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Based on kernel version 4.8. Page generated on 2016-10-06 23:16 EST.

2		      Overview of the Linux Virtual File System
4		Original author: Richard Gooch <rgooch@atnf.csiro.au>
6			  Last updated on June 24, 2007.
8	  Copyright (C) 1999 Richard Gooch
9	  Copyright (C) 2005 Pekka Enberg
11	  This file is released under the GPLv2.
14	Introduction
15	============
17	The Virtual File System (also known as the Virtual Filesystem Switch)
18	is the software layer in the kernel that provides the filesystem
19	interface to userspace programs. It also provides an abstraction
20	within the kernel which allows different filesystem implementations to
21	coexist.
23	VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24	on are called from a process context. Filesystem locking is described
25	in the document Documentation/filesystems/Locking.
28	Directory Entry Cache (dcache)
29	------------------------------
31	The VFS implements the open(2), stat(2), chmod(2), and similar system
32	calls. The pathname argument that is passed to them is used by the VFS
33	to search through the directory entry cache (also known as the dentry
34	cache or dcache). This provides a very fast look-up mechanism to
35	translate a pathname (filename) into a specific dentry. Dentries live
36	in RAM and are never saved to disc: they exist only for performance.
38	The dentry cache is meant to be a view into your entire filespace. As
39	most computers cannot fit all dentries in the RAM at the same time,
40	some bits of the cache are missing. In order to resolve your pathname
41	into a dentry, the VFS may have to resort to creating dentries along
42	the way, and then loading the inode. This is done by looking up the
43	inode.
46	The Inode Object
47	----------------
49	An individual dentry usually has a pointer to an inode. Inodes are
50	filesystem objects such as regular files, directories, FIFOs and other
51	beasts.  They live either on the disc (for block device filesystems)
52	or in the memory (for pseudo filesystems). Inodes that live on the
53	disc are copied into the memory when required and changes to the inode
54	are written back to disc. A single inode can be pointed to by multiple
55	dentries (hard links, for example, do this).
57	To look up an inode requires that the VFS calls the lookup() method of
58	the parent directory inode. This method is installed by the specific
59	filesystem implementation that the inode lives in. Once the VFS has
60	the required dentry (and hence the inode), we can do all those boring
61	things like open(2) the file, or stat(2) it to peek at the inode
62	data. The stat(2) operation is fairly simple: once the VFS has the
63	dentry, it peeks at the inode data and passes some of it back to
64	userspace.
67	The File Object
68	---------------
70	Opening a file requires another operation: allocation of a file
71	structure (this is the kernel-side implementation of file
72	descriptors). The freshly allocated file structure is initialized with
73	a pointer to the dentry and a set of file operation member functions.
74	These are taken from the inode data. The open() file method is then
75	called so the specific filesystem implementation can do its work. You
76	can see that this is another switch performed by the VFS. The file
77	structure is placed into the file descriptor table for the process.
79	Reading, writing and closing files (and other assorted VFS operations)
80	is done by using the userspace file descriptor to grab the appropriate
81	file structure, and then calling the required file structure method to
82	do whatever is required. For as long as the file is open, it keeps the
83	dentry in use, which in turn means that the VFS inode is still in use.
86	Registering and Mounting a Filesystem
87	=====================================
89	To register and unregister a filesystem, use the following API
90	functions:
92	   #include <linux/fs.h>
94	   extern int register_filesystem(struct file_system_type *);
95	   extern int unregister_filesystem(struct file_system_type *);
97	The passed struct file_system_type describes your filesystem. When a
98	request is made to mount a filesystem onto a directory in your namespace,
99	the VFS will call the appropriate mount() method for the specific
100	filesystem.  New vfsmount referring to the tree returned by ->mount()
101	will be attached to the mountpoint, so that when pathname resolution
102	reaches the mountpoint it will jump into the root of that vfsmount.
104	You can see all filesystems that are registered to the kernel in the
105	file /proc/filesystems.
