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Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.

1	
2		      Overview of the Linux Virtual File System
3	
4		Original author: Richard Gooch <rgooch@atnf.csiro.au>
5	
6			  Last updated on June 24, 2007.
7	
8	  Copyright (C) 1999 Richard Gooch
9	  Copyright (C) 2005 Pekka Enberg
10	
11	  This file is released under the GPLv2.
12	
13	
14	Introduction
15	============
16	
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.
22	
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.
26	
27	
28	Directory Entry Cache (dcache)
29	------------------------------
30	
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.
37	
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.
44	
45	
46	The Inode Object
47	----------------
48	
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).
56	
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.
65	
66	
67	The File Object
68	---------------
69	
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.
78	
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.
84	
85	
86	Registering and Mounting a Filesystem
87	=====================================
88	
89	To register and unregister a filesystem, use the following API
90	functions:
91	
92	   #include <linux/fs.h>
93	
94	   extern int register_filesystem(struct file_system_type *);
95	   extern int unregister_filesystem(struct file_system_type *);
96	
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.
103	
104	You can see all filesystems that are registered to the kernel in the
105	file /proc/filesystems.
106	
107	
108	struct file_system_type
109	-----------------------
110	
111	This describes the filesystem. As of kernel 2.6.39, the following
112	members are defined:
113	
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	};
126	
127	  name: the name of the filesystem type, such as "ext2", "iso9660",
128		"msdos" and so on
129	
130	  fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131	
132	  mount: the method to call when a new instance of this
133		filesystem should be mounted
134	
135	  kill_sb: the method to call when an instance of this filesystem
136		should be shut down
137	
138	  owner: for internal VFS use: you should initialize this to THIS_MODULE in
139	  	most cases.
140	
141	  next: for internal VFS use: you should initialize this to NULL
142	
143	  s_lock_key, s_umount_key: lockdep-specific
144	
145	The mount() method has the following arguments:
146	
147	  struct file_system_type *fs_type: describes the filesystem, partly initialized
148	  	by the specific filesystem code
149	
150	  int flags: mount flags
151	
152	  const char *dev_name: the device name we are mounting.
153	
154	  void *data: arbitrary mount options, usually comes as an ASCII
155		string (see "Mount Options" section)
156	
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).
160	
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.
166	
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.
171	
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.
176	
177	Usually, a filesystem uses one of the generic mount() implementations
178	and provides a fill_super() callback instead. The generic variants are:
179	
180	  mount_bdev: mount a filesystem residing on a block device
181	
182	  mount_nodev: mount a filesystem that is not backed by a device
183	
184	  mount_single: mount a filesystem which shares the instance between
185	  	all mounts
186	
187	A fill_super() callback implementation has the following arguments:
188	
189	  struct super_block *sb: the superblock structure. The callback
190	  	must initialize this properly.
191	
192	  void *data: arbitrary mount options, usually comes as an ASCII
193		string (see "Mount Options" section)
194	
195	  int silent: whether or not to be silent on error
196	
197	
198	The Superblock Object
199	=====================
200	
201	A superblock object represents a mounted filesystem.
202	
203	
204	struct super_operations
205	-----------------------
206	
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:
209	
210	struct super_operations {
211	        struct inode *(*alloc_inode)(struct super_block *sb);
212	        void (*destroy_inode)(struct inode *);
213	
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 *);
226	
227	        int (*show_options)(struct seq_file *, struct dentry *);
228	
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	};
234	
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).
239	
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.
245	
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.
250	
251	  dirty_inode: this method is called by the VFS to mark an inode dirty.
252	
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.
256	
257	  drop_inode: called when the last access to the inode is dropped,
258		with the inode->i_lock spinlock held.
259	
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)
264	
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. 
269	
270	  delete_inode: called when the VFS wants to delete an inode
271	
272	  put_super: called when the VFS wishes to free the superblock
273		(i.e. unmount). This is called with the superblock lock held
274	
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.
278	
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).
282	
283	  unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284	  	again.
285	
286	  statfs: called when the VFS needs to get filesystem statistics.
287	
288	  remount_fs: called when the filesystem is remounted. This is called
289		with the kernel lock held
290	
291	  clear_inode: called then the VFS clears the inode. Optional
292	
293	  umount_begin: called when the VFS is unmounting a filesystem.
294	
295	  show_options: called by the VFS to show mount options for
296		/proc/<pid>/mounts.  (see "Mount Options" section)
297	
298	  quota_read: called by the VFS to read from filesystem quota file.
299	
300	  quota_write: called by the VFS to write to filesystem quota file.
301	
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.
305	
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.
310	
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.
315	
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.
321	
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.
