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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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) (const struct path *, struct kstat *, u32, unsigned int);
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	  get_link: called by the VFS to follow a symbolic link to the
455		inode it points to.  Only required if you want to support
456		symbolic links.  This method returns the symlink body
457		to traverse (and possibly resets the current position with
458		nd_jump_link()).  If the body won't go away until the inode
459		is gone, nothing else is needed; if it needs to be otherwise
460		pinned, arrange for its release by having get_link(..., ..., done)
461		do set_delayed_call(done, destructor, argument).
462		In that case destructor(argument) will be called once VFS is
463		done with the body you've returned.
464		May be called in RCU mode; that is indicated by NULL dentry
465		argument.  If request can't be handled without leaving RCU mode,
466		have it return ERR_PTR(-ECHILD).
468	  readlink: this is now just an override for use by readlink(2) for the
469		cases when ->get_link uses nd_jump_link() or object is not in
470		fact a symlink.  Normally filesystems should only implement
471		->get_link for symlinks and readlink(2) will automatically use
472		that.
474	  permission: called by the VFS to check for access rights on a POSIX-like
475	  	filesystem.
477		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
478	        mode, the filesystem must check the permission without blocking or
479		storing to the inode.
481		If a situation is encountered that rcu-walk cannot handle, return
482		-ECHILD and it will be called again in ref-walk mode.
484	  setattr: called by the VFS to set attributes for a file. This method
485	  	is called by chmod(2) and related system calls.
487	  getattr: called by the VFS to get attributes of a file. This method
488	  	is called by stat(2) and related system calls.
490	  listxattr: called by the VFS to list all extended attributes for a
491		given file. This method is called by the listxattr(2) system call.
493	  update_time: called by the VFS to update a specific time or the i_version of
494	  	an inode.  If this is not defined the VFS will update the inode itself
495	  	and call mark_inode_dirty_sync.
497	  atomic_open: called on the last component of an open.  Using this optional
498	  	method the filesystem can look up, possibly create and open the file in
499	  	one atomic operation.  If it cannot perform this (e.g. the file type
500	  	turned out to be wrong) it may signal this by returning 1 instead of
501		usual 0 or -ve .  This method is only called if the last component is
502		negative or needs lookup.  Cached positive dentries are still handled by
503		f_op->open().  If the file was created, the FILE_CREATED flag should be
504		set in "opened".  In case of O_EXCL the method must only succeed if the
505		file didn't exist and hence FILE_CREATED shall always be set on success.
507	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
508		atomically creating, opening and unlinking a file in given directory.
510	The Address Space Object
511	========================
513	The address space object is used to group and manage pages in the page
514	cache.  It can be used to keep track of the pages in a file (or
515	anything else) and also track the mapping of sections of the file into
516	process address spaces.
518	There are a number of distinct yet related services that an
519	address-space can provide.  These include communicating memory
520	pressure, page lookup by address, and keeping track of pages tagged as
521	Dirty or Writeback.
523	The first can be used independently to the others.  The VM can try to
524	either write dirty pages in order to clean them, or release clean
525	pages in order to reuse them.  To do this it can call the ->writepage
526	method on dirty pages, and ->releasepage on clean pages with
527	PagePrivate set. Clean pages without PagePrivate and with no external
528	references will be released without notice being given to the
529	address_space.
531	To achieve this functionality, pages need to be placed on an LRU with
532	lru_cache_add and mark_page_active needs to be called whenever the
533	page is used.
535	Pages are normally kept in a radix tree index by ->index. This tree
536	maintains information about the PG_Dirty and PG_Writeback status of
537	each page, so that pages with either of these flags can be found
538	quickly.
540	The Dirty tag is primarily used by mpage_writepages - the default
541	->writepages method.  It uses the tag to find dirty pages to call
542	->writepage on.  If mpage_writepages is not used (i.e. the address
543	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
544	almost unused.  write_inode_now and sync_inode do use it (through
545	__sync_single_inode) to check if ->writepages has been successful in
546	writing out the whole address_space.
