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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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 inode_alloc() 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	};
368	
369	Again, all methods are called without any locks being held, unless
370	otherwise noted.
371	
372	  create: called by the open(2) and creat(2) system calls. Only
373		required if you want to support regular files. The dentry you
374		get should not have an inode (i.e. it should be a negative
375		dentry). Here you will probably call d_instantiate() with the
376		dentry and the newly created inode
377	
378	  lookup: called when the VFS needs to look up an inode in a parent
379		directory. The name to look for is found in the dentry. This
380		method must call d_add() to insert the found inode into the
381		dentry. The "i_count" field in the inode structure should be
382		incremented. If the named inode does not exist a NULL inode
383		should be inserted into the dentry (this is called a negative
384		dentry). Returning an error code from this routine must only
385		be done on a real error, otherwise creating inodes with system
386		calls like create(2), mknod(2), mkdir(2) and so on will fail.
387		If you wish to overload the dentry methods then you should
388		initialise the "d_dop" field in the dentry; this is a pointer
389		to a struct "dentry_operations".
390		This method is called with the directory inode semaphore held
391	
392	  link: called by the link(2) system call. Only required if you want
393		to support hard links. You will probably need to call
394		d_instantiate() just as you would in the create() method
395	
396	  unlink: called by the unlink(2) system call. Only required if you
397		want to support deleting inodes
398	
399	  symlink: called by the symlink(2) system call. Only required if you
400		want to support symlinks. You will probably need to call
401		d_instantiate() just as you would in the create() method
402	
403	  mkdir: called by the mkdir(2) system call. Only required if you want
404		to support creating subdirectories. You will probably need to
405		call d_instantiate() just as you would in the create() method
406	
407	  rmdir: called by the rmdir(2) system call. Only required if you want
408		to support deleting subdirectories
409	
410	  mknod: called by the mknod(2) system call to create a device (char,
411		block) inode or a named pipe (FIFO) or socket. Only required
412		if you want to support creating these types of inodes. You
413		will probably need to call d_instantiate() just as you would
414		in the create() method
415	
416	  rename: called by the rename(2) system call to rename the object to
417		have the parent and name given by the second inode and dentry.
418	
419	  rename2: this has an additional flags argument compared to rename.
420		If no flags are supported by the filesystem then this method
421		need not be implemented.  If some flags are supported then the
422		filesystem must return -EINVAL for any unsupported or unknown
423		flags.  Currently the following flags are implemented:
424		(1) RENAME_NOREPLACE: this flag indicates that if the target
425		of the rename exists the rename should fail with -EEXIST
426		instead of replacing the target.  The VFS already checks for
427		existence, so for local filesystems the RENAME_NOREPLACE
428		implementation is equivalent to plain rename.
429		(2) RENAME_EXCHANGE: exchange source and target.  Both must
430		exist; this is checked by the VFS.  Unlike plain rename,
431		source and target may be of different type.
432	
433	  readlink: called by the readlink(2) system call. Only required if
434		you want to support reading symbolic links
435	
436	  follow_link: called by the VFS to follow a symbolic link to the
437		inode it points to.  Only required if you want to support
438		symbolic links.  This method returns a void pointer cookie
439		that is passed to put_link().
440	
441	  put_link: called by the VFS to release resources allocated by
442	  	follow_link().  The cookie returned by follow_link() is passed
443	  	to this method as the last parameter.  It is used by
444	  	filesystems such as NFS where page cache is not stable
445	  	(i.e. page that was installed when the symbolic link walk
446	  	started might not be in the page cache at the end of the
447	  	walk).
448	
449	  permission: called by the VFS to check for access rights on a POSIX-like
450	  	filesystem.
451	
452		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
453	        mode, the filesystem must check the permission without blocking or
454		storing to the inode.
455	
456		If a situation is encountered that rcu-walk cannot handle, return
457		-ECHILD and it will be called again in ref-walk mode.
458	
459	  setattr: called by the VFS to set attributes for a file. This method
460	  	is called by chmod(2) and related system calls.
461	
462	  getattr: called by the VFS to get attributes of a file. This method
463	  	is called by stat(2) and related system calls.
464	
465	  setxattr: called by the VFS to set an extended attribute for a file.
466	  	Extended attribute is a name:value pair associated with an
467	  	inode. This method is called by setxattr(2) system call.