325	
326	
327	The Inode Object
328	================
329	
330	An inode object represents an object within the filesystem.
331	
332	
333	struct inode_operations
334	-----------------------
335	
336	This describes how the VFS can manipulate an inode in your
337	filesystem. As of kernel 2.6.22, the following members are defined:
338	
339	struct inode_operations {
340		int (*create) (struct inode *,struct dentry *, umode_t, bool);
341		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
342		int (*link) (struct dentry *,struct inode *,struct dentry *);
343		int (*unlink) (struct inode *,struct dentry *);
344		int (*symlink) (struct inode *,struct dentry *,const char *);
345		int (*mkdir) (struct inode *,struct dentry *,umode_t);
346		int (*rmdir) (struct inode *,struct dentry *);
347		int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
348		int (*rename) (struct inode *, struct dentry *,
349				struct inode *, struct dentry *);
350		int (*rename2) (struct inode *, struct dentry *,
351				struct inode *, struct dentry *, unsigned int);
352		int (*readlink) (struct dentry *, char __user *,int);
353	        void * (*follow_link) (struct dentry *, struct nameidata *);
354	        void (*put_link) (struct dentry *, struct nameidata *, void *);
355		int (*permission) (struct inode *, int);
356		int (*get_acl)(struct inode *, int);
357		int (*setattr) (struct dentry *, struct iattr *);
358		int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
359		int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
360		ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
361		ssize_t (*listxattr) (struct dentry *, char *, size_t);
362		int (*removexattr) (struct dentry *, const char *);
363		void (*update_time)(struct inode *, struct timespec *, int);
364		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
365				unsigned open_flag, umode_t create_mode, int *opened);
366		int (*tmpfile) (struct inode *, struct dentry *, umode_t);
367		int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
368	};
369	
370	Again, all methods are called without any locks being held, unless
371	otherwise noted.
372	
373	  create: called by the open(2) and creat(2) system calls. Only
374		required if you want to support regular files. The dentry you
375		get should not have an inode (i.e. it should be a negative
376		dentry). Here you will probably call d_instantiate() with the
377		dentry and the newly created inode
378	
379	  lookup: called when the VFS needs to look up an inode in a parent
380		directory. The name to look for is found in the dentry. This
381		method must call d_add() to insert the found inode into the
382		dentry. The "i_count" field in the inode structure should be
383		incremented. If the named inode does not exist a NULL inode
384		should be inserted into the dentry (this is called a negative
385		dentry). Returning an error code from this routine must only
386		be done on a real error, otherwise creating inodes with system
387		calls like create(2), mknod(2), mkdir(2) and so on will fail.
388		If you wish to overload the dentry methods then you should
389		initialise the "d_dop" field in the dentry; this is a pointer
390		to a struct "dentry_operations".
391		This method is called with the directory inode semaphore held
392	
393	  link: called by the link(2) system call. Only required if you want
394		to support hard links. You will probably need to call
395		d_instantiate() just as you would in the create() method
396	
397	  unlink: called by the unlink(2) system call. Only required if you
398		want to support deleting inodes
399	
400	  symlink: called by the symlink(2) system call. Only required if you
401		want to support symlinks. You will probably need to call
402		d_instantiate() just as you would in the create() method
403	
404	  mkdir: called by the mkdir(2) system call. Only required if you want
405		to support creating subdirectories. You will probably need to
406		call d_instantiate() just as you would in the create() method
407	
408	  rmdir: called by the rmdir(2) system call. Only required if you want
409		to support deleting subdirectories
410	
411	  mknod: called by the mknod(2) system call to create a device (char,
412		block) inode or a named pipe (FIFO) or socket. Only required
413		if you want to support creating these types of inodes. You
414		will probably need to call d_instantiate() just as you would
415		in the create() method
416	
417	  rename: called by the rename(2) system call to rename the object to
418		have the parent and name given by the second inode and dentry.
419	
420	  rename2: this has an additional flags argument compared to rename.
421		If no flags are supported by the filesystem then this method
422		need not be implemented.  If some flags are supported then the
423		filesystem must return -EINVAL for any unsupported or unknown
424		flags.  Currently the following flags are implemented:
425		(1) RENAME_NOREPLACE: this flag indicates that if the target
426		of the rename exists the rename should fail with -EEXIST
427		instead of replacing the target.  The VFS already checks for
428		existence, so for local filesystems the RENAME_NOREPLACE
429		implementation is equivalent to plain rename.
430		(2) RENAME_EXCHANGE: exchange source and target.  Both must
431		exist; this is checked by the VFS.  Unlike plain rename,
432		source and target may be of different type.
433	
434	  readlink: called by the readlink(2) system call. Only required if
435		you want to support reading symbolic links
436	
437	  follow_link: called by the VFS to follow a symbolic link to the
438		inode it points to.  Only required if you want to support
439		symbolic links.  This method returns a void pointer cookie
440		that is passed to put_link().
441	
442	  put_link: called by the VFS to release resources allocated by
443	  	follow_link().  The cookie returned by follow_link() is passed
444	  	to this method as the last parameter.  It is used by
445	  	filesystems such as NFS where page cache is not stable
446	  	(i.e. page that was installed when the symbolic link walk
447	  	started might not be in the page cache at the end of the
448	  	walk).