548	The Writeback tag is used by filemap*wait* and sync_page* functions,
549	via filemap_fdatawait_range, to wait for all writeback to complete.
551	An address_space handler may attach extra information to a page,
552	typically using the 'private' field in the 'struct page'.  If such
553	information is attached, the PG_Private flag should be set.  This will
554	cause various VM routines to make extra calls into the address_space
555	handler to deal with that data.
557	An address space acts as an intermediate between storage and
558	application.  Data is read into the address space a whole page at a
559	time, and provided to the application either by copying of the page,
560	or by memory-mapping the page.
561	Data is written into the address space by the application, and then
562	written-back to storage typically in whole pages, however the
563	address_space has finer control of write sizes.
565	The read process essentially only requires 'readpage'.  The write
566	process is more complicated and uses write_begin/write_end or
567	set_page_dirty to write data into the address_space, and writepage
568	and writepages to writeback data to storage.
570	Adding and removing pages to/from an address_space is protected by the
571	inode's i_mutex.
573	When data is written to a page, the PG_Dirty flag should be set.  It
574	typically remains set until writepage asks for it to be written.  This
575	should clear PG_Dirty and set PG_Writeback.  It can be actually
576	written at any point after PG_Dirty is clear.  Once it is known to be
577	safe, PG_Writeback is cleared.
579	Writeback makes use of a writeback_control structure to direct the
580	operations.  This gives the the writepage and writepages operations some
581	information about the nature of and reason for the writeback request,
582	and the constraints under which it is being done.  It is also used to
583	return information back to the caller about the result of a writepage or
584	writepages request.
586	Handling errors during writeback
587	--------------------------------
588	Most applications that do buffered I/O will periodically call a file
589	synchronization call (fsync, fdatasync, msync or sync_file_range) to
590	ensure that data written has made it to the backing store.  When there
591	is an error during writeback, they expect that error to be reported when
592	a file sync request is made.  After an error has been reported on one
593	request, subsequent requests on the same file descriptor should return
594	0, unless further writeback errors have occurred since the previous file
595	syncronization.
597	Ideally, the kernel would report errors only on file descriptions on
598	which writes were done that subsequently failed to be written back.  The
599	generic pagecache infrastructure does not track the file descriptions
600	that have dirtied each individual page however, so determining which
601	file descriptors should get back an error is not possible.
603	Instead, the generic writeback error tracking infrastructure in the
604	kernel settles for reporting errors to fsync on all file descriptions
605	that were open at the time that the error occurred.  In a situation with
606	multiple writers, all of them will get back an error on a subsequent fsync,
607	even if all of the writes done through that particular file descriptor
608	succeeded (or even if there were no writes on that file descriptor at all).
610	Filesystems that wish to use this infrastructure should call
611	mapping_set_error to record the error in the address_space when it
612	occurs.  Then, after writing back data from the pagecache in their
613	file->fsync operation, they should call file_check_and_advance_wb_err to
614	ensure that the struct file's error cursor has advanced to the correct
615	point in the stream of errors emitted by the backing device(s).