468	
469	  getxattr: called by the VFS to retrieve the value of an extended
470	  	attribute name. This method is called by getxattr(2) function
471	  	call.
472	
473	  listxattr: called by the VFS to list all extended attributes for a
474	  	given file. This method is called by listxattr(2) system call.
475	
476	  removexattr: called by the VFS to remove an extended attribute from
477	  	a file. This method is called by removexattr(2) system call.
478	
479	  update_time: called by the VFS to update a specific time or the i_version of
480	  	an inode.  If this is not defined the VFS will update the inode itself
481	  	and call mark_inode_dirty_sync.
482	
483	  atomic_open: called on the last component of an open.  Using this optional
484	  	method the filesystem can look up, possibly create and open the file in
485	  	one atomic operation.  If it cannot perform this (e.g. the file type
486	  	turned out to be wrong) it may signal this by returning 1 instead of
487		usual 0 or -ve .  This method is only called if the last component is
488		negative or needs lookup.  Cached positive dentries are still handled by
489		f_op->open().  If the file was created, the FILE_CREATED flag should be
490		set in "opened".  In case of O_EXCL the method must only succeed if the
491		file didn't exist and hence FILE_CREATED shall always be set on success.
492	
493	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
494		atomically creating, opening and unlinking a file in given directory.
495	
496	The Address Space Object
497	========================
498	
499	The address space object is used to group and manage pages in the page
500	cache.  It can be used to keep track of the pages in a file (or
501	anything else) and also track the mapping of sections of the file into
502	process address spaces.
503	
504	There are a number of distinct yet related services that an
505	address-space can provide.  These include communicating memory
506	pressure, page lookup by address, and keeping track of pages tagged as
507	Dirty or Writeback.
508	
509	The first can be used independently to the others.  The VM can try to
510	either write dirty pages in order to clean them, or release clean
511	pages in order to reuse them.  To do this it can call the ->writepage
512	method on dirty pages, and ->releasepage on clean pages with
513	PagePrivate set. Clean pages without PagePrivate and with no external
514	references will be released without notice being given to the
515	address_space.
516	
517	To achieve this functionality, pages need to be placed on an LRU with
518	lru_cache_add and mark_page_active needs to be called whenever the
519	page is used.
520	
521	Pages are normally kept in a radix tree index by ->index. This tree
522	maintains information about the PG_Dirty and PG_Writeback status of
523	each page, so that pages with either of these flags can be found
524	quickly.
525	
526	The Dirty tag is primarily used by mpage_writepages - the default
527	->writepages method.  It uses the tag to find dirty pages to call
528	->writepage on.  If mpage_writepages is not used (i.e. the address
529	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
530	almost unused.  write_inode_now and sync_inode do use it (through
531	__sync_single_inode) to check if ->writepages has been successful in
532	writing out the whole address_space.
533	
534	The Writeback tag is used by filemap*wait* and sync_page* functions,
535	via filemap_fdatawait_range, to wait for all writeback to
536	complete.  While waiting ->sync_page (if defined) will be called on
537	each page that is found to require writeback.
538	
539	An address_space handler may attach extra information to a page,
540	typically using the 'private' field in the 'struct page'.  If such
541	information is attached, the PG_Private flag should be set.  This will
542	cause various VM routines to make extra calls into the address_space
543	handler to deal with that data.
544	
545	An address space acts as an intermediate between storage and
546	application.  Data is read into the address space a whole page at a
547	time, and provided to the application either by copying of the page,
548	or by memory-mapping the page.
549	Data is written into the address space by the application, and then
550	written-back to storage typically in whole pages, however the
551	address_space has finer control of write sizes.
552	
553	The read process essentially only requires 'readpage'.  The write
554	process is more complicated and uses write_begin/write_end or
555	set_page_dirty to write data into the address_space, and writepage,
556	sync_page, and writepages to writeback data to storage.
557	
558	Adding and removing pages to/from an address_space is protected by the
559	inode's i_mutex.
560	
561	When data is written to a page, the PG_Dirty flag should be set.  It
562	typically remains set until writepage asks for it to be written.  This
563	should clear PG_Dirty and set PG_Writeback.  It can be actually
564	written at any point after PG_Dirty is clear.  Once it is known to be
565	safe, PG_Writeback is cleared.