449	
450	  permission: called by the VFS to check for access rights on a POSIX-like
451	  	filesystem.
452	
453		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
454	        mode, the filesystem must check the permission without blocking or
455		storing to the inode.
456	
457		If a situation is encountered that rcu-walk cannot handle, return
458		-ECHILD and it will be called again in ref-walk mode.
459	
460	  setattr: called by the VFS to set attributes for a file. This method
461	  	is called by chmod(2) and related system calls.
462	
463	  getattr: called by the VFS to get attributes of a file. This method
464	  	is called by stat(2) and related system calls.
465	
466	  setxattr: called by the VFS to set an extended attribute for a file.
467	  	Extended attribute is a name:value pair associated with an
468	  	inode. This method is called by setxattr(2) system call.
469	
470	  getxattr: called by the VFS to retrieve the value of an extended
471	  	attribute name. This method is called by getxattr(2) function
472	  	call.
473	
474	  listxattr: called by the VFS to list all extended attributes for a
475	  	given file. This method is called by listxattr(2) system call.
476	
477	  removexattr: called by the VFS to remove an extended attribute from
478	  	a file. This method is called by removexattr(2) system call.
479	
480	  update_time: called by the VFS to update a specific time or the i_version of
481	  	an inode.  If this is not defined the VFS will update the inode itself
482	  	and call mark_inode_dirty_sync.
483	
484	  atomic_open: called on the last component of an open.  Using this optional
485	  	method the filesystem can look up, possibly create and open the file in
486	  	one atomic operation.  If it cannot perform this (e.g. the file type
487	  	turned out to be wrong) it may signal this by returning 1 instead of
488		usual 0 or -ve .  This method is only called if the last component is
489		negative or needs lookup.  Cached positive dentries are still handled by
490		f_op->open().  If the file was created, the FILE_CREATED flag should be
491		set in "opened".  In case of O_EXCL the method must only succeed if the
492		file didn't exist and hence FILE_CREATED shall always be set on success.
493	
494	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
495		atomically creating, opening and unlinking a file in given directory.
496	
497	The Address Space Object
498	========================
499	
500	The address space object is used to group and manage pages in the page
501	cache.  It can be used to keep track of the pages in a file (or
502	anything else) and also track the mapping of sections of the file into
503	process address spaces.
504	
505	There are a number of distinct yet related services that an
506	address-space can provide.  These include communicating memory
507	pressure, page lookup by address, and keeping track of pages tagged as
508	Dirty or Writeback.
509	
510	The first can be used independently to the others.  The VM can try to
511	either write dirty pages in order to clean them, or release clean
512	pages in order to reuse them.  To do this it can call the ->writepage
513	method on dirty pages, and ->releasepage on clean pages with
514	PagePrivate set. Clean pages without PagePrivate and with no external
515	references will be released without notice being given to the
516	address_space.
517	
518	To achieve this functionality, pages need to be placed on an LRU with
519	lru_cache_add and mark_page_active needs to be called whenever the
520	page is used.
521	
522	Pages are normally kept in a radix tree index by ->index. This tree
523	maintains information about the PG_Dirty and PG_Writeback status of
524	each page, so that pages with either of these flags can be found
525	quickly.
526	
527	The Dirty tag is primarily used by mpage_writepages - the default
528	->writepages method.  It uses the tag to find dirty pages to call
529	->writepage on.  If mpage_writepages is not used (i.e. the address
530	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
531	almost unused.  write_inode_now and sync_inode do use it (through
532	__sync_single_inode) to check if ->writepages has been successful in
533	writing out the whole address_space.
534	
535	The Writeback tag is used by filemap*wait* and sync_page* functions,
536	via filemap_fdatawait_range, to wait for all writeback to
537	complete.  While waiting ->sync_page (if defined) will be called on
538	each page that is found to require writeback.
539	
540	An address_space handler may attach extra information to a page,
541	typically using the 'private' field in the 'struct page'.  If such
542	information is attached, the PG_Private flag should be set.  This will
543	cause various VM routines to make extra calls into the address_space
544	handler to deal with that data.
545	
546	An address space acts as an intermediate between storage and
547	application.  Data is read into the address space a whole page at a
548	time, and provided to the application either by copying of the page,
549	or by memory-mapping the page.
550	Data is written into the address space by the application, and then
551	written-back to storage typically in whole pages, however the
552	address_space has finer control of write sizes.
553	
554	The read process essentially only requires 'readpage'.  The write
555	process is more complicated and uses write_begin/write_end or
556	set_page_dirty to write data into the address_space, and writepage,
557	sync_page, and writepages to writeback data to storage.
558	
559	Adding and removing pages to/from an address_space is protected by the
560	inode's i_mutex.
