstr is the name to compare it with.
1001		Must be constant and idempotent, and should not take locks if
1002		possible, and should not or store into the dentry.
1003		Should not dereference pointers outside the dentry without
1004		lots of care (eg.  d_parent, d_inode, d_name should not be used).
1006		However, our vfsmount is pinned, and RCU held, so the dentries and
1007		inodes won't disappear, neither will our sb or filesystem module.
1008		->d_sb may be used.
1010		It is a tricky calling convention because it needs to be called under
1011		"rcu-walk", ie. without any locks or references on things.
1013	  d_delete: called when the last reference to a dentry is dropped and the
1014		dcache is deciding whether or not to cache it. Return 1 to delete
1015		immediately, or 0 to cache the dentry. Default is NULL which means to
1016		always cache a reachable dentry. d_delete must be constant and
1017		idempotent.
1019	  d_init: called when a dentry is allocated
1021	  d_release: called when a dentry is really deallocated
1023	  d_iput: called when a dentry loses its inode (just prior to its
1024		being deallocated). The default when this is NULL is that the
1025		VFS calls iput(). If you define this method, you must call
1026		iput() yourself
1028	  d_dname: called when the pathname of a dentry should be generated.
1029		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1030		pathname generation. (Instead of doing it when dentry is created,
1031		it's done only when the path is needed.). Real filesystems probably
1032		dont want to use it, because their dentries are present in global
1033		dcache hash, so their hash should be an invariant. As no lock is
1034		held, d_dname() should not try to modify the dentry itself, unless
1035		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1036		tricky. The correct way to return for example "Hello" is to put it
1037		at the end of the buffer, and returns a pointer to the first char.
1038		dynamic_dname() helper function is provided to take care of this.
1040		Example :
1042		static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1043		{
1044			return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1045					dentry->d_inode->i_ino);
1046		}
1048	  d_automount: called when an automount dentry is to be traversed (optional).
1049		This should create a new VFS mount record and return the record to the
1050		caller.  The caller is supplied with a path parameter giving the
1051		automount directory to describe the automount target and the parent
1052		VFS mount record to provide inheritable mount parameters.  NULL should
1053		be returned if someone else managed to make the automount first.  If
1054		the vfsmount creation failed, then an error code should be returned.
1055		If -EISDIR is returned, then the directory will be treated as an
1056		ordinary directory and returned to pathwalk to continue walking.
1058		If a vfsmount is returned, the caller will attempt to mount it on the
1059		mountpoint and will remove the vfsmount from its expiration list in
1060		the case of failure.  The vfsmount should be returned with 2 refs on
1061		it to prevent automatic expiration - the caller will clean up the
1062		additional ref.
1064		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1065		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1066		inode being added.
1068	  d_manage: called to allow the filesystem to manage the transition from a
1069		dentry (optional).  This allows autofs, for example, to hold up clients
1070		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1071		past and construct the subtree there.  0 should be returned to let the
1072		calling process continue.  -EISDIR can be returned to tell pathwalk to
1073		use this directory as an ordinary directory and to ignore anything
1074		mounted on it and not to check the automount flag.  Any other error
1075		code will abort pathwalk completely.
1077		If the 'rcu_walk' parameter is true, then the caller is doing a
1078		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1079		and the caller can be asked to leave it and call again by returning
1080		-ECHILD.  -EISDIR may also be returned to tell pathwalk to
1081		ignore d_automount or any mounts.
1083		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1084		dentry being transited from.
1086	  d_real: overlay/union type filesystems implement this method to return one of
1087		the underlying dentries hidden by the overlay.  It is used in three
1088		different modes:
1090		Called from open it may need to copy-up the file depending on the
1091		supplied open flags.  This mode is selected with a non-zero flags
1092		argument.  In this mode the d_real method can return an error.