566	
567	Writeback makes use of a writeback_control structure...
568	
569	struct address_space_operations
570	-------------------------------
571	
572	This describes how the VFS can manipulate mapping of a file to page cache in
573	your filesystem. The following members are defined:
574	
575	struct address_space_operations {
576		int (*writepage)(struct page *page, struct writeback_control *wbc);
577		int (*readpage)(struct file *, struct page *);
578		int (*writepages)(struct address_space *, struct writeback_control *);
579		int (*set_page_dirty)(struct page *page);
580		int (*readpages)(struct file *filp, struct address_space *mapping,
581				struct list_head *pages, unsigned nr_pages);
582		int (*write_begin)(struct file *, struct address_space *mapping,
583					loff_t pos, unsigned len, unsigned flags,
584					struct page **pagep, void **fsdata);
585		int (*write_end)(struct file *, struct address_space *mapping,
586					loff_t pos, unsigned len, unsigned copied,
587					struct page *page, void *fsdata);
588		sector_t (*bmap)(struct address_space *, sector_t);
589		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
590		int (*releasepage) (struct page *, int);
591		void (*freepage)(struct page *);
592		ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
593				loff_t offset, unsigned long nr_segs);
594		struct page* (*get_xip_page)(struct address_space *, sector_t,
595				int);
596		/* migrate the contents of a page to the specified target */
597		int (*migratepage) (struct page *, struct page *);
598		int (*launder_page) (struct page *);
599		int (*is_partially_uptodate) (struct page *, unsigned long,
600						unsigned long);
601		void (*is_dirty_writeback) (struct page *, bool *, bool *);
602		int (*error_remove_page) (struct mapping *mapping, struct page *page);
603		int (*swap_activate)(struct file *);
604		int (*swap_deactivate)(struct file *);
605	};
606	
607	  writepage: called by the VM to write a dirty page to backing store.
608	      This may happen for data integrity reasons (i.e. 'sync'), or
609	      to free up memory (flush).  The difference can be seen in
610	      wbc->sync_mode.
611	      The PG_Dirty flag has been cleared and PageLocked is true.
612	      writepage should start writeout, should set PG_Writeback,
613	      and should make sure the page is unlocked, either synchronously
614	      or asynchronously when the write operation completes.
615	
616	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
617	      try too hard if there are problems, and may choose to write out
618	      other pages from the mapping if that is easier (e.g. due to
619	      internal dependencies).  If it chooses not to start writeout, it
620	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
621	      calling ->writepage on that page.
622	
623	      See the file "Locking" for more details.
624	
625	  readpage: called by the VM to read a page from backing store.
626	       The page will be Locked when readpage is called, and should be
627	       unlocked and marked uptodate once the read completes.
628	       If ->readpage discovers that it needs to unlock the page for
629	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
630	       In this case, the page will be relocated, relocked and if
631	       that all succeeds, ->readpage will be called again.
632	
633	  writepages: called by the VM to write out pages associated with the
634	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
635	  	the writeback_control will specify a range of pages that must be
636	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
637		and that many pages should be written if possible.
638		If no ->writepages is given, then mpage_writepages is used
639	  	instead.  This will choose pages from the address space that are
640	  	tagged as DIRTY and will pass them to ->writepage.
641	
642	  set_page_dirty: called by the VM to set a page dirty.
643	        This is particularly needed if an address space attaches
644	        private data to a page, and that data needs to be updated when
645	        a page is dirtied.  This is called, for example, when a memory
646		mapped page gets modified.
647		If defined, it should set the PageDirty flag, and the
648	        PAGECACHE_TAG_DIRTY tag in the radix tree.
649	
650	  readpages: called by the VM to read pages associated with the address_space
651	  	object. This is essentially just a vector version of
652	  	readpage.  Instead of just one page, several pages are
653	  	requested.
654		readpages is only used for read-ahead, so read errors are
655	  	ignored.  If anything goes wrong, feel free to give up.
656	
657	  write_begin:
658		Called by the generic buffered write code to ask the filesystem to
659		prepare to write len bytes at the given offset in the file. The
660		address_space should check that the write will be able to complete,
661		by allocating space if necessary and doing any other internal
662		housekeeping.  If the write will update parts of any basic-blocks on
663		storage, then those blocks should be pre-read (if they haven't been
664		read already) so that the updated blocks can be written out properly.