561	
562	When data is written to a page, the PG_Dirty flag should be set.  It
563	typically remains set until writepage asks for it to be written.  This
564	should clear PG_Dirty and set PG_Writeback.  It can be actually
565	written at any point after PG_Dirty is clear.  Once it is known to be
566	safe, PG_Writeback is cleared.
567	
568	Writeback makes use of a writeback_control structure...
569	
570	struct address_space_operations
571	-------------------------------
572	
573	This describes how the VFS can manipulate mapping of a file to page cache in
574	your filesystem. The following members are defined:
575	
576	struct address_space_operations {
577		int (*writepage)(struct page *page, struct writeback_control *wbc);
578		int (*readpage)(struct file *, struct page *);
579		int (*writepages)(struct address_space *, struct writeback_control *);
580		int (*set_page_dirty)(struct page *page);
581		int (*readpages)(struct file *filp, struct address_space *mapping,
582				struct list_head *pages, unsigned nr_pages);
583		int (*write_begin)(struct file *, struct address_space *mapping,
584					loff_t pos, unsigned len, unsigned flags,
585					struct page **pagep, void **fsdata);
586		int (*write_end)(struct file *, struct address_space *mapping,
587					loff_t pos, unsigned len, unsigned copied,
588					struct page *page, void *fsdata);
589		sector_t (*bmap)(struct address_space *, sector_t);
590		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
591		int (*releasepage) (struct page *, int);
592		void (*freepage)(struct page *);
593		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t offset);
594		/* migrate the contents of a page to the specified target */
595		int (*migratepage) (struct page *, struct page *);
596		int (*launder_page) (struct page *);
597		int (*is_partially_uptodate) (struct page *, unsigned long,
598						unsigned long);
599		void (*is_dirty_writeback) (struct page *, bool *, bool *);
600		int (*error_remove_page) (struct mapping *mapping, struct page *page);
601		int (*swap_activate)(struct file *);
602		int (*swap_deactivate)(struct file *);
603	};
604	
605	  writepage: called by the VM to write a dirty page to backing store.
606	      This may happen for data integrity reasons (i.e. 'sync'), or
607	      to free up memory (flush).  The difference can be seen in
608	      wbc->sync_mode.
609	      The PG_Dirty flag has been cleared and PageLocked is true.
610	      writepage should start writeout, should set PG_Writeback,
611	      and should make sure the page is unlocked, either synchronously
612	      or asynchronously when the write operation completes.
613	
614	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
615	      try too hard if there are problems, and may choose to write out
616	      other pages from the mapping if that is easier (e.g. due to
617	      internal dependencies).  If it chooses not to start writeout, it
618	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
619	      calling ->writepage on that page.
620	
621	      See the file "Locking" for more details.
622	
623	  readpage: called by the VM to read a page from backing store.
624	       The page will be Locked when readpage is called, and should be
625	       unlocked and marked uptodate once the read completes.
626	       If ->readpage discovers that it needs to unlock the page for
627	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
628	       In this case, the page will be relocated, relocked and if
629	       that all succeeds, ->readpage will be called again.
630	
631	  writepages: called by the VM to write out pages associated with the
632	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
633	  	the writeback_control will specify a range of pages that must be
634	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
635		and that many pages should be written if possible.
636		If no ->writepages is given, then mpage_writepages is used
637	  	instead.  This will choose pages from the address space that are
638	  	tagged as DIRTY and will pass them to ->writepage.
639	
640	  set_page_dirty: called by the VM to set a page dirty.
641	        This is particularly needed if an address space attaches
642	        private data to a page, and that data needs to be updated when
643	        a page is dirtied.  This is called, for example, when a memory
644		mapped page gets modified.
645		If defined, it should set the PageDirty flag, and the
646	        PAGECACHE_TAG_DIRTY tag in the radix tree.
647	
648	  readpages: called by the VM to read pages associated with the address_space
649	  	object. This is essentially just a vector version of
650	  	readpage.  Instead of just one page, several pages are
651	  	requested.
652		readpages is only used for read-ahead, so read errors are
653	  	ignored.  If anything goes wrong, feel free to give up.
654	
655	  write_begin:
656		Called by the generic buffered write code to ask the filesystem to
657		prepare to write len bytes at the given offset in the file. The
658		address_space should check that the write will be able to complete,
659		by allocating space if necessary and doing any other internal
660		housekeeping.  If the write will update parts of any basic-blocks on
661		storage, then those blocks should be pre-read (if they haven't been
662		read already) so that the updated blocks can be written out properly.
663	
664	        The filesystem must return the locked pagecache page for the specified
665		offset, in *pagep, for the caller to write into.
666	
667		It must be able to cope with short writes (where the length passed to
668		write_begin is greater than the number of bytes copied into the page).
669	
670		flags is a field for AOP_FLAG_xxx flags, described in
671		include/linux/fs.h.