108	struct file_system_type
109	-----------------------
111	This describes the filesystem. As of kernel 2.6.39, the following
112	members are defined:
114	struct file_system_type {
115		const char *name;
116		int fs_flags;
117	        struct dentry *(*mount) (struct file_system_type *, int,
118	                       const char *, void *);
119	        void (*kill_sb) (struct super_block *);
120	        struct module *owner;
121	        struct file_system_type * next;
122	        struct list_head fs_supers;
123		struct lock_class_key s_lock_key;
124		struct lock_class_key s_umount_key;
125	};
127	  name: the name of the filesystem type, such as "ext2", "iso9660",
128		"msdos" and so on
130	  fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
132	  mount: the method to call when a new instance of this
133		filesystem should be mounted
135	  kill_sb: the method to call when an instance of this filesystem
136		should be shut down
138	  owner: for internal VFS use: you should initialize this to THIS_MODULE in
139	  	most cases.
141	  next: for internal VFS use: you should initialize this to NULL
143	  s_lock_key, s_umount_key: lockdep-specific
145	The mount() method has the following arguments:
147	  struct file_system_type *fs_type: describes the filesystem, partly initialized
148	  	by the specific filesystem code
150	  int flags: mount flags
152	  const char *dev_name: the device name we are mounting.
154	  void *data: arbitrary mount options, usually comes as an ASCII
155		string (see "Mount Options" section)
157	The mount() method must return the root dentry of the tree requested by
158	caller.  An active reference to its superblock must be grabbed and the
159	superblock must be locked.  On failure it should return ERR_PTR(error).
161	The arguments match those of mount(2) and their interpretation
162	depends on filesystem type.  E.g. for block filesystems, dev_name is
163	interpreted as block device name, that device is opened and if it
164	contains a suitable filesystem image the method creates and initializes
165	struct super_block accordingly, returning its root dentry to caller.
167	->mount() may choose to return a subtree of existing filesystem - it
168	doesn't have to create a new one.  The main result from the caller's
169	point of view is a reference to dentry at the root of (sub)tree to
170	be attached; creation of new superblock is a common side effect.
172	The most interesting member of the superblock structure that the
173	mount() method fills in is the "s_op" field. This is a pointer to
174	a "struct super_operations" which describes the next level of the
175	filesystem implementation.
177	Usually, a filesystem uses one of the generic mount() implementations
178	and provides a fill_super() callback instead. The generic variants are:
180	  mount_bdev: mount a filesystem residing on a block device
182	  mount_nodev: mount a filesystem that is not backed by a device
184	  mount_single: mount a filesystem which shares the instance between
185	  	all mounts
187	A fill_super() callback implementation has the following arguments:
189	  struct super_block *sb: the superblock structure. The callback
190	  	must initialize this properly.
192	  void *data: arbitrary mount options, usually comes as an ASCII
193		string (see "Mount Options" section)
195	  int silent: whether or not to be silent on error
198	The Superblock Object
199	=====================
201	A superblock object represents a mounted filesystem.
204	struct super_operations
205	-----------------------
207	This describes how the VFS can manipulate the superblock of your
208	filesystem. As of kernel 2.6.22, the following members are defined:
210	struct super_operations {
211	        struct inode *(*alloc_inode)(struct super_block *sb);
212	        void (*destroy_inode)(struct inode *);
214	        void (*dirty_inode) (struct inode *, int flags);
215	        int (*write_inode) (struct inode *, int);
216	        void (*drop_inode) (struct inode *);
217	        void (*delete_inode) (struct inode *);
218	        void (*put_super) (struct super_block *);
219	        int (*sync_fs)(struct super_block *sb, int wait);
220	        int (*freeze_fs) (struct super_block *);
221	        int (*unfreeze_fs) (struct super_block *);
222	        int (*statfs) (struct dentry *, struct kstatfs *);
223	        int (*remount_fs) (struct super_block *, int *, char *);
224	        void (*clear_inode) (struct inode *);
225	        void (*umount_begin) (struct super_block *);
227	        int (*show_options)(struct seq_file *, struct dentry *);
229	        ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230	        ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
231		int (*nr_cached_objects)(struct super_block *);
232		void (*free_cached_objects)(struct super_block *, int);
233	};
235	All methods are called without any locks being held, unless otherwise
236	noted. This means that most methods can block safely. All methods are
237	only called from a process context (i.e. not from an interrupt handler
238	or bottom half).