665	
666	        The filesystem must return the locked pagecache page for the specified
667		offset, in *pagep, for the caller to write into.
668	
669		It must be able to cope with short writes (where the length passed to
670		write_begin is greater than the number of bytes copied into the page).
671	
672		flags is a field for AOP_FLAG_xxx flags, described in
673		include/linux/fs.h.
674	
675	        A void * may be returned in fsdata, which then gets passed into
676	        write_end.
677	
678	        Returns 0 on success; < 0 on failure (which is the error code), in
679		which case write_end is not called.
680	
681	  write_end: After a successful write_begin, and data copy, write_end must
682	        be called. len is the original len passed to write_begin, and copied
683	        is the amount that was able to be copied (copied == len is always true
684		if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
685	
686	        The filesystem must take care of unlocking the page and releasing it
687	        refcount, and updating i_size.
688	
689	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
690	        that were able to be copied into pagecache.
691	
692	  bmap: called by the VFS to map a logical block offset within object to
693	  	physical block number. This method is used by the FIBMAP
694	  	ioctl and for working with swap-files.  To be able to swap to
695	  	a file, the file must have a stable mapping to a block
696	  	device.  The swap system does not go through the filesystem
697	  	but instead uses bmap to find out where the blocks in the file
698	  	are and uses those addresses directly.
699	
700	
701	  invalidatepage: If a page has PagePrivate set, then invalidatepage
702	        will be called when part or all of the page is to be removed
703		from the address space.  This generally corresponds to either a
704		truncation, punch hole  or a complete invalidation of the address
705		space (in the latter case 'offset' will always be 0 and 'length'
706		will be PAGE_CACHE_SIZE). Any private data associated with the page
707		should be updated to reflect this truncation.  If offset is 0 and
708		length is PAGE_CACHE_SIZE, then the private data should be released,
709		because the page must be able to be completely discarded.  This may
710		be done by calling the ->releasepage function, but in this case the
711		release MUST succeed.
712	
713	  releasepage: releasepage is called on PagePrivate pages to indicate
714	        that the page should be freed if possible.  ->releasepage
715	        should remove any private data from the page and clear the
716	        PagePrivate flag. If releasepage() fails for some reason, it must
717		indicate failure with a 0 return value.
718		releasepage() is used in two distinct though related cases.  The
719		first is when the VM finds a clean page with no active users and
720	        wants to make it a free page.  If ->releasepage succeeds, the
721	        page will be removed from the address_space and become free.
722	
723		The second case is when a request has been made to invalidate
724	        some or all pages in an address_space.  This can happen
725	        through the fadvice(POSIX_FADV_DONTNEED) system call or by the
726	        filesystem explicitly requesting it as nfs and 9fs do (when
727	        they believe the cache may be out of date with storage) by
728	        calling invalidate_inode_pages2().
729		If the filesystem makes such a call, and needs to be certain
730	        that all pages are invalidated, then its releasepage will
731	        need to ensure this.  Possibly it can clear the PageUptodate
732	        bit if it cannot free private data yet.
733	
734	  freepage: freepage is called once the page is no longer visible in
735	        the page cache in order to allow the cleanup of any private
736		data. Since it may be called by the memory reclaimer, it
737		should not assume that the original address_space mapping still
738		exists, and it should not block.
739	
740	  direct_IO: called by the generic read/write routines to perform
741	        direct_IO - that is IO requests which bypass the page cache
742	        and transfer data directly between the storage and the
743	        application's address space.
744	
745	  get_xip_page: called by the VM to translate a block number to a page.
746		The page is valid until the corresponding filesystem is unmounted.
747		Filesystems that want to use execute-in-place (XIP) need to implement
748		it.  An example implementation can be found in fs/ext2/xip.c.
749	
750	  migrate_page:  This is used to compact the physical memory usage.
751	        If the VM wants to relocate a page (maybe off a memory card
752	        that is signalling imminent failure) it will pass a new page
753		and an old page to this function.  migrate_page should
754		transfer any private data across and update any references
755	        that it has to the page.
756	
757	  launder_page: Called before freeing a page - it writes back the dirty page. To
758	  	prevent redirtying the page, it is kept locked during the whole
759		operation.
760	
761	  is_partially_uptodate: Called by the VM when reading a file through the
762		pagecache when the underlying blocksize != pagesize. If the required
763		block is up to date then the read can complete without needing the IO
764		to bring the whole page up to date.