617	struct address_space_operations
618	-------------------------------
620	This describes how the VFS can manipulate mapping of a file to page cache in
621	your filesystem. The following members are defined:
623	struct address_space_operations {
624		int (*writepage)(struct page *page, struct writeback_control *wbc);
625		int (*readpage)(struct file *, struct page *);
626		int (*writepages)(struct address_space *, struct writeback_control *);
627		int (*set_page_dirty)(struct page *page);
628		int (*readpages)(struct file *filp, struct address_space *mapping,
629				struct list_head *pages, unsigned nr_pages);
630		int (*write_begin)(struct file *, struct address_space *mapping,
631					loff_t pos, unsigned len, unsigned flags,
632					struct page **pagep, void **fsdata);
633		int (*write_end)(struct file *, struct address_space *mapping,
634					loff_t pos, unsigned len, unsigned copied,
635					struct page *page, void *fsdata);
636		sector_t (*bmap)(struct address_space *, sector_t);
637		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
638		int (*releasepage) (struct page *, int);
639		void (*freepage)(struct page *);
640		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
641		/* isolate a page for migration */
642		bool (*isolate_page) (struct page *, isolate_mode_t);
643		/* migrate the contents of a page to the specified target */
644		int (*migratepage) (struct page *, struct page *);
645		/* put migration-failed page back to right list */
646		void (*putback_page) (struct page *);
647		int (*launder_page) (struct page *);
649		int (*is_partially_uptodate) (struct page *, unsigned long,
650						unsigned long);
651		void (*is_dirty_writeback) (struct page *, bool *, bool *);
652		int (*error_remove_page) (struct mapping *mapping, struct page *page);
653		int (*swap_activate)(struct file *);
654		int (*swap_deactivate)(struct file *);
655	};
657	  writepage: called by the VM to write a dirty page to backing store.
658	      This may happen for data integrity reasons (i.e. 'sync'), or
659	      to free up memory (flush).  The difference can be seen in
660	      wbc->sync_mode.
661	      The PG_Dirty flag has been cleared and PageLocked is true.
662	      writepage should start writeout, should set PG_Writeback,
663	      and should make sure the page is unlocked, either synchronously
664	      or asynchronously when the write operation completes.
666	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
667	      try too hard if there are problems, and may choose to write out
668	      other pages from the mapping if that is easier (e.g. due to
669	      internal dependencies).  If it chooses not to start writeout, it
670	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
671	      calling ->writepage on that page.
673	      See the file "Locking" for more details.
675	  readpage: called by the VM to read a page from backing store.
676	       The page will be Locked when readpage is called, and should be
677	       unlocked and marked uptodate once the read completes.
678	       If ->readpage discovers that it needs to unlock the page for
679	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
680	       In this case, the page will be relocated, relocked and if
681	       that all succeeds, ->readpage will be called again.
683	  writepages: called by the VM to write out pages associated with the
684	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
685	  	the writeback_control will specify a range of pages that must be
686	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
687		and that many pages should be written if possible.
688		If no ->writepages is given, then mpage_writepages is used
689	  	instead.  This will choose pages from the address space that are
690	  	tagged as DIRTY and will pass them to ->writepage.
692	  set_page_dirty: called by the VM to set a page dirty.
693	        This is particularly needed if an address space attaches
694	        private data to a page, and that data needs to be updated when
695	        a page is dirtied.  This is called, for example, when a memory
696		mapped page gets modified.
697		If defined, it should set the PageDirty flag, and the
698	        PAGECACHE_TAG_DIRTY tag in the radix tree.
700	  readpages: called by the VM to read pages associated with the address_space
701	  	object. This is essentially just a vector version of
702	  	readpage.  Instead of just one page, several pages are
703	  	requested.
704		readpages is only used for read-ahead, so read errors are
705	  	ignored.  If anything goes wrong, feel free to give up.
707	  write_begin:
708		Called by the generic buffered write code to ask the filesystem to
709		prepare to write len bytes at the given offset in the file. The
710		address_space should check that the write will be able to complete,
711		by allocating space if necessary and doing any other internal
712		housekeeping.  If the write will update parts of any basic-blocks on
713		storage, then those blocks should be pre-read (if they haven't been
714		read already) so that the updated blocks can be written out properly.
716	        The filesystem must return the locked pagecache page for the specified
717		offset, in *pagep, for the caller to write into.
719		It must be able to cope with short writes (where the length passed to
720		write_begin is greater than the number of bytes copied into the page).
722		flags is a field for AOP_FLAG_xxx flags, described in
723		include/linux/fs.h.
725	        A void * may be returned in fsdata, which then gets passed into
726	        write_end.
