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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 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 (*readlink) (struct dentry *, char __user *,int);
351	        void * (*follow_link) (struct dentry *, struct nameidata *);
352	        void (*put_link) (struct dentry *, struct nameidata *, void *);
353		int (*permission) (struct inode *, int);
354		int (*get_acl)(struct inode *, int);
355		int (*setattr) (struct dentry *, struct iattr *);
356		int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
357		int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
358		ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
359		ssize_t (*listxattr) (struct dentry *, char *, size_t);
360		int (*removexattr) (struct dentry *, const char *);
361		void (*update_time)(struct inode *, struct timespec *, int);
362		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
363				unsigned open_flag, umode_t create_mode, int *opened);
364		int (*tmpfile) (struct inode *, struct dentry *, umode_t);
365	};
366	
367	Again, all methods are called without any locks being held, unless
368	otherwise noted.
369	
370	  create: called by the open(2) and creat(2) system calls. Only
371		required if you want to support regular files. The dentry you
372		get should not have an inode (i.e. it should be a negative
373		dentry). Here you will probably call d_instantiate() with the
374		dentry and the newly created inode
375	
376	  lookup: called when the VFS needs to look up an inode in a parent
377		directory. The name to look for is found in the dentry. This
378		method must call d_add() to insert the found inode into the
379		dentry. The "i_count" field in the inode structure should be
380		incremented. If the named inode does not exist a NULL inode
381		should be inserted into the dentry (this is called a negative
382		dentry). Returning an error code from this routine must only
383		be done on a real error, otherwise creating inodes with system
384		calls like create(2), mknod(2), mkdir(2) and so on will fail.
385		If you wish to overload the dentry methods then you should
386		initialise the "d_dop" field in the dentry; this is a pointer
387		to a struct "dentry_operations".
388		This method is called with the directory inode semaphore held
389	
390	  link: called by the link(2) system call. Only required if you want
391		to support hard links. You will probably need to call
392		d_instantiate() just as you would in the create() method
393	
394	  unlink: called by the unlink(2) system call. Only required if you
395		want to support deleting inodes
396	
397	  symlink: called by the symlink(2) system call. Only required if you
398		want to support symlinks. You will probably need to call
399		d_instantiate() just as you would in the create() method
400	
401	  mkdir: called by the mkdir(2) system call. Only required if you want
402		to support creating subdirectories. You will probably need to
403		call d_instantiate() just as you would in the create() method
404	
405	  rmdir: called by the rmdir(2) system call. Only required if you want
406		to support deleting subdirectories
407	
408	  mknod: called by the mknod(2) system call to create a device (char,
409		block) inode or a named pipe (FIFO) or socket. Only required
410		if you want to support creating these types of inodes. You
411		will probably need to call d_instantiate() just as you would
412		in the create() method
413	
414	  rename: called by the rename(2) system call to rename the object to
415		have the parent and name given by the second inode and dentry.
416	
417	  readlink: called by the readlink(2) system call. Only required if
418		you want to support reading symbolic links
419	
420	  follow_link: called by the VFS to follow a symbolic link to the
421		inode it points to.  Only required if you want to support
422		symbolic links.  This method returns a void pointer cookie
423		that is passed to put_link().
424	
425	  put_link: called by the VFS to release resources allocated by
426	  	follow_link().  The cookie returned by follow_link() is passed
427	  	to this method as the last parameter.  It is used by
428	  	filesystems such as NFS where page cache is not stable
429	  	(i.e. page that was installed when the symbolic link walk
430	  	started might not be in the page cache at the end of the
431	  	walk).
432	
433	  permission: called by the VFS to check for access rights on a POSIX-like
434	  	filesystem.
435	
436		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
437	        mode, the filesystem must check the permission without blocking or
438		storing to the inode.
439	
440		If a situation is encountered that rcu-walk cannot handle, return
441		-ECHILD and it will be called again in ref-walk mode.
442	
443	  setattr: called by the VFS to set attributes for a file. This method
444	  	is called by chmod(2) and related system calls.
445	
446	  getattr: called by the VFS to get attributes of a file. This method
447	  	is called by stat(2) and related system calls.