765	
766	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
767		The VM uses dirty and writeback information to determine if it needs
768		to stall to allow flushers a chance to complete some IO. Ordinarily
769		it can use PageDirty and PageWriteback but some filesystems have
770		more complex state (unstable pages in NFS prevent reclaim) or
771		do not set those flags due to locking problems (jbd). This callback
772		allows a filesystem to indicate to the VM if a page should be
773		treated as dirty or writeback for the purposes of stalling.
774	
775	  error_remove_page: normally set to generic_error_remove_page if truncation
776		is ok for this address space. Used for memory failure handling.
777		Setting this implies you deal with pages going away under you,
778		unless you have them locked or reference counts increased.
779	
780	  swap_activate: Called when swapon is used on a file to allocate
781		space if necessary and pin the block lookup information in
782		memory. A return value of zero indicates success,
783		in which case this file can be used to back swapspace. The
784		swapspace operations will be proxied to this address space's
785		->swap_{out,in} methods.
786	
787	  swap_deactivate: Called during swapoff on files where swap_activate
788		was successful.
789	
790	
791	The File Object
792	===============
793	
794	A file object represents a file opened by a process.
795	
796	
797	struct file_operations
798	----------------------
799	
800	This describes how the VFS can manipulate an open file. As of kernel
801	3.12, the following members are defined:
802	
803	struct file_operations {
804		struct module *owner;
805		loff_t (*llseek) (struct file *, loff_t, int);
806		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
807		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
808		ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
809		ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
810		int (*iterate) (struct file *, struct dir_context *);
811		unsigned int (*poll) (struct file *, struct poll_table_struct *);
812		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
813		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
814		int (*mmap) (struct file *, struct vm_area_struct *);
815		int (*open) (struct inode *, struct file *);
816		int (*flush) (struct file *);
817		int (*release) (struct inode *, struct file *);
818		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
819		int (*aio_fsync) (struct kiocb *, int datasync);
820		int (*fasync) (int, struct file *, int);
821		int (*lock) (struct file *, int, struct file_lock *);
822		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
823		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
824		int (*check_flags)(int);
825		int (*flock) (struct file *, int, struct file_lock *);
826		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
827		ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
828		int (*setlease)(struct file *, long arg, struct file_lock **);
829		long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
830		int (*show_fdinfo)(struct seq_file *m, struct file *f);
831	};
832	
833	Again, all methods are called without any locks being held, unless
834	otherwise noted.
835	
836	  llseek: called when the VFS needs to move the file position index
837	
838	  read: called by read(2) and related system calls
839	
840	  aio_read: called by io_submit(2) and other asynchronous I/O operations
841	
842	  write: called by write(2) and related system calls
843	
844	  aio_write: called by io_submit(2) and other asynchronous I/O operations
845	
846	  iterate: called when the VFS needs to read the directory contents
847	
848	  poll: called by the VFS when a process wants to check if there is
849		activity on this file and (optionally) go to sleep until there
850		is activity. Called by the select(2) and poll(2) system calls
851	
852	  unlocked_ioctl: called by the ioctl(2) system call.
853	
854	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
855	 	 are used on 64 bit kernels.
856	
857	  mmap: called by the mmap(2) system call
858	
859	  open: called by the VFS when an inode should be opened. When the VFS
860		opens a file, it creates a new "struct file". It then calls the
861		open method for the newly allocated file structure. You might
862		think that the open method really belongs in
863		"struct inode_operations", and you may be right. I think it's
864		done the way it is because it makes filesystems simpler to
865		implement. The open() method is a good place to initialize the
866		"private_data" member in the file structure if you want to point
867		to a device structure
868	
869	  flush: called by the close(2) system call to flush a file
870	
871	  release: called when the last reference to an open file is closed
872	
873	  fsync: called by the fsync(2) system call
874	
875	  fasync: called by the fcntl(2) system call when asynchronous
876		(non-blocking) mode is enabled for a file
877	
878	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
879	  	commands
880	
881	  get_unmapped_area: called by the mmap(2) system call
882	
883	  check_flags: called by the fcntl(2) system call for F_SETFL command
884	
885	  flock: called by the flock(2) system call
886	
887	  splice_write: called by the VFS to splice data from a pipe to a file. This
888			method is used by the splice(2) system call
889	
890	  splice_read: called by the VFS to splice data from file to a pipe. This
891		       method is used by the splice(2) system call
892	
893	  setlease: called by the VFS to set or release a file lock lease.