728	        Returns 0 on success; < 0 on failure (which is the error code), in
729		which case write_end is not called.
731	  write_end: After a successful write_begin, and data copy, write_end must
732	        be called. len is the original len passed to write_begin, and copied
733	        is the amount that was able to be copied.
735	        The filesystem must take care of unlocking the page and releasing it
736	        refcount, and updating i_size.
738	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
739	        that were able to be copied into pagecache.
741	  bmap: called by the VFS to map a logical block offset within object to
742	  	physical block number. This method is used by the FIBMAP
743	  	ioctl and for working with swap-files.  To be able to swap to
744	  	a file, the file must have a stable mapping to a block
745	  	device.  The swap system does not go through the filesystem
746	  	but instead uses bmap to find out where the blocks in the file
747	  	are and uses those addresses directly.
749	  invalidatepage: If a page has PagePrivate set, then invalidatepage
750	        will be called when part or all of the page is to be removed
751		from the address space.  This generally corresponds to either a
752		truncation, punch hole  or a complete invalidation of the address
753		space (in the latter case 'offset' will always be 0 and 'length'
754		will be PAGE_SIZE). Any private data associated with the page
755		should be updated to reflect this truncation.  If offset is 0 and
756		length is PAGE_SIZE, then the private data should be released,
757		because the page must be able to be completely discarded.  This may
758		be done by calling the ->releasepage function, but in this case the
759		release MUST succeed.
761	  releasepage: releasepage is called on PagePrivate pages to indicate
762	        that the page should be freed if possible.  ->releasepage
763	        should remove any private data from the page and clear the
764	        PagePrivate flag. If releasepage() fails for some reason, it must
765		indicate failure with a 0 return value.
766		releasepage() is used in two distinct though related cases.  The
767		first is when the VM finds a clean page with no active users and
768	        wants to make it a free page.  If ->releasepage succeeds, the
769	        page will be removed from the address_space and become free.
771		The second case is when a request has been made to invalidate
772	        some or all pages in an address_space.  This can happen
773	        through the fadvise(POSIX_FADV_DONTNEED) system call or by the
774	        filesystem explicitly requesting it as nfs and 9fs do (when
775	        they believe the cache may be out of date with storage) by
776	        calling invalidate_inode_pages2().
777		If the filesystem makes such a call, and needs to be certain
778	        that all pages are invalidated, then its releasepage will
779	        need to ensure this.  Possibly it can clear the PageUptodate
780	        bit if it cannot free private data yet.
782	  freepage: freepage is called once the page is no longer visible in
783	        the page cache in order to allow the cleanup of any private
784		data. Since it may be called by the memory reclaimer, it
785		should not assume that the original address_space mapping still
786		exists, and it should not block.
788	  direct_IO: called by the generic read/write routines to perform
789	        direct_IO - that is IO requests which bypass the page cache
790	        and transfer data directly between the storage and the
791	        application's address space.
793	  isolate_page: Called by the VM when isolating a movable non-lru page.
794		If page is successfully isolated, VM marks the page as PG_isolated
795		via __SetPageIsolated.
797	  migrate_page:  This is used to compact the physical memory usage.
798	        If the VM wants to relocate a page (maybe off a memory card
799	        that is signalling imminent failure) it will pass a new page
800		and an old page to this function.  migrate_page should
801		transfer any private data across and update any references
802	        that it has to the page.
804	  putback_page: Called by the VM when isolated page's migration fails.
806	  launder_page: Called before freeing a page - it writes back the dirty page. To
807	  	prevent redirtying the page, it is kept locked during the whole
808		operation.
810	  is_partially_uptodate: Called by the VM when reading a file through the
811		pagecache when the underlying blocksize != pagesize. If the required
812		block is up to date then the read can complete without needing the IO
813		to bring the whole page up to date.