240	  alloc_inode: this method is called by alloc_inode() to allocate memory
241	 	for struct inode and initialize it.  If this function is not
242	 	defined, a simple 'struct inode' is allocated.  Normally
243	 	alloc_inode will be used to allocate a larger structure which
244	 	contains a 'struct inode' embedded within it.
246	  destroy_inode: this method is called by destroy_inode() to release
247	  	resources allocated for struct inode.  It is only required if
248	  	->alloc_inode was defined and simply undoes anything done by
249		->alloc_inode.
251	  dirty_inode: this method is called by the VFS to mark an inode dirty.
253	  write_inode: this method is called when the VFS needs to write an
254		inode to disc.  The second parameter indicates whether the write
255		should be synchronous or not, not all filesystems check this flag.
257	  drop_inode: called when the last access to the inode is dropped,
258		with the inode->i_lock spinlock held.
260		This method should be either NULL (normal UNIX filesystem
261		semantics) or "generic_delete_inode" (for filesystems that do not
262		want to cache inodes - causing "delete_inode" to always be
263		called regardless of the value of i_nlink)
265		The "generic_delete_inode()" behavior is equivalent to the
266		old practice of using "force_delete" in the put_inode() case,
267		but does not have the races that the "force_delete()" approach
268		had. 
270	  delete_inode: called when the VFS wants to delete an inode
272	  put_super: called when the VFS wishes to free the superblock
273		(i.e. unmount). This is called with the superblock lock held
275	  sync_fs: called when VFS is writing out all dirty data associated with
276	  	a superblock. The second parameter indicates whether the method
277		should wait until the write out has been completed. Optional.
279	  freeze_fs: called when VFS is locking a filesystem and
280	  	forcing it into a consistent state.  This method is currently
281	  	used by the Logical Volume Manager (LVM).
283	  unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284	  	again.
286	  statfs: called when the VFS needs to get filesystem statistics.
288	  remount_fs: called when the filesystem is remounted. This is called
289		with the kernel lock held
291	  clear_inode: called then the VFS clears the inode. Optional
293	  umount_begin: called when the VFS is unmounting a filesystem.
295	  show_options: called by the VFS to show mount options for
296		/proc/<pid>/mounts.  (see "Mount Options" section)
298	  quota_read: called by the VFS to read from filesystem quota file.
300	  quota_write: called by the VFS to write to filesystem quota file.
302	  nr_cached_objects: called by the sb cache shrinking function for the
303		filesystem to return the number of freeable cached objects it contains.
304		Optional.
306	  free_cache_objects: called by the sb cache shrinking function for the
307		filesystem to scan the number of objects indicated to try to free them.
308		Optional, but any filesystem implementing this method needs to also
309		implement ->nr_cached_objects for it to be called correctly.
311		We can't do anything with any errors that the filesystem might
312		encountered, hence the void return type. This will never be called if
313		the VM is trying to reclaim under GFP_NOFS conditions, hence this
314		method does not need to handle that situation itself.
316		Implementations must include conditional reschedule calls inside any
317		scanning loop that is done. This allows the VFS to determine
318		appropriate scan batch sizes without having to worry about whether
319		implementations will cause holdoff problems due to large scan batch
320		sizes.
322	Whoever sets up the inode is responsible for filling in the "i_op" field. This
323	is a pointer to a "struct inode_operations" which describes the methods that
324	can be performed on individual inodes.
327	The Inode Object
328	================
330	An inode object represents an object within the filesystem.
333	struct inode_operations
334	-----------------------
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:
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		const char *(*get_link) (struct dentry *, struct inode *,
354					 struct delayed_call *);
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	};
369	Again, all methods are called without any locks being held, unless
370	otherwise noted.