672	
673	        A void * may be returned in fsdata, which then gets passed into
674	        write_end.
675	
676	        Returns 0 on success; < 0 on failure (which is the error code), in
677		which case write_end is not called.
678	
679	  write_end: After a successful write_begin, and data copy, write_end must
680	        be called. len is the original len passed to write_begin, and copied
681	        is the amount that was able to be copied (copied == len is always true
682		if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
683	
684	        The filesystem must take care of unlocking the page and releasing it
685	        refcount, and updating i_size.
686	
687	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
688	        that were able to be copied into pagecache.
689	
690	  bmap: called by the VFS to map a logical block offset within object to
691	  	physical block number. This method is used by the FIBMAP
692	  	ioctl and for working with swap-files.  To be able to swap to
693	  	a file, the file must have a stable mapping to a block
694	  	device.  The swap system does not go through the filesystem
695	  	but instead uses bmap to find out where the blocks in the file
696	  	are and uses those addresses directly.
697	
698	  dentry_open: *WARNING: probably going away soon, do not use!* This is an
699		alternative to f_op->open(), the difference is that this method may open
700		a file not necessarily originating from the same filesystem as the one
701		i_op->open() was called on.  It may be useful for stacking filesystems
702		which want to allow native I/O directly on underlying files.
703	
704	
705	  invalidatepage: If a page has PagePrivate set, then invalidatepage
706	        will be called when part or all of the page is to be removed
707		from the address space.  This generally corresponds to either a
708		truncation, punch hole  or a complete invalidation of the address
709		space (in the latter case 'offset' will always be 0 and 'length'
710		will be PAGE_CACHE_SIZE). Any private data associated with the page
711		should be updated to reflect this truncation.  If offset is 0 and
712		length is PAGE_CACHE_SIZE, then the private data should be released,
713		because the page must be able to be completely discarded.  This may
714		be done by calling the ->releasepage function, but in this case the
715		release MUST succeed.
716	
717	  releasepage: releasepage is called on PagePrivate pages to indicate
718	        that the page should be freed if possible.  ->releasepage
719	        should remove any private data from the page and clear the
720	        PagePrivate flag. If releasepage() fails for some reason, it must
721		indicate failure with a 0 return value.
722		releasepage() is used in two distinct though related cases.  The
723		first is when the VM finds a clean page with no active users and
724	        wants to make it a free page.  If ->releasepage succeeds, the
725	        page will be removed from the address_space and become free.
726	
727		The second case is when a request has been made to invalidate
728	        some or all pages in an address_space.  This can happen
729	        through the fadvice(POSIX_FADV_DONTNEED) system call or by the
730	        filesystem explicitly requesting it as nfs and 9fs do (when
731	        they believe the cache may be out of date with storage) by
732	        calling invalidate_inode_pages2().
733		If the filesystem makes such a call, and needs to be certain
734	        that all pages are invalidated, then its releasepage will
735	        need to ensure this.  Possibly it can clear the PageUptodate
736	        bit if it cannot free private data yet.
737	
738	  freepage: freepage is called once the page is no longer visible in
739	        the page cache in order to allow the cleanup of any private
740		data. Since it may be called by the memory reclaimer, it
741		should not assume that the original address_space mapping still
742		exists, and it should not block.
743	
744	  direct_IO: called by the generic read/write routines to perform
745	        direct_IO - that is IO requests which bypass the page cache
746	        and transfer data directly between the storage and the
747	        application's address space.
748	
749	  migrate_page:  This is used to compact the physical memory usage.
750	        If the VM wants to relocate a page (maybe off a memory card
751	        that is signalling imminent failure) it will pass a new page
752		and an old page to this function.  migrate_page should
753		transfer any private data across and update any references
754	        that it has to the page.
755	
756	  launder_page: Called before freeing a page - it writes back the dirty page. To
757	  	prevent redirtying the page, it is kept locked during the whole
758		operation.
759	
760	  is_partially_uptodate: Called by the VM when reading a file through the
761		pagecache when the underlying blocksize != pagesize. If the required
762		block is up to date then the read can complete without needing the IO
763		to bring the whole page up to date.
764	
765	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
766		The VM uses dirty and writeback information to determine if it needs
767		to stall to allow flushers a chance to complete some IO. Ordinarily
768		it can use PageDirty and PageWriteback but some filesystems have
769		more complex state (unstable pages in NFS prevent reclaim) or
770		do not set those flags due to locking problems (jbd). This callback
771		allows a filesystem to indicate to the VM if a page should be
772		treated as dirty or writeback for the purposes of stalling.
773	
774	  error_remove_page: normally set to generic_error_remove_page if truncation
775		is ok for this address space. Used for memory failure handling.
776		Setting this implies you deal with pages going away under you,
777		unless you have them locked or reference counts increased.