815	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
816		The VM uses dirty and writeback information to determine if it needs
817		to stall to allow flushers a chance to complete some IO. Ordinarily
818		it can use PageDirty and PageWriteback but some filesystems have
819		more complex state (unstable pages in NFS prevent reclaim) or
820		do not set those flags due to locking problems. This callback
821		allows a filesystem to indicate to the VM if a page should be
822		treated as dirty or writeback for the purposes of stalling.
824	  error_remove_page: normally set to generic_error_remove_page if truncation
825		is ok for this address space. Used for memory failure handling.
826		Setting this implies you deal with pages going away under you,
827		unless you have them locked or reference counts increased.
829	  swap_activate: Called when swapon is used on a file to allocate
830		space if necessary and pin the block lookup information in
831		memory. A return value of zero indicates success,
832		in which case this file can be used to back swapspace.
834	  swap_deactivate: Called during swapoff on files where swap_activate
835		was successful.
838	The File Object
839	===============
841	A file object represents a file opened by a process. This is also known
842	as an "open file description" in POSIX parlance.
845	struct file_operations
846	----------------------
848	This describes how the VFS can manipulate an open file. As of kernel
849	4.1, the following members are defined:
851	struct file_operations {
852		struct module *owner;
853		loff_t (*llseek) (struct file *, loff_t, int);
854		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
855		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
856		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
857		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
858		int (*iterate) (struct file *, struct dir_context *);
859		unsigned int (*poll) (struct file *, struct poll_table_struct *);
860		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
861		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
862		int (*mmap) (struct file *, struct vm_area_struct *);
863		int (*mremap)(struct file *, struct vm_area_struct *);
864		int (*open) (struct inode *, struct file *);
865		int (*flush) (struct file *, fl_owner_t id);
866		int (*release) (struct inode *, struct file *);
867		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
868		int (*fasync) (int, struct file *, int);
869		int (*lock) (struct file *, int, struct file_lock *);
870		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
871		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
872		int (*check_flags)(int);
873		int (*flock) (struct file *, int, struct file_lock *);
874		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
875		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
876		int (*setlease)(struct file *, long, struct file_lock **, void **);
877		long (*fallocate)(struct file *file, int mode, loff_t offset,
878				  loff_t len);
879		void (*show_fdinfo)(struct seq_file *m, struct file *f);
880	#ifndef CONFIG_MMU
881		unsigned (*mmap_capabilities)(struct file *);
882	#endif
883	};
885	Again, all methods are called without any locks being held, unless
886	otherwise noted.
888	  llseek: called when the VFS needs to move the file position index
890	  read: called by read(2) and related system calls
892	  read_iter: possibly asynchronous read with iov_iter as destination
894	  write: called by write(2) and related system calls
896	  write_iter: possibly asynchronous write with iov_iter as source
898	  iterate: called when the VFS needs to read the directory contents
900	  poll: called by the VFS when a process wants to check if there is
901		activity on this file and (optionally) go to sleep until there
902		is activity. Called by the select(2) and poll(2) system calls
904	  unlocked_ioctl: called by the ioctl(2) system call.
906	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
907	 	 are used on 64 bit kernels.
909	  mmap: called by the mmap(2) system call
911	  open: called by the VFS when an inode should be opened. When the VFS
912		opens a file, it creates a new "struct file". It then calls the
913		open method for the newly allocated file structure. You might
914		think that the open method really belongs in
915		"struct inode_operations", and you may be right. I think it's
916		done the way it is because it makes filesystems simpler to
917		implement. The open() method is a good place to initialize the
918		"private_data" member in the file structure if you want to point
919		to a device structure
921	  flush: called by the close(2) system call to flush a file
923	  release: called when the last reference to an open file is closed
925	  fsync: called by the fsync(2) system call. Also see the section above
926		 entitled "Handling errors during writeback".