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
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
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
396	  unlink: called by the unlink(2) system call. Only required if you
397		want to support deleting inodes
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
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
407	  rmdir: called by the rmdir(2) system call. Only required if you want
408		to support deleting subdirectories
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
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.
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.
433	  readlink: called by the readlink(2) system call. Only required if
434		you want to support reading symbolic links
436	  get_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 the symlink body
439		to traverse (and possibly resets the current position with
440		nd_jump_link()).  If the body won't go away until the inode
441		is gone, nothing else is needed; if it needs to be otherwise
442		pinned, arrange for its release by having get_link(..., ..., done)
443		do set_delayed_call(done, destructor, argument).
444		In that case destructor(argument) will be called once VFS is
445		done with the body you've returned.
446		May be called in RCU mode; that is indicated by NULL dentry
447		argument.  If request can't be handled without leaving RCU mode,
448		have it return ERR_PTR(-ECHILD).
450	  permission: called by the VFS to check for access rights on a POSIX-like
451	  	filesystem.
453		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
454	        mode, the filesystem must check the permission without blocking or
455		storing to the inode.
457		If a situation is encountered that rcu-walk cannot handle, return
458		-ECHILD and it will be called again in ref-walk mode.
460	  setattr: called by the VFS to set attributes for a file. This method
461	  	is called by chmod(2) and related system calls.
463	  getattr: called by the VFS to get attributes of a file. This method
464	  	is called by stat(2) and related system calls.
466	  setxattr: called by the VFS to set an extended attribute for a file.
467	  	Extended attribute is a name:value pair associated with an
468	  	inode. This method is called by setxattr(2) system call.
470	  getxattr: called by the VFS to retrieve the value of an extended
471	  	attribute name. This method is called by getxattr(2) function
472	  	call.
474	  listxattr: called by the VFS to list all extended attributes for a
475	  	given file. This method is called by listxattr(2) system call.
477	  removexattr: called by the VFS to remove an extended attribute from
478	  	a file. This method is called by removexattr(2) system call.
480	  update_time: called by the VFS to update a specific time or the i_version of
481	  	an inode.  If this is not defined the VFS will update the inode itself
482	  	and call mark_inode_dirty_sync.
484	  atomic_open: called on the last component of an open.  Using this optional
485	  	method the filesystem can look up, possibly create and open the file in
486	  	one atomic operation.  If it cannot perform this (e.g. the file type
487	  	turned out to be wrong) it may signal this by returning 1 instead of
488		usual 0 or -ve .  This method is only called if the last component is
489		negative or needs lookup.  Cached positive dentries are still handled by
490		f_op->open().  If the file was created, the FILE_CREATED flag should be
491		set in "opened".  In case of O_EXCL the method must only succeed if the
492		file didn't exist and hence FILE_CREATED shall always be set on success.
494	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
495		atomically creating, opening and unlinking a file in given directory.
497	The Address Space Object
498	========================
500	The address space object is used to group and manage pages in the page
501	cache.  It can be used to keep track of the pages in a file (or
502	anything else) and also track the mapping of sections of the file into
503	process address spaces.
505	There are a number of distinct yet related services that an
506	address-space can provide.  These include communicating memory
507	pressure, page lookup by address, and keeping track of pages tagged as
508	Dirty or Writeback.
510	The first can be used independently to the others.  The VM can try to
511	either write dirty pages in order to clean them, or release clean
512	pages in order to reuse them.  To do this it can call the ->writepage
513	method on dirty pages, and ->releasepage on clean pages with
514	PagePrivate set. Clean pages without PagePrivate and with no external
515	references will be released without notice being given to the
516	address_space.
518	To achieve this functionality, pages need to be placed on an LRU with
519	lru_cache_add and mark_page_active needs to be called whenever the
520	page is used.
522	Pages are normally kept in a radix tree index by ->index. This tree
523	maintains information about the PG_Dirty and PG_Writeback status of
524	each page, so that pages with either of these flags can be found
525	quickly.