778	
779	  swap_activate: Called when swapon is used on a file to allocate
780		space if necessary and pin the block lookup information in
781		memory. A return value of zero indicates success,
782		in which case this file can be used to back swapspace. The
783		swapspace operations will be proxied to this address space's
784		->swap_{out,in} methods.
785	
786	  swap_deactivate: Called during swapoff on files where swap_activate
787		was successful.
788	
789	
790	The File Object
791	===============
792	
793	A file object represents a file opened by a process.
794	
795	
796	struct file_operations
797	----------------------
798	
799	This describes how the VFS can manipulate an open file. As of kernel
800	3.12, the following members are defined:
801	
802	struct file_operations {
803		struct module *owner;
804		loff_t (*llseek) (struct file *, loff_t, int);
805		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
806		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
807		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
808		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
809		int (*iterate) (struct file *, struct dir_context *);
810		unsigned int (*poll) (struct file *, struct poll_table_struct *);
811		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
812		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
813		int (*mmap) (struct file *, struct vm_area_struct *);
814		int (*open) (struct inode *, struct file *);
815		int (*flush) (struct file *);
816		int (*release) (struct inode *, struct file *);
817		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
818		int (*aio_fsync) (struct kiocb *, int datasync);
819		int (*fasync) (int, struct file *, int);
820		int (*lock) (struct file *, int, struct file_lock *);
821		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
822		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
823		int (*check_flags)(int);
824		int (*flock) (struct file *, int, struct file_lock *);
825		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
826		ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
827		int (*setlease)(struct file *, long arg, struct file_lock **, void **);
828		long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
829		void (*show_fdinfo)(struct seq_file *m, struct file *f);
830	};
831	
832	Again, all methods are called without any locks being held, unless
833	otherwise noted.
834	
835	  llseek: called when the VFS needs to move the file position index
836	
837	  read: called by read(2) and related system calls
838	
839	  read_iter: possibly asynchronous read with iov_iter as destination
840	
841	  write: called by write(2) and related system calls
842	
843	  write_iter: possibly asynchronous write with iov_iter as source
844	
845	  iterate: called when the VFS needs to read the directory contents
846	
847	  poll: called by the VFS when a process wants to check if there is
848		activity on this file and (optionally) go to sleep until there
849		is activity. Called by the select(2) and poll(2) system calls
850	
851	  unlocked_ioctl: called by the ioctl(2) system call.
852	
853	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
854	 	 are used on 64 bit kernels.
855	
856	  mmap: called by the mmap(2) system call
857	
858	  open: called by the VFS when an inode should be opened. When the VFS
859		opens a file, it creates a new "struct file". It then calls the
860		open method for the newly allocated file structure. You might
861		think that the open method really belongs in
862		"struct inode_operations", and you may be right. I think it's
863		done the way it is because it makes filesystems simpler to
864		implement. The open() method is a good place to initialize the
865		"private_data" member in the file structure if you want to point
866		to a device structure
867	
868	  flush: called by the close(2) system call to flush a file
869	
870	  release: called when the last reference to an open file is closed
871	
872	  fsync: called by the fsync(2) system call
873	
874	  fasync: called by the fcntl(2) system call when asynchronous
875		(non-blocking) mode is enabled for a file
876	
877	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
878	  	commands
879	
880	  get_unmapped_area: called by the mmap(2) system call
881	
882	  check_flags: called by the fcntl(2) system call for F_SETFL command
883	
884	  flock: called by the flock(2) system call
885	
886	  splice_write: called by the VFS to splice data from a pipe to a file. This
887			method is used by the splice(2) system call
888	
889	  splice_read: called by the VFS to splice data from file to a pipe. This
890		       method is used by the splice(2) system call
891	
892	  setlease: called by the VFS to set or release a file lock lease. setlease
893		    implementations should call generic_setlease to record or remove
894		    the lease in the inode after setting it.
895	
896	  fallocate: called by the VFS to preallocate blocks or punch a hole.
897	
898	Note that the file operations are implemented by the specific
899	filesystem in which the inode resides. When opening a device node
900	(character or block special) most filesystems will call special
901	support routines in the VFS which will locate the required device
902	driver information. These support routines replace the filesystem file
903	operations with those for the device driver, and then proceed to call
904	the new open() method for the file. This is how opening a device file
905	in the filesystem eventually ends up calling the device driver open()
906	method.