928	  fasync: called by the fcntl(2) system call when asynchronous
929		(non-blocking) mode is enabled for a file
931	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
932	  	commands
934	  get_unmapped_area: called by the mmap(2) system call
936	  check_flags: called by the fcntl(2) system call for F_SETFL command
938	  flock: called by the flock(2) system call
940	  splice_write: called by the VFS to splice data from a pipe to a file. This
941			method is used by the splice(2) system call
943	  splice_read: called by the VFS to splice data from file to a pipe. This
944		       method is used by the splice(2) system call
946	  setlease: called by the VFS to set or release a file lock lease. setlease
947		    implementations should call generic_setlease to record or remove
948		    the lease in the inode after setting it.
950	  fallocate: called by the VFS to preallocate blocks or punch a hole.
952	Note that the file operations are implemented by the specific
953	filesystem in which the inode resides. When opening a device node
954	(character or block special) most filesystems will call special
955	support routines in the VFS which will locate the required device
956	driver information. These support routines replace the filesystem file
957	operations with those for the device driver, and then proceed to call
958	the new open() method for the file. This is how opening a device file
959	in the filesystem eventually ends up calling the device driver open()
960	method.
963	Directory Entry Cache (dcache)
964	==============================
967	struct dentry_operations
968	------------------------
970	This describes how a filesystem can overload the standard dentry
971	operations. Dentries and the dcache are the domain of the VFS and the
972	individual filesystem implementations. Device drivers have no business
973	here. These methods may be set to NULL, as they are either optional or
974	the VFS uses a default. As of kernel 2.6.22, the following members are
975	defined:
977	struct dentry_operations {
978		int (*d_revalidate)(struct dentry *, unsigned int);
979		int (*d_weak_revalidate)(struct dentry *, unsigned int);
980		int (*d_hash)(const struct dentry *, struct qstr *);
981		int (*d_compare)(const struct dentry *,
982				unsigned int, const char *, const struct qstr *);
983		int (*d_delete)(const struct dentry *);
984		int (*d_init)(struct dentry *);
985		void (*d_release)(struct dentry *);
986		void (*d_iput)(struct dentry *, struct inode *);
987		char *(*d_dname)(struct dentry *, char *, int);
988		struct vfsmount *(*d_automount)(struct path *);
989		int (*d_manage)(const struct path *, bool);
990		struct dentry *(*d_real)(struct dentry *, const struct inode *,
991					 unsigned int, unsigned int);
992	};
994	  d_revalidate: called when the VFS needs to revalidate a dentry. This
995		is called whenever a name look-up finds a dentry in the
996		dcache. Most local filesystems leave this as NULL, because all their
997		dentries in the dcache are valid. Network filesystems are different
998		since things can change on the server without the client necessarily
999		being aware of it.
1001		This function should return a positive value if the dentry is still
1002		valid, and zero or a negative error code if it isn't.
1004		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
1005		If in rcu-walk mode, the filesystem must revalidate the dentry without
1006		blocking or storing to the dentry, d_parent and d_inode should not be
1007		used without care (because they can change and, in d_inode case, even
1008		become NULL under us).
1010		If a situation is encountered that rcu-walk cannot handle, return
1011		-ECHILD and it will be called again in ref-walk mode.
1013	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
1014		This is called when a path-walk ends at dentry that was not acquired by
1015		doing a lookup in the parent directory. This includes "/", "." and "..",
1016		as well as procfs-style symlinks and mountpoint traversal.
1018		In this case, we are less concerned with whether the dentry is still
1019		fully correct, but rather that the inode is still valid. As with
1020		d_revalidate, most local filesystems will set this to NULL since their
1021		dcache entries are always valid.
1023		This function has the same return code semantics as d_revalidate.
1025		d_weak_revalidate is only called after leaving rcu-walk mode.
1027	  d_hash: called when the VFS adds a dentry to the hash table. The first
1028		dentry passed to d_hash is the parent directory that the name is
1029		to be hashed into.
1031		Same locking and synchronisation rules as d_compare regarding
1032		what is safe to dereference etc.