527	The Dirty tag is primarily used by mpage_writepages - the default
528	->writepages method.  It uses the tag to find dirty pages to call
529	->writepage on.  If mpage_writepages is not used (i.e. the address
530	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
531	almost unused.  write_inode_now and sync_inode do use it (through
532	__sync_single_inode) to check if ->writepages has been successful in
533	writing out the whole address_space.
535	The Writeback tag is used by filemap*wait* and sync_page* functions,
536	via filemap_fdatawait_range, to wait for all writeback to complete.
538	An address_space handler may attach extra information to a page,
539	typically using the 'private' field in the 'struct page'.  If such
540	information is attached, the PG_Private flag should be set.  This will
541	cause various VM routines to make extra calls into the address_space
542	handler to deal with that data.
544	An address space acts as an intermediate between storage and
545	application.  Data is read into the address space a whole page at a
546	time, and provided to the application either by copying of the page,
547	or by memory-mapping the page.
548	Data is written into the address space by the application, and then
549	written-back to storage typically in whole pages, however the
550	address_space has finer control of write sizes.
552	The read process essentially only requires 'readpage'.  The write
553	process is more complicated and uses write_begin/write_end or
554	set_page_dirty to write data into the address_space, and writepage
555	and writepages to writeback data to storage.
557	Adding and removing pages to/from an address_space is protected by the
558	inode's i_mutex.
560	When data is written to a page, the PG_Dirty flag should be set.  It
561	typically remains set until writepage asks for it to be written.  This
562	should clear PG_Dirty and set PG_Writeback.  It can be actually
563	written at any point after PG_Dirty is clear.  Once it is known to be
564	safe, PG_Writeback is cleared.
566	Writeback makes use of a writeback_control structure...
568	struct address_space_operations
569	-------------------------------
571	This describes how the VFS can manipulate mapping of a file to page cache in
572	your filesystem. The following members are defined:
574	struct address_space_operations {
575		int (*writepage)(struct page *page, struct writeback_control *wbc);
576		int (*readpage)(struct file *, struct page *);
577		int (*writepages)(struct address_space *, struct writeback_control *);
578		int (*set_page_dirty)(struct page *page);
579		int (*readpages)(struct file *filp, struct address_space *mapping,
580				struct list_head *pages, unsigned nr_pages);
581		int (*write_begin)(struct file *, struct address_space *mapping,
582					loff_t pos, unsigned len, unsigned flags,
583					struct page **pagep, void **fsdata);
584		int (*write_end)(struct file *, struct address_space *mapping,
585					loff_t pos, unsigned len, unsigned copied,
586					struct page *page, void *fsdata);
587		sector_t (*bmap)(struct address_space *, sector_t);
588		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
589		int (*releasepage) (struct page *, int);
590		void (*freepage)(struct page *);
591		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
592		/* isolate a page for migration */
593		bool (*isolate_page) (struct page *, isolate_mode_t);
594		/* migrate the contents of a page to the specified target */
595		int (*migratepage) (struct page *, struct page *);
596		/* put migration-failed page back to right list */
597		void (*putback_page) (struct page *);
598		int (*launder_page) (struct page *);
600		int (*is_partially_uptodate) (struct page *, unsigned long,
601						unsigned long);
602		void (*is_dirty_writeback) (struct page *, bool *, bool *);
603		int (*error_remove_page) (struct mapping *mapping, struct page *page);
604		int (*swap_activate)(struct file *);
605		int (*swap_deactivate)(struct file *);
606	};
608	  writepage: called by the VM to write a dirty page to backing store.
609	      This may happen for data integrity reasons (i.e. 'sync'), or
610	      to free up memory (flush).  The difference can be seen in
611	      wbc->sync_mode.
612	      The PG_Dirty flag has been cleared and PageLocked is true.
613	      writepage should start writeout, should set PG_Writeback,
614	      and should make sure the page is unlocked, either synchronously
615	      or asynchronously when the write operation completes.
617	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
618	      try too hard if there are problems, and may choose to write out
619	      other pages from the mapping if that is easier (e.g. due to
620	      internal dependencies).  If it chooses not to start writeout, it
621	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
622	      calling ->writepage on that page.