1094		Called from file_dentry() it returns the real dentry matching the inode
1095		argument.  The real dentry may be from a lower layer already copied up,
1096		but still referenced from the file.  This mode is selected with a
1097		non-NULL inode argument.  This will always succeed.
1099		With NULL inode and zero flags the topmost real underlying dentry is
1100		returned.  This will always succeed.
1102		This method is never called with both non-NULL inode and non-zero flags.
1104	Each dentry has a pointer to its parent dentry, as well as a hash list
1105	of child dentries. Child dentries are basically like files in a
1106	directory.
1109	Directory Entry Cache API
1110	--------------------------
1112	There are a number of functions defined which permit a filesystem to
1113	manipulate dentries:
1115	  dget: open a new handle for an existing dentry (this just increments
1116		the usage count)
1118	  dput: close a handle for a dentry (decrements the usage count). If
1119		the usage count drops to 0, and the dentry is still in its
1120		parent's hash, the "d_delete" method is called to check whether
1121		it should be cached. If it should not be cached, or if the dentry
1122		is not hashed, it is deleted. Otherwise cached dentries are put
1123		into an LRU list to be reclaimed on memory shortage.
1125	  d_drop: this unhashes a dentry from its parents hash list. A
1126		subsequent call to dput() will deallocate the dentry if its
1127		usage count drops to 0
1129	  d_delete: delete a dentry. If there are no other open references to
1130		the dentry then the dentry is turned into a negative dentry
1131		(the d_iput() method is called). If there are other
1132		references, then d_drop() is called instead
1134	  d_add: add a dentry to its parents hash list and then calls
1135		d_instantiate()
1137	  d_instantiate: add a dentry to the alias hash list for the inode and
1138		updates the "d_inode" member. The "i_count" member in the
1139		inode structure should be set/incremented. If the inode
1140		pointer is NULL, the dentry is called a "negative
1141		dentry". This function is commonly called when an inode is
1142		created for an existing negative dentry
1144	  d_lookup: look up a dentry given its parent and path name component
1145		It looks up the child of that given name from the dcache
1146		hash table. If it is found, the reference count is incremented
1147		and the dentry is returned. The caller must use dput()
1148		to free the dentry when it finishes using it.
1150	Mount Options
1151	=============
1153	Parsing options
1154	---------------
1156	On mount and remount the filesystem is passed a string containing a
1157	comma separated list of mount options.  The options can have either of
1158	these forms:
1160	  option
1161	  option=value
1163	The <linux/parser.h> header defines an API that helps parse these
1164	options.  There are plenty of examples on how to use it in existing
1165	filesystems.
1167	Showing options
1168	---------------
1170	If a filesystem accepts mount options, it must define show_options()
1171	to show all the currently active options.  The rules are:
1173	  - options MUST be shown which are not default or their values differ
1174	    from the default
1176	  - options MAY be shown which are enabled by default or have their
1177	    default value
1179	Options used only internally between a mount helper and the kernel
1180	(such as file descriptors), or which only have an effect during the
1181	mounting (such as ones controlling the creation of a journal) are exempt
1182	from the above rules.
1184	The underlying reason for the above rules is to make sure, that a
1185	mount can be accurately replicated (e.g. umounting and mounting again)
1186	based on the information found in /proc/mounts.
1188	A simple method of saving options at mount/remount time and showing
1189	them is provided with the save_mount_options() and
1190	generic_show_options() helper functions.  Please note, that using
1191	these may have drawbacks.  For more info see header comments for these
1192	functions in fs/namespace.c.
1194	Resources
1195	=========
1197	(Note some of these resources are not up-to-date with the latest kernel
1198	 version.)
1200	Creating Linux virtual filesystems. 2002
1201	    <http://lwn.net/Articles/13325/>
1203	The Linux Virtual File-system Layer by Neil Brown. 1999
1204	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1206	A tour of the Linux VFS by Michael K. Johnson. 1996
1207	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1209	A small trail through the Linux kernel by Andries Brouwer. 2001
1210	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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