448	
449	  setxattr: called by the VFS to set an extended attribute for a file.
450	  	Extended attribute is a name:value pair associated with an
451	  	inode. This method is called by setxattr(2) system call.
452	
453	  getxattr: called by the VFS to retrieve the value of an extended
454	  	attribute name. This method is called by getxattr(2) function
455	  	call.
456	
457	  listxattr: called by the VFS to list all extended attributes for a
458	  	given file. This method is called by listxattr(2) system call.
459	
460	  removexattr: called by the VFS to remove an extended attribute from
461	  	a file. This method is called by removexattr(2) system call.
462	
463	  update_time: called by the VFS to update a specific time or the i_version of
464	  	an inode.  If this is not defined the VFS will update the inode itself
465	  	and call mark_inode_dirty_sync.
466	
467	  atomic_open: called on the last component of an open.  Using this optional
468	  	method the filesystem can look up, possibly create and open the file in
469	  	one atomic operation.  If it cannot perform this (e.g. the file type
470	  	turned out to be wrong) it may signal this by returning 1 instead of
471		usual 0 or -ve .  This method is only called if the last component is
472		negative or needs lookup.  Cached positive dentries are still handled by
473		f_op->open().  If the file was created, the FILE_CREATED flag should be
474		set in "opened".  In case of O_EXCL the method must only succeed if the
475		file didn't exist and hence FILE_CREATED shall always be set on success.
476	
477	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
478		atomically creating, opening and unlinking a file in given directory.
479	
480	The Address Space Object
481	========================
482	
483	The address space object is used to group and manage pages in the page
484	cache.  It can be used to keep track of the pages in a file (or
485	anything else) and also track the mapping of sections of the file into
486	process address spaces.
487	
488	There are a number of distinct yet related services that an
489	address-space can provide.  These include communicating memory
490	pressure, page lookup by address, and keeping track of pages tagged as
491	Dirty or Writeback.
492	
493	The first can be used independently to the others.  The VM can try to
494	either write dirty pages in order to clean them, or release clean
495	pages in order to reuse them.  To do this it can call the ->writepage
496	method on dirty pages, and ->releasepage on clean pages with
497	PagePrivate set. Clean pages without PagePrivate and with no external
498	references will be released without notice being given to the
499	address_space.
500	
501	To achieve this functionality, pages need to be placed on an LRU with
502	lru_cache_add and mark_page_active needs to be called whenever the
503	page is used.
504	
505	Pages are normally kept in a radix tree index by ->index. This tree
506	maintains information about the PG_Dirty and PG_Writeback status of
507	each page, so that pages with either of these flags can be found
508	quickly.
509	
510	The Dirty tag is primarily used by mpage_writepages - the default
511	->writepages method.  It uses the tag to find dirty pages to call
512	->writepage on.  If mpage_writepages is not used (i.e. the address
513	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
514	almost unused.  write_inode_now and sync_inode do use it (through
515	__sync_single_inode) to check if ->writepages has been successful in
516	writing out the whole address_space.
517	
518	The Writeback tag is used by filemap*wait* and sync_page* functions,
519	via filemap_fdatawait_range, to wait for all writeback to
520	complete.  While waiting ->sync_page (if defined) will be called on
521	each page that is found to require writeback.
522	
523	An address_space handler may attach extra information to a page,
524	typically using the 'private' field in the 'struct page'.  If such
525	information is attached, the PG_Private flag should be set.  This will
526	cause various VM routines to make extra calls into the address_space
527	handler to deal with that data.
528	
529	An address space acts as an intermediate between storage and
530	application.  Data is read into the address space a whole page at a
531	time, and provided to the application either by copying of the page,
532	or by memory-mapping the page.
533	Data is written into the address space by the application, and then
534	written-back to storage typically in whole pages, however the
535	address_space has finer control of write sizes.
536	
537	The read process essentially only requires 'readpage'.  The write
538	process is more complicated and uses write_begin/write_end or
539	set_page_dirty to write data into the address_space, and writepage,
540	sync_page, and writepages to writeback data to storage.