624	      See the file "Locking" for more details.
626	  readpage: called by the VM to read a page from backing store.
627	       The page will be Locked when readpage is called, and should be
628	       unlocked and marked uptodate once the read completes.
629	       If ->readpage discovers that it needs to unlock the page for
630	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
631	       In this case, the page will be relocated, relocked and if
632	       that all succeeds, ->readpage will be called again.
634	  writepages: called by the VM to write out pages associated with the
635	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
636	  	the writeback_control will specify a range of pages that must be
637	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
638		and that many pages should be written if possible.
639		If no ->writepages is given, then mpage_writepages is used
640	  	instead.  This will choose pages from the address space that are
641	  	tagged as DIRTY and will pass them to ->writepage.
643	  set_page_dirty: called by the VM to set a page dirty.
644	        This is particularly needed if an address space attaches
645	        private data to a page, and that data needs to be updated when
646	        a page is dirtied.  This is called, for example, when a memory
647		mapped page gets modified.
648		If defined, it should set the PageDirty flag, and the
649	        PAGECACHE_TAG_DIRTY tag in the radix tree.
651	  readpages: called by the VM to read pages associated with the address_space
652	  	object. This is essentially just a vector version of
653	  	readpage.  Instead of just one page, several pages are
654	  	requested.
655		readpages is only used for read-ahead, so read errors are
656	  	ignored.  If anything goes wrong, feel free to give up.
658	  write_begin:
659		Called by the generic buffered write code to ask the filesystem to
660		prepare to write len bytes at the given offset in the file. The
661		address_space should check that the write will be able to complete,
662		by allocating space if necessary and doing any other internal
663		housekeeping.  If the write will update parts of any basic-blocks on
664		storage, then those blocks should be pre-read (if they haven't been
665		read already) so that the updated blocks can be written out properly.
667	        The filesystem must return the locked pagecache page for the specified
668		offset, in *pagep, for the caller to write into.
670		It must be able to cope with short writes (where the length passed to
671		write_begin is greater than the number of bytes copied into the page).
673		flags is a field for AOP_FLAG_xxx flags, described in
674		include/linux/fs.h.
676	        A void * may be returned in fsdata, which then gets passed into
677	        write_end.
679	        Returns 0 on success; < 0 on failure (which is the error code), in
680		which case write_end is not called.
682	  write_end: After a successful write_begin, and data copy, write_end must
683	        be called. len is the original len passed to write_begin, and copied
684	        is the amount that was able to be copied (copied == len is always true
685		if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
687	        The filesystem must take care of unlocking the page and releasing it
688	        refcount, and updating i_size.
690	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
691	        that were able to be copied into pagecache.
693	  bmap: called by the VFS to map a logical block offset within object to
694	  	physical block number. This method is used by the FIBMAP
695	  	ioctl and for working with swap-files.  To be able to swap to
696	  	a file, the file must have a stable mapping to a block
697	  	device.  The swap system does not go through the filesystem
698	  	but instead uses bmap to find out where the blocks in the file
699	  	are and uses those addresses directly.
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_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_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.
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.
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.
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.
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.
745	  isolate_page: Called by the VM when isolating a movable non-lru page.
746		If page is successfully isolated, VM marks the page as PG_isolated
747		via __SetPageIsolated.
749	  migrate_page:  This is used to compact the physical memory usage.
750	        If the VM wants to relocate a page (maybe off a memory card
751	        that is signalling imminent failure) it will pass a new page
752		and an old page to this function.  migrate_page should
753		transfer any private data across and update any references
754	        that it has to the page.
756	  putback_page: Called by the VM when isolated page's migration fails.
758	  launder_page: Called before freeing a page - it writes back the dirty page. To
759	  	prevent redirtying the page, it is kept locked during the whole
760		operation.
762	  is_partially_uptodate: Called by the VM when reading a file through the
763		pagecache when the underlying blocksize != pagesize. If the required
764		block is up to date then the read can complete without needing the IO
765		to bring the whole page up to date.