894		    setlease has the file_lock_lock held and must not sleep.
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.
1052	
1053		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1054		dentry being transited from.
1055	
1056	Example :
1057	
1058	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1059	{
1060		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1061					dentry->d_inode->i_ino);
1062	}
1063	
1064	Each dentry has a pointer to its parent dentry, as well as a hash list
1065	of child dentries. Child dentries are basically like files in a
1066	directory.
1067	
1068	
1069	Directory Entry Cache API
1070	--------------------------
1071	
1072	There are a number of functions defined which permit a filesystem to
1073	manipulate dentries:
1074	
1075	  dget: open a new handle for an existing dentry (this just increments
1076		the usage count)
1077	
1078	  dput: close a handle for a dentry (decrements the usage count). If
1079		the usage count drops to 0, and the dentry is still in its
1080		parent's hash, the "d_delete" method is called to check whether
1081		it should be cached. If it should not be cached, or if the dentry
1082		is not hashed, it is deleted. Otherwise cached dentries are put
1083		into an LRU list to be reclaimed on memory shortage.
1084	
1085	  d_drop: this unhashes a dentry from its parents hash list. A
1086		subsequent call to dput() will deallocate the dentry if its
1087		usage count drops to 0
1088	
1089	  d_delete: delete a dentry. If there are no other open references to
1090		the dentry then the dentry is turned into a negative dentry
1091		(the d_iput() method is called). If there are other
1092		references, then d_drop() is called instead
1093	
1094	  d_add: add a dentry to its parents hash list and then calls
1095		d_instantiate()
1096	
1097	  d_instantiate: add a dentry to the alias hash list for the inode and
1098		updates the "d_inode" member. The "i_count" member in the
1099		inode structure should be set/incremented. If the inode
1100		pointer is NULL, the dentry is called a "negative
1101		dentry". This function is commonly called when an inode is
1102		created for an existing negative dentry
1103	
1104	  d_lookup: look up a dentry given its parent and path name component
1105		It looks up the child of that given name from the dcache
1106		hash table. If it is found, the reference count is incremented
1107		and the dentry is returned. The caller must use dput()
1108		to free the dentry when it finishes using it.
1109	
1110	Mount Options
1111	=============
1112	
1113	Parsing options
1114	---------------
1115	
1116	On mount and remount the filesystem is passed a string containing a
1117	comma separated list of mount options.  The options can have either of
1118	these forms:
1119	
1120	  option
1121	  option=value
1122	
1123	The <linux/parser.h> header defines an API that helps parse these
1124	options.  There are plenty of examples on how to use it in existing
1125	filesystems.
1126	
1127	Showing options
1128	---------------
1129	
1130	If a filesystem accepts mount options, it must define show_options()
1131	to show all the currently active options.  The rules are:
1132	
1133	  - options MUST be shown which are not default or their values differ
1134	    from the default
1135	
1136	  - options MAY be shown which are enabled by default or have their
1137	    default value
1138	
1139	Options used only internally between a mount helper and the kernel
1140	(such as file descriptors), or which only have an effect during the
1141	mounting (such as ones controlling the creation of a journal) are exempt
1142	from the above rules.
1143	
1144	The underlying reason for the above rules is to make sure, that a
1145	mount can be accurately replicated (e.g. umounting and mounting again)
1146	based on the information found in /proc/mounts.
1147	
1148	A simple method of saving options at mount/remount time and showing
1149	them is provided with the save_mount_options() and
1150	generic_show_options() helper functions.  Please note, that using
1151	these may have drawbacks.  For more info see header comments for these
1152	functions in fs/namespace.c.
1153	
1154	Resources
1155	=========
1156	
1157	(Note some of these resources are not up-to-date with the latest kernel
1158	 version.)
1159	
1160	Creating Linux virtual filesystems. 2002
1161	    <http://lwn.net/Articles/13325/>
1162	
1163	The Linux Virtual File-system Layer by Neil Brown. 1999
1164	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1165	
1166	A tour of the Linux VFS by Michael K. Johnson. 1996
1167	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1168	
1169	A small trail through the Linux kernel by Andries Brouwer. 2001
1170	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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