541	
542	Adding and removing pages to/from an address_space is protected by the
543	inode's i_mutex.
544	
545	When data is written to a page, the PG_Dirty flag should be set.  It
546	typically remains set until writepage asks for it to be written.  This
547	should clear PG_Dirty and set PG_Writeback.  It can be actually
548	written at any point after PG_Dirty is clear.  Once it is known to be
549	safe, PG_Writeback is cleared.
550	
551	Writeback makes use of a writeback_control structure...
552	
553	struct address_space_operations
554	-------------------------------
555	
556	This describes how the VFS can manipulate mapping of a file to page cache in
557	your filesystem. The following members are defined:
558	
559	struct address_space_operations {
560		int (*writepage)(struct page *page, struct writeback_control *wbc);
561		int (*readpage)(struct file *, struct page *);
562		int (*writepages)(struct address_space *, struct writeback_control *);
563		int (*set_page_dirty)(struct page *page);
564		int (*readpages)(struct file *filp, struct address_space *mapping,
565				struct list_head *pages, unsigned nr_pages);
566		int (*write_begin)(struct file *, struct address_space *mapping,
567					loff_t pos, unsigned len, unsigned flags,
568					struct page **pagep, void **fsdata);
569		int (*write_end)(struct file *, struct address_space *mapping,
570					loff_t pos, unsigned len, unsigned copied,
571					struct page *page, void *fsdata);
572		sector_t (*bmap)(struct address_space *, sector_t);
573		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
574		int (*releasepage) (struct page *, int);
575		void (*freepage)(struct page *);
576		ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
577				loff_t offset, unsigned long nr_segs);
578		struct page* (*get_xip_page)(struct address_space *, sector_t,
579				int);
580		/* migrate the contents of a page to the specified target */
581		int (*migratepage) (struct page *, struct page *);
582		int (*launder_page) (struct page *);
583		int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
584						unsigned long);
585		void (*is_dirty_writeback) (struct page *, bool *, bool *);
586		int (*error_remove_page) (struct mapping *mapping, struct page *page);
587		int (*swap_activate)(struct file *);
588		int (*swap_deactivate)(struct file *);
589	};
590	
591	  writepage: called by the VM to write a dirty page to backing store.
592	      This may happen for data integrity reasons (i.e. 'sync'), or
593	      to free up memory (flush).  The difference can be seen in
594	      wbc->sync_mode.
595	      The PG_Dirty flag has been cleared and PageLocked is true.
596	      writepage should start writeout, should set PG_Writeback,
597	      and should make sure the page is unlocked, either synchronously
598	      or asynchronously when the write operation completes.
599	
600	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
601	      try too hard if there are problems, and may choose to write out
602	      other pages from the mapping if that is easier (e.g. due to
603	      internal dependencies).  If it chooses not to start writeout, it
604	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
605	      calling ->writepage on that page.
606	
607	      See the file "Locking" for more details.
608	
609	  readpage: called by the VM to read a page from backing store.
610	       The page will be Locked when readpage is called, and should be
611	       unlocked and marked uptodate once the read completes.
612	       If ->readpage discovers that it needs to unlock the page for
613	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
614	       In this case, the page will be relocated, relocked and if
615	       that all succeeds, ->readpage will be called again.
616	
617	  writepages: called by the VM to write out pages associated with the
618	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
619	  	the writeback_control will specify a range of pages that must be
620	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
621		and that many pages should be written if possible.
622		If no ->writepages is given, then mpage_writepages is used
623	  	instead.  This will choose pages from the address space that are
624	  	tagged as DIRTY and will pass them to ->writepage.
625	
626	  set_page_dirty: called by the VM to set a page dirty.
627	        This is particularly needed if an address space attaches
628	        private data to a page, and that data needs to be updated when
629	        a page is dirtied.  This is called, for example, when a memory
630		mapped page gets modified.
631		If defined, it should set the PageDirty flag, and the
632	        PAGECACHE_TAG_DIRTY tag in the radix tree.
633	
634	  readpages: called by the VM to read pages associated with the address_space
635	  	object. This is essentially just a vector version of
636	  	readpage.  Instead of just one page, several pages are
637	  	requested.