1034	  d_compare: called to compare a dentry name with a given name. The first
1035		dentry is the parent of the dentry to be compared, the second is
1036		the child dentry. len and name string are properties of the dentry
1037		to be compared. qstr is the name to compare it with.
1039		Must be constant and idempotent, and should not take locks if
1040		possible, and should not or store into the dentry.
1041		Should not dereference pointers outside the dentry without
1042		lots of care (eg.  d_parent, d_inode, d_name should not be used).
1044		However, our vfsmount is pinned, and RCU held, so the dentries and
1045		inodes won't disappear, neither will our sb or filesystem module.
1046		->d_sb may be used.
1048		It is a tricky calling convention because it needs to be called under
1049		"rcu-walk", ie. without any locks or references on things.
1051	  d_delete: called when the last reference to a dentry is dropped and the
1052		dcache is deciding whether or not to cache it. Return 1 to delete
1053		immediately, or 0 to cache the dentry. Default is NULL which means to
1054		always cache a reachable dentry. d_delete must be constant and
1055		idempotent.
1057	  d_init: called when a dentry is allocated
1059	  d_release: called when a dentry is really deallocated
1061	  d_iput: called when a dentry loses its inode (just prior to its
1062		being deallocated). The default when this is NULL is that the
1063		VFS calls iput(). If you define this method, you must call
1064		iput() yourself
1066	  d_dname: called when the pathname of a dentry should be generated.
1067		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1068		pathname generation. (Instead of doing it when dentry is created,
1069		it's done only when the path is needed.). Real filesystems probably
1070		dont want to use it, because their dentries are present in global
1071		dcache hash, so their hash should be an invariant. As no lock is
1072		held, d_dname() should not try to modify the dentry itself, unless
1073		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1074		tricky. The correct way to return for example "Hello" is to put it
1075		at the end of the buffer, and returns a pointer to the first char.
1076		dynamic_dname() helper function is provided to take care of this.
1078		Example :
1080		static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1081		{
1082			return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1083					dentry->d_inode->i_ino);
1084		}
1086	  d_automount: called when an automount dentry is to be traversed (optional).
1087		This should create a new VFS mount record and return the record to the
1088		caller.  The caller is supplied with a path parameter giving the
1089		automount directory to describe the automount target and the parent
1090		VFS mount record to provide inheritable mount parameters.  NULL should
1091		be returned if someone else managed to make the automount first.  If
1092		the vfsmount creation failed, then an error code should be returned.
1093		If -EISDIR is returned, then the directory will be treated as an
1094		ordinary directory and returned to pathwalk to continue walking.
1096		If a vfsmount is returned, the caller will attempt to mount it on the
1097		mountpoint and will remove the vfsmount from its expiration list in
1098		the case of failure.  The vfsmount should be returned with 2 refs on
1099		it to prevent automatic expiration - the caller will clean up the
1100		additional ref.
1102		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1103		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1104		inode being added.
1106	  d_manage: called to allow the filesystem to manage the transition from a
1107		dentry (optional).  This allows autofs, for example, to hold up clients
1108		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1109		past and construct the subtree there.  0 should be returned to let the
1110		calling process continue.  -EISDIR can be returned to tell pathwalk to
1111		use this directory as an ordinary directory and to ignore anything
1112		mounted on it and not to check the automount flag.  Any other error
1113		code will abort pathwalk completely.
1115		If the 'rcu_walk' parameter is true, then the caller is doing a
1116		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1117		and the caller can be asked to leave it and call again by returning
1118		-ECHILD.  -EISDIR may also be returned to tell pathwalk to
1119		ignore d_automount or any mounts.
1121		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1122		dentry being transited from.
1124	  d_real: overlay/union type filesystems implement this method to return one of
1125		the underlying dentries hidden by the overlay.  It is used in three
1126		different modes:
1128		Called from open it may need to copy-up the file depending on the
1129		supplied open flags.  This mode is selected with a non-zero flags
1130		argument.  In this mode the d_real method can return an error.