907	
908	
909	Directory Entry Cache (dcache)
910	==============================
911	
912	
913	struct dentry_operations
914	------------------------
915	
916	This describes how a filesystem can overload the standard dentry
917	operations. Dentries and the dcache are the domain of the VFS and the
918	individual filesystem implementations. Device drivers have no business
919	here. These methods may be set to NULL, as they are either optional or
920	the VFS uses a default. As of kernel 2.6.22, the following members are
921	defined:
922	
923	struct dentry_operations {
924		int (*d_revalidate)(struct dentry *, unsigned int);
925		int (*d_weak_revalidate)(struct dentry *, unsigned int);
926		int (*d_hash)(const struct dentry *, struct qstr *);
927		int (*d_compare)(const struct dentry *, const struct dentry *,
928				unsigned int, const char *, const struct qstr *);
929		int (*d_delete)(const struct dentry *);
930		void (*d_release)(struct dentry *);
931		void (*d_iput)(struct dentry *, struct inode *);
932		char *(*d_dname)(struct dentry *, char *, int);
933		struct vfsmount *(*d_automount)(struct path *);
934		int (*d_manage)(struct dentry *, bool);
935	};
936	
937	  d_revalidate: called when the VFS needs to revalidate a dentry. This
938		is called whenever a name look-up finds a dentry in the
939		dcache. Most local filesystems leave this as NULL, because all their
940		dentries in the dcache are valid. Network filesystems are different
941		since things can change on the server without the client necessarily
942		being aware of it.
943	
944		This function should return a positive value if the dentry is still
945		valid, and zero or a negative error code if it isn't.
946	
947		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
948		If in rcu-walk mode, the filesystem must revalidate the dentry without
949		blocking or storing to the dentry, d_parent and d_inode should not be
950		used without care (because they can change and, in d_inode case, even
951		become NULL under us).
952	
953		If a situation is encountered that rcu-walk cannot handle, return
954		-ECHILD and it will be called again in ref-walk mode.
955	
956	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
957		This is called when a path-walk ends at dentry that was not acquired by
958		doing a lookup in the parent directory. This includes "/", "." and "..",
959		as well as procfs-style symlinks and mountpoint traversal.
960	
961		In this case, we are less concerned with whether the dentry is still
962		fully correct, but rather that the inode is still valid. As with
963		d_revalidate, most local filesystems will set this to NULL since their
964		dcache entries are always valid.
965	
966		This function has the same return code semantics as d_revalidate.
967	
968		d_weak_revalidate is only called after leaving rcu-walk mode.
969	
970	  d_hash: called when the VFS adds a dentry to the hash table. The first
971		dentry passed to d_hash is the parent directory that the name is
972		to be hashed into.
973	
974		Same locking and synchronisation rules as d_compare regarding
975		what is safe to dereference etc.
976	
977	  d_compare: called to compare a dentry name with a given name. The first
978		dentry is the parent of the dentry to be compared, the second is
979		the child dentry. len and name string are properties of the dentry
980		to be compared. qstr is the name to compare it with.
981	
982		Must be constant and idempotent, and should not take locks if
983		possible, and should not or store into the dentry.
984		Should not dereference pointers outside the dentry without
985		lots of care (eg.  d_parent, d_inode, d_name should not be used).
986	
987		However, our vfsmount is pinned, and RCU held, so the dentries and
988		inodes won't disappear, neither will our sb or filesystem module.
989		->d_sb may be used.
990	
991		It is a tricky calling convention because it needs to be called under
992		"rcu-walk", ie. without any locks or references on things.
993	
994	  d_delete: called when the last reference to a dentry is dropped and the
995		dcache is deciding whether or not to cache it. Return 1 to delete
996		immediately, or 0 to cache the dentry. Default is NULL which means to
997		always cache a reachable dentry. d_delete must be constant and
998		idempotent.
999	
1000	  d_release: called when a dentry is really deallocated
1001	
1002	  d_iput: called when a dentry loses its inode (just prior to its
1003		being deallocated). The default when this is NULL is that the
1004		VFS calls iput(). If you define this method, you must call
1005		iput() yourself
1006	
1007	  d_dname: called when the pathname of a dentry should be generated.
1008		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1009		pathname generation. (Instead of doing it when dentry is created,
1010		it's done only when the path is needed.). Real filesystems probably
1011		dont want to use it, because their dentries are present in global
1012		dcache hash, so their hash should be an invariant. As no lock is
1013		held, d_dname() should not try to modify the dentry itself, unless
1014		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1015		tricky. The correct way to return for example "Hello" is to put it
1016		at the end of the buffer, and returns a pointer to the first char.
1017		dynamic_dname() helper function is provided to take care of this.
1018	
1019	  d_automount: called when an automount dentry is to be traversed (optional).
1020		This should create a new VFS mount record and return the record to the
1021		caller.  The caller is supplied with a path parameter giving the
1022		automount directory to describe the automount target and the parent
1023		VFS mount record to provide inheritable mount parameters.  NULL should
1024		be returned if someone else managed to make the automount first.  If
1025		the vfsmount creation failed, then an error code should be returned.
1026		If -EISDIR is returned, then the directory will be treated as an
1027		ordinary directory and returned to pathwalk to continue walking.
1028	
1029		If a vfsmount is returned, the caller will attempt to mount it on the
1030		mountpoint and will remove the vfsmount from its expiration list in
1031		the case of failure.  The vfsmount should be returned with 2 refs on
1032		it to prevent automatic expiration - the caller will clean up the
1033		additional ref.