638		readpages is only used for read-ahead, so read errors are
639	  	ignored.  If anything goes wrong, feel free to give up.
640	
641	  write_begin:
642		Called by the generic buffered write code to ask the filesystem to
643		prepare to write len bytes at the given offset in the file. The
644		address_space should check that the write will be able to complete,
645		by allocating space if necessary and doing any other internal
646		housekeeping.  If the write will update parts of any basic-blocks on
647		storage, then those blocks should be pre-read (if they haven't been
648		read already) so that the updated blocks can be written out properly.
649	
650	        The filesystem must return the locked pagecache page for the specified
651		offset, in *pagep, for the caller to write into.
652	
653		It must be able to cope with short writes (where the length passed to
654		write_begin is greater than the number of bytes copied into the page).
655	
656		flags is a field for AOP_FLAG_xxx flags, described in
657		include/linux/fs.h.
658	
659	        A void * may be returned in fsdata, which then gets passed into
660	        write_end.
661	
662	        Returns 0 on success; < 0 on failure (which is the error code), in
663		which case write_end is not called.
664	
665	  write_end: After a successful write_begin, and data copy, write_end must
666	        be called. len is the original len passed to write_begin, and copied
667	        is the amount that was able to be copied (copied == len is always true
668		if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
669	
670	        The filesystem must take care of unlocking the page and releasing it
671	        refcount, and updating i_size.
672	
673	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
674	        that were able to be copied into pagecache.
675	
676	  bmap: called by the VFS to map a logical block offset within object to
677	  	physical block number. This method is used by the FIBMAP
678	  	ioctl and for working with swap-files.  To be able to swap to
679	  	a file, the file must have a stable mapping to a block
680	  	device.  The swap system does not go through the filesystem
681	  	but instead uses bmap to find out where the blocks in the file
682	  	are and uses those addresses directly.
683	
684	
685	  invalidatepage: If a page has PagePrivate set, then invalidatepage
686	        will be called when part or all of the page is to be removed
687		from the address space.  This generally corresponds to either a
688		truncation, punch hole  or a complete invalidation of the address
689		space (in the latter case 'offset' will always be 0 and 'length'
690		will be PAGE_CACHE_SIZE). Any private data associated with the page
691		should be updated to reflect this truncation.  If offset is 0 and
692		length is PAGE_CACHE_SIZE, then the private data should be released,
693		because the page must be able to be completely discarded.  This may
694		be done by calling the ->releasepage function, but in this case the
695		release MUST succeed.
696	
697	  releasepage: releasepage is called on PagePrivate pages to indicate
698	        that the page should be freed if possible.  ->releasepage
699	        should remove any private data from the page and clear the
700	        PagePrivate flag. If releasepage() fails for some reason, it must
701		indicate failure with a 0 return value.
702		releasepage() is used in two distinct though related cases.  The
703		first is when the VM finds a clean page with no active users and
704	        wants to make it a free page.  If ->releasepage succeeds, the
705	        page will be removed from the address_space and become free.
706	
707		The second case is when a request has been made to invalidate
708	        some or all pages in an address_space.  This can happen
709	        through the fadvice(POSIX_FADV_DONTNEED) system call or by the
710	        filesystem explicitly requesting it as nfs and 9fs do (when
711	        they believe the cache may be out of date with storage) by
712	        calling invalidate_inode_pages2().
713		If the filesystem makes such a call, and needs to be certain
714	        that all pages are invalidated, then its releasepage will
715	        need to ensure this.  Possibly it can clear the PageUptodate
716	        bit if it cannot free private data yet.
717	
718	  freepage: freepage is called once the page is no longer visible in
719	        the page cache in order to allow the cleanup of any private
720		data. Since it may be called by the memory reclaimer, it
721		should not assume that the original address_space mapping still
722		exists, and it should not block.
723	
724	  direct_IO: called by the generic read/write routines to perform
725	        direct_IO - that is IO requests which bypass the page cache
726	        and transfer data directly between the storage and the
727	        application's address space.