1132		Called from file_dentry() it returns the real dentry matching the inode
1133		argument.  The real dentry may be from a lower layer already copied up,
1134		but still referenced from the file.  This mode is selected with a
1135		non-NULL inode argument.  This will always succeed.
1137		With NULL inode and zero flags the topmost real underlying dentry is
1138		returned.  This will always succeed.
1140		This method is never called with both non-NULL inode and non-zero flags.
1142	Each dentry has a pointer to its parent dentry, as well as a hash list
1143	of child dentries. Child dentries are basically like files in a
1144	directory.
1147	Directory Entry Cache API
1148	--------------------------
1150	There are a number of functions defined which permit a filesystem to
1151	manipulate dentries:
1153	  dget: open a new handle for an existing dentry (this just increments
1154		the usage count)
1156	  dput: close a handle for a dentry (decrements the usage count). If
1157		the usage count drops to 0, and the dentry is still in its
1158		parent's hash, the "d_delete" method is called to check whether
1159		it should be cached. If it should not be cached, or if the dentry
1160		is not hashed, it is deleted. Otherwise cached dentries are put
1161		into an LRU list to be reclaimed on memory shortage.
1163	  d_drop: this unhashes a dentry from its parents hash list. A
1164		subsequent call to dput() will deallocate the dentry if its
1165		usage count drops to 0
1167	  d_delete: delete a dentry. If there are no other open references to
1168		the dentry then the dentry is turned into a negative dentry
1169		(the d_iput() method is called). If there are other
1170		references, then d_drop() is called instead
1172	  d_add: add a dentry to its parents hash list and then calls
1173		d_instantiate()
1175	  d_instantiate: add a dentry to the alias hash list for the inode and
1176		updates the "d_inode" member. The "i_count" member in the
1177		inode structure should be set/incremented. If the inode
1178		pointer is NULL, the dentry is called a "negative
1179		dentry". This function is commonly called when an inode is
1180		created for an existing negative dentry
1182	  d_lookup: look up a dentry given its parent and path name component
1183		It looks up the child of that given name from the dcache
1184		hash table. If it is found, the reference count is incremented
1185		and the dentry is returned. The caller must use dput()
1186		to free the dentry when it finishes using it.
1188	Mount Options
1189	=============
1191	Parsing options
1192	---------------
1194	On mount and remount the filesystem is passed a string containing a
1195	comma separated list of mount options.  The options can have either of
1196	these forms:
1198	  option
1199	  option=value
1201	The <linux/parser.h> header defines an API that helps parse these
1202	options.  There are plenty of examples on how to use it in existing
1203	filesystems.
1205	Showing options
1206	---------------
1208	If a filesystem accepts mount options, it must define show_options()
1209	to show all the currently active options.  The rules are:
1211	  - options MUST be shown which are not default or their values differ
1212	    from the default
1214	  - options MAY be shown which are enabled by default or have their
1215	    default value
1217	Options used only internally between a mount helper and the kernel
1218	(such as file descriptors), or which only have an effect during the
1219	mounting (such as ones controlling the creation of a journal) are exempt
1220	from the above rules.
1222	The underlying reason for the above rules is to make sure, that a
1223	mount can be accurately replicated (e.g. umounting and mounting again)
1224	based on the information found in /proc/mounts.
1226	Resources
1227	=========
1229	(Note some of these resources are not up-to-date with the latest kernel
1230	 version.)
1232	Creating Linux virtual filesystems. 2002
1233	    <http://lwn.net/Articles/13325/>
1235	The Linux Virtual File-system Layer by Neil Brown. 1999
1236	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1238	A tour of the Linux VFS by Michael K. Johnson. 1996
1239	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1241	A small trail through the Linux kernel by Andries Brouwer. 2001
1242	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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