1034	
1035		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1036		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1037		inode being added.
1038	
1039	  d_manage: called to allow the filesystem to manage the transition from a
1040		dentry (optional).  This allows autofs, for example, to hold up clients
1041		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1042		past and construct the subtree there.  0 should be returned to let the
1043		calling process continue.  -EISDIR can be returned to tell pathwalk to
1044		use this directory as an ordinary directory and to ignore anything
1045		mounted on it and not to check the automount flag.  Any other error
1046		code will abort pathwalk completely.
1047	
1048		If the 'rcu_walk' parameter is true, then the caller is doing a
1049		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1050		and the caller can be asked to leave it and call again by returning
1051		-ECHILD.  -EISDIR may also be returned to tell pathwalk to
1052		ignore d_automount or any mounts.
1053	
1054		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1055		dentry being transited from.
1056	
1057	Example :
1058	
1059	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1060	{
1061		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1062					dentry->d_inode->i_ino);
1063	}
1064	
1065	Each dentry has a pointer to its parent dentry, as well as a hash list
1066	of child dentries. Child dentries are basically like files in a
1067	directory.
1068	
1069	
1070	Directory Entry Cache API
1071	--------------------------
1072	
1073	There are a number of functions defined which permit a filesystem to
1074	manipulate dentries:
1075	
1076	  dget: open a new handle for an existing dentry (this just increments
1077		the usage count)
1078	
1079	  dput: close a handle for a dentry (decrements the usage count). If
1080		the usage count drops to 0, and the dentry is still in its
1081		parent's hash, the "d_delete" method is called to check whether
1082		it should be cached. If it should not be cached, or if the dentry
1083		is not hashed, it is deleted. Otherwise cached dentries are put
1084		into an LRU list to be reclaimed on memory shortage.
1085	
1086	  d_drop: this unhashes a dentry from its parents hash list. A
1087		subsequent call to dput() will deallocate the dentry if its
1088		usage count drops to 0
1089	
1090	  d_delete: delete a dentry. If there are no other open references to
1091		the dentry then the dentry is turned into a negative dentry
1092		(the d_iput() method is called). If there are other
1093		references, then d_drop() is called instead
1094	
1095	  d_add: add a dentry to its parents hash list and then calls
1096		d_instantiate()
1097	
1098	  d_instantiate: add a dentry to the alias hash list for the inode and
1099		updates the "d_inode" member. The "i_count" member in the
1100		inode structure should be set/incremented. If the inode
1101		pointer is NULL, the dentry is called a "negative
1102		dentry". This function is commonly called when an inode is
1103		created for an existing negative dentry
1104	
1105	  d_lookup: look up a dentry given its parent and path name component
1106		It looks up the child of that given name from the dcache
1107		hash table. If it is found, the reference count is incremented
1108		and the dentry is returned. The caller must use dput()
1109		to free the dentry when it finishes using it.
1110	
1111	Mount Options
1112	=============
1113	
1114	Parsing options
1115	---------------
1116	
1117	On mount and remount the filesystem is passed a string containing a
1118	comma separated list of mount options.  The options can have either of
1119	these forms:
1120	
1121	  option
1122	  option=value
1123	
1124	The <linux/parser.h> header defines an API that helps parse these
1125	options.  There are plenty of examples on how to use it in existing
1126	filesystems.
1127	
1128	Showing options
1129	---------------
1130	
1131	If a filesystem accepts mount options, it must define show_options()
1132	to show all the currently active options.  The rules are:
1133	
1134	  - options MUST be shown which are not default or their values differ
1135	    from the default
1136	
1137	  - options MAY be shown which are enabled by default or have their
1138	    default value
1139	
1140	Options used only internally between a mount helper and the kernel
1141	(such as file descriptors), or which only have an effect during the
1142	mounting (such as ones controlling the creation of a journal) are exempt
1143	from the above rules.
1144	
1145	The underlying reason for the above rules is to make sure, that a
1146	mount can be accurately replicated (e.g. umounting and mounting again)
1147	based on the information found in /proc/mounts.
1148	
1149	A simple method of saving options at mount/remount time and showing
1150	them is provided with the save_mount_options() and
1151	generic_show_options() helper functions.  Please note, that using
1152	these may have drawbacks.  For more info see header comments for these
1153	functions in fs/namespace.c.
1154	
1155	Resources
1156	=========
1157	
1158	(Note some of these resources are not up-to-date with the latest kernel
1159	 version.)
1160	
1161	Creating Linux virtual filesystems. 2002
1162	    <http://lwn.net/Articles/13325/>
1163	
1164	The Linux Virtual File-system Layer by Neil Brown. 1999
1165	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1166	
1167	A tour of the Linux VFS by Michael K. Johnson. 1996
1168	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1169	
1170	A small trail through the Linux kernel by Andries Brouwer. 2001
1171	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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