728	
729	  get_xip_page: called by the VM to translate a block number to a page.
730		The page is valid until the corresponding filesystem is unmounted.
731		Filesystems that want to use execute-in-place (XIP) need to implement
732		it.  An example implementation can be found in fs/ext2/xip.c.
733	
734	  migrate_page:  This is used to compact the physical memory usage.
735	        If the VM wants to relocate a page (maybe off a memory card
736	        that is signalling imminent failure) it will pass a new page
737		and an old page to this function.  migrate_page should
738		transfer any private data across and update any references
739	        that it has to the page.
740	
741	  launder_page: Called before freeing a page - it writes back the dirty page. To
742	  	prevent redirtying the page, it is kept locked during the whole
743		operation.
744	
745	  is_partially_uptodate: Called by the VM when reading a file through the
746		pagecache when the underlying blocksize != pagesize. If the required
747		block is up to date then the read can complete without needing the IO
748		to bring the whole page up to date.
749	
750	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
751		The VM uses dirty and writeback information to determine if it needs
752		to stall to allow flushers a chance to complete some IO. Ordinarily
753		it can use PageDirty and PageWriteback but some filesystems have
754		more complex state (unstable pages in NFS prevent reclaim) or
755		do not set those flags due to locking problems (jbd). This callback
756		allows a filesystem to indicate to the VM if a page should be
757		treated as dirty or writeback for the purposes of stalling.
758	
759	  error_remove_page: normally set to generic_error_remove_page if truncation
760		is ok for this address space. Used for memory failure handling.
761		Setting this implies you deal with pages going away under you,
762		unless you have them locked or reference counts increased.
763	
764	  swap_activate: Called when swapon is used on a file to allocate
765		space if necessary and pin the block lookup information in
766		memory. A return value of zero indicates success,
767		in which case this file can be used to back swapspace. The
768		swapspace operations will be proxied to this address space's
769		->swap_{out,in} methods.
770	
771	  swap_deactivate: Called during swapoff on files where swap_activate
772		was successful.
773	
774	
775	The File Object
776	===============
777	
778	A file object represents a file opened by a process.
779	
780	
781	struct file_operations
782	----------------------
783	
784	This describes how the VFS can manipulate an open file. As of kernel
785	3.5, the following members are defined:
786	
787	struct file_operations {
788		struct module *owner;
789		loff_t (*llseek) (struct file *, loff_t, int);
790		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
791		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
792		ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
793		ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
794		int (*iterate) (struct file *, struct dir_context *);
795		unsigned int (*poll) (struct file *, struct poll_table_struct *);
796		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
797		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
798		int (*mmap) (struct file *, struct vm_area_struct *);
799		int (*open) (struct inode *, struct file *);
800		int (*flush) (struct file *);
801		int (*release) (struct inode *, struct file *);
802		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
803		int (*aio_fsync) (struct kiocb *, int datasync);
804		int (*fasync) (int, struct file *, int);
805		int (*lock) (struct file *, int, struct file_lock *);
806		ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
807		ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
808		ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
809		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
810		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
811		int (*check_flags)(int);
812		int (*flock) (struct file *, int, struct file_lock *);
813		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
814		ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
815		int (*setlease)(struct file *, long arg, struct file_lock **);
816		long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
817	};
818	
819	Again, all methods are called without any locks being held, unless
820	otherwise noted.
821	
822	  llseek: called when the VFS needs to move the file position index
823	
824	  read: called by read(2) and related system calls
825	
826	  aio_read: called by io_submit(2) and other asynchronous I/O operations
827	
828	  write: called by write(2) and related system calls
829	
830	  aio_write: called by io_submit(2) and other asynchronous I/O operations
831	
832	  iterate: called when the VFS needs to read the directory contents
833	
834	  poll: called by the VFS when a process wants to check if there is
835		activity on this file and (optionally) go to sleep until there
836		is activity. Called by the select(2) and poll(2) system calls
837	
838	  unlocked_ioctl: called by the ioctl(2) system call.
839	
840	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
841	 	 are used on 64 bit kernels.
842	
843	  mmap: called by the mmap(2) system call
844	
845	  open: called by the VFS when an inode should be opened. When the VFS
846		opens a file, it creates a new "struct file". It then calls the
847		open method for the newly allocated file structure. You might
848		think that the open method really belongs in
849		"struct inode_operations", and you may be right. I think it's
850		done the way it is because it makes filesystems simpler to
851		implement. The open() method is a good place to initialize the
852		"private_data" member in the file structure if you want to point
853		to a device structure
854	
855	  flush: called by the close(2) system call to flush a file
856	
857	  release: called when the last reference to an open file is closed
858	
859	  fsync: called by the fsync(2) system call
860	
861	  fasync: called by the fcntl(2) system call when asynchronous
862		(non-blocking) mode is enabled for a file
863	
864	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
865	  	commands
866	
867	  readv: called by the readv(2) system call
868	
869	  writev: called by the writev(2) system call
870	
871	  sendfile: called by the sendfile(2) system call
872	
873	  get_unmapped_area: called by the mmap(2) system call
874	
875	  check_flags: called by the fcntl(2) system call for F_SETFL command
876	
877	  flock: called by the flock(2) system call
878	
879	  splice_write: called by the VFS to splice data from a pipe to a file. This
880			method is used by the splice(2) system call
881	
882	  splice_read: called by the VFS to splice data from file to a pipe. This
883		       method is used by the splice(2) system call
884	
885	  setlease: called by the VFS to set or release a file lock lease.
886		    setlease has the file_lock_lock held and must not sleep.
887	
888	  fallocate: called by the VFS to preallocate blocks or punch a hole.
889	
890	Note that the file operations are implemented by the specific
891	filesystem in which the inode resides. When opening a device node
892	(character or block special) most filesystems will call special
893	support routines in the VFS which will locate the required device
894	driver information. These support routines replace the filesystem file
895	operations with those for the device driver, and then proceed to call
896	the new open() method for the file. This is how opening a device file
897	in the filesystem eventually ends up calling the device driver open()
898	method.
899	
900	
901	Directory Entry Cache (dcache)
902	==============================
903	
904	
905	struct dentry_operations
906	------------------------
907	
908	This describes how a filesystem can overload the standard dentry
909	operations. Dentries and the dcache are the domain of the VFS and the
910	individual filesystem implementations. Device drivers have no business
911	here. These methods may be set to NULL, as they are either optional or
912	the VFS uses a default. As of kernel 2.6.22, the following members are
913	defined:
914	
915	struct dentry_operations {
916		int (*d_revalidate)(struct dentry *, unsigned int);
917		int (*d_weak_revalidate)(struct dentry *, unsigned int);
918		int (*d_hash)(const struct dentry *, struct qstr *);
919		int (*d_compare)(const struct dentry *, const struct dentry *,
920				unsigned int, const char *, const struct qstr *);
921		int (*d_delete)(const struct dentry *);
922		void (*d_release)(struct dentry *);
923		void (*d_iput)(struct dentry *, struct inode *);
924		char *(*d_dname)(struct dentry *, char *, int);
925		struct vfsmount *(*d_automount)(struct path *);
926		int (*d_manage)(struct dentry *, bool);
927	};
928	
929	  d_revalidate: called when the VFS needs to revalidate a dentry. This
930		is called whenever a name look-up finds a dentry in the
931		dcache. Most local filesystems leave this as NULL, because all their
932		dentries in the dcache are valid. Network filesystems are different
933		since things can change on the server without the client necessarily
934		being aware of it.
935	
936		This function should return a positive value if the dentry is still
937		valid, and zero or a negative error code if it isn't.
938	
939		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
940		If in rcu-walk mode, the filesystem must revalidate the dentry without
941		blocking or storing to the dentry, d_parent and d_inode should not be
942		used without care (because they can change and, in d_inode case, even
943		become NULL under us).
944	
945		If a situation is encountered that rcu-walk cannot handle, return
946		-ECHILD and it will be called again in ref-walk mode.
947	
948	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
949		This is called when a path-walk ends at dentry that was not acquired by
950		doing a lookup in the parent directory. This includes "/", "." and "..",
951		as well as procfs-style symlinks and mountpoint traversal.
952	
953		In this case, we are less concerned with whether the dentry is still
954		fully correct, but rather that the inode is still valid. As with
955		d_revalidate, most local filesystems will set this to NULL since their
956		dcache entries are always valid.
957	
958		This function has the same return code semantics as d_revalidate.
959	
960		d_weak_revalidate is only called after leaving rcu-walk mode.
961	
962	  d_hash: called when the VFS adds a dentry to the hash table. The first
963		dentry passed to d_hash is the parent directory that the name is
964		to be hashed into.
965	
966		Same locking and synchronisation rules as d_compare regarding
967		what is safe to dereference etc.
968	
969	  d_compare: called to compare a dentry name with a given name. The first
970		dentry is the parent of the dentry to be compared, the second is
971		the child dentry. len and name string are properties of the dentry
972		to be compared. qstr is the name to compare it with.
973	
974		Must be constant and idempotent, and should not take locks if
975		possible, and should not or store into the dentry.
976		Should not dereference pointers outside the dentry without
977		lots of care (eg.  d_parent, d_inode, d_name should not be used).
978	
979		However, our vfsmount is pinned, and RCU held, so the dentries and
980		inodes won't disappear, neither will our sb or filesystem module.
981		->d_sb may be used.
982	
983		It is a tricky calling convention because it needs to be called under
984		"rcu-walk", ie. without any locks or references on things.
985	
986	  d_delete: called when the last reference to a dentry is dropped and the
987		dcache is deciding whether or not to cache it. Return 1 to delete
988		immediately, or 0 to cache the dentry. Default is NULL which means to
989		always cache a reachable dentry. d_delete must be constant and
990		idempotent.
991	
992	  d_release: called when a dentry is really deallocated
993	
994	  d_iput: called when a dentry loses its inode (just prior to its
995		being deallocated). The default when this is NULL is that the
996		VFS calls iput(). If you define this method, you must call
997		iput() yourself
998	
999	  d_dname: called when the pathname of a dentry should be generated.
1000		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1001		pathname generation. (Instead of doing it when dentry is created,
1002		it's done only when the path is needed.). Real filesystems probably
1003		dont want to use it, because their dentries are present in global
1004		dcache hash, so their hash should be an invariant. As no lock is
1005		held, d_dname() should not try to modify the dentry itself, unless
1006		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1007		tricky. The correct way to return for example "Hello" is to put it
1008		at the end of the buffer, and returns a pointer to the first char.
1009		dynamic_dname() helper function is provided to take care of this.
1010	
1011	  d_automount: called when an automount dentry is to be traversed (optional).
1012		This should create a new VFS mount record and return the record to the
1013		caller.  The caller is supplied with a path parameter giving the
1014		automount directory to describe the automount target and the parent
1015		VFS mount record to provide inheritable mount parameters.  NULL should
1016		be returned if someone else managed to make the automount first.  If
1017		the vfsmount creation failed, then an error code should be returned.
1018		If -EISDIR is returned, then the directory will be treated as an
1019		ordinary directory and returned to pathwalk to continue walking.
1020	
1021		If a vfsmount is returned, the caller will attempt to mount it on the
1022		mountpoint and will remove the vfsmount from its expiration list in
1023		the case of failure.  The vfsmount should be returned with 2 refs on
1024		it to prevent automatic expiration - the caller will clean up the
1025		additional ref.
1026	
1027		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1028		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1029		inode being added.
1030	
1031	  d_manage: called to allow the filesystem to manage the transition from a
1032		dentry (optional).  This allows autofs, for example, to hold up clients
1033		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1034		past and construct the subtree there.  0 should be returned to let the
1035		calling process continue.  -EISDIR can be returned to tell pathwalk to
1036		use this directory as an ordinary directory and to ignore anything
1037		mounted on it and not to check the automount flag.  Any other error
1038		code will abort pathwalk completely.
1039	
1040		If the 'rcu_walk' parameter is true, then the caller is doing a
1041		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1042		and the caller can be asked to leave it and call again by returning
1043		-ECHILD.
1044	
1045		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1046		dentry being transited from.
1047	
1048	Example :
1049	
1050	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1051	{
1052		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1053					dentry->d_inode->i_ino);
1054	}
1055	
1056	Each dentry has a pointer to its parent dentry, as well as a hash list
1057	of child dentries. Child dentries are basically like files in a
1058	directory.
1059	
1060	
1061	Directory Entry Cache API
1062	--------------------------
1063	
1064	There are a number of functions defined which permit a filesystem to
1065	manipulate dentries:
1066	
1067	  dget: open a new handle for an existing dentry (this just increments
1068		the usage count)
1069	
1070	  dput: close a handle for a dentry (decrements the usage count). If
1071		the usage count drops to 0, and the dentry is still in its
1072		parent's hash, the "d_delete" method is called to check whether
1073		it should be cached. If it should not be cached, or if the dentry
1074		is not hashed, it is deleted. Otherwise cached dentries are put
1075		into an LRU list to be reclaimed on memory shortage.
1076	
1077	  d_drop: this unhashes a dentry from its parents hash list. A
1078		subsequent call to dput() will deallocate the dentry if its
1079		usage count drops to 0
1080	
1081	  d_delete: delete a dentry. If there are no other open references to
1082		the dentry then the dentry is turned into a negative dentry
1083		(the d_iput() method is called). If there are other
1084		references, then d_drop() is called instead
1085	
1086	  d_add: add a dentry to its parents hash list and then calls
1087		d_instantiate()
1088	
1089	  d_instantiate: add a dentry to the alias hash list for the inode and
1090		updates the "d_inode" member. The "i_count" member in the
1091		inode structure should be set/incremented. If the inode
1092		pointer is NULL, the dentry is called a "negative
1093		dentry". This function is commonly called when an inode is
1094		created for an existing negative dentry
1095	
1096	  d_lookup: look up a dentry given its parent and path name component
1097		It looks up the child of that given name from the dcache
1098		hash table. If it is found, the reference count is incremented
1099		and the dentry is returned. The caller must use dput()
1100		to free the dentry when it finishes using it.
1101	
1102	Mount Options
1103	=============
1104	
1105	Parsing options
1106	---------------
1107	
1108	On mount and remount the filesystem is passed a string containing a
1109	comma separated list of mount options.  The options can have either of
1110	these forms:
1111	
1112	  option
1113	  option=value
1114	
1115	The <linux/parser.h> header defines an API that helps parse these
1116	options.  There are plenty of examples on how to use it in existing
1117	filesystems.
1118	
1119	Showing options
1120	---------------
1121	
1122	If a filesystem accepts mount options, it must define show_options()
1123	to show all the currently active options.  The rules are:
1124	
1125	  - options MUST be shown which are not default or their values differ
1126	    from the default
1127	
1128	  - options MAY be shown which are enabled by default or have their
1129	    default value
1130	
1131	Options used only internally between a mount helper and the kernel
1132	(such as file descriptors), or which only have an effect during the
1133	mounting (such as ones controlling the creation of a journal) are exempt
1134	from the above rules.
1135	
1136	The underlying reason for the above rules is to make sure, that a
1137	mount can be accurately replicated (e.g. umounting and mounting again)
1138	based on the information found in /proc/mounts.
1139	
1140	A simple method of saving options at mount/remount time and showing
1141	them is provided with the save_mount_options() and
1142	generic_show_options() helper functions.  Please note, that using
1143	these may have drawbacks.  For more info see header comments for these
1144	functions in fs/namespace.c.
1145	
1146	Resources
1147	=========
1148	
1149	(Note some of these resources are not up-to-date with the latest kernel
1150	 version.)
1151	
1152	Creating Linux virtual filesystems. 2002
1153	    <http://lwn.net/Articles/13325/>
1154	
1155	The Linux Virtual File-system Layer by Neil Brown. 1999
1156	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1157	
1158	A tour of the Linux VFS by Michael K. Johnson. 1996
1159	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1160	
1161	A small trail through the Linux kernel by Andries Brouwer. 2001
1162	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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