767	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
768		The VM uses dirty and writeback information to determine if it needs
769		to stall to allow flushers a chance to complete some IO. Ordinarily
770		it can use PageDirty and PageWriteback but some filesystems have
771		more complex state (unstable pages in NFS prevent reclaim) or
772		do not set those flags due to locking problems. This callback
773		allows a filesystem to indicate to the VM if a page should be
774		treated as dirty or writeback for the purposes of stalling.
776	  error_remove_page: normally set to generic_error_remove_page if truncation
777		is ok for this address space. Used for memory failure handling.
778		Setting this implies you deal with pages going away under you,
779		unless you have them locked or reference counts increased.
781	  swap_activate: Called when swapon is used on a file to allocate
782		space if necessary and pin the block lookup information in
783		memory. A return value of zero indicates success,
784		in which case this file can be used to back swapspace. The
785		swapspace operations will be proxied to this address space's
786		->swap_{out,in} methods.
788	  swap_deactivate: Called during swapoff on files where swap_activate
789		was successful.
792	The File Object
793	===============
795	A file object represents a file opened by a process.
798	struct file_operations
799	----------------------
801	This describes how the VFS can manipulate an open file. As of kernel
802	4.1, the following members are defined:
804	struct file_operations {
805		struct module *owner;
806		loff_t (*llseek) (struct file *, loff_t, int);
807		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
808		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
809		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
810		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
811		int (*iterate) (struct file *, struct dir_context *);
812		unsigned int (*poll) (struct file *, struct poll_table_struct *);
813		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
814		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
815		int (*mmap) (struct file *, struct vm_area_struct *);
816		int (*mremap)(struct file *, struct vm_area_struct *);
817		int (*open) (struct inode *, struct file *);
818		int (*flush) (struct file *, fl_owner_t id);
819		int (*release) (struct inode *, struct file *);
820		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
821		int (*aio_fsync) (struct kiocb *, int datasync);
822		int (*fasync) (int, struct file *, int);
823		int (*lock) (struct file *, int, struct file_lock *);
824		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
825		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
826		int (*check_flags)(int);
827		int (*flock) (struct file *, int, struct file_lock *);
828		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
829		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
830		int (*setlease)(struct file *, long, struct file_lock **, void **);
831		long (*fallocate)(struct file *file, int mode, loff_t offset,
832				  loff_t len);
833		void (*show_fdinfo)(struct seq_file *m, struct file *f);
834	#ifndef CONFIG_MMU
835		unsigned (*mmap_capabilities)(struct file *);
836	#endif
837	};
839	Again, all methods are called without any locks being held, unless
840	otherwise noted.
842	  llseek: called when the VFS needs to move the file position index
844	  read: called by read(2) and related system calls
846	  read_iter: possibly asynchronous read with iov_iter as destination
848	  write: called by write(2) and related system calls
850	  write_iter: possibly asynchronous write with iov_iter as source
852	  iterate: called when the VFS needs to read the directory contents
854	  poll: called by the VFS when a process wants to check if there is
855		activity on this file and (optionally) go to sleep until there
856		is activity. Called by the select(2) and poll(2) system calls
858	  unlocked_ioctl: called by the ioctl(2) system call.
860	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
861	 	 are used on 64 bit kernels.
863	  mmap: called by the mmap(2) system call
865	  open: called by the VFS when an inode should be opened. When the VFS
866		opens a file, it creates a new "struct file". It then calls the
867		open method for the newly allocated file structure. You might
868		think that the open method really belongs in
869		"struct inode_operations", and you may be right. I think it's
870		done the way it is because it makes filesystems simpler to
871		implement. The open() method is a good place to initialize the
872		"private_data" member in the file structure if you want to point
873		to a device structure
875	  flush: called by the close(2) system call to flush a file
877	  release: called when the last reference to an open file is closed
879	  fsync: called by the fsync(2) system call
881	  fasync: called by the fcntl(2) system call when asynchronous
882		(non-blocking) mode is enabled for a file
884	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
885	  	commands
887	  get_unmapped_area: called by the mmap(2) system call
889	  check_flags: called by the fcntl(2) system call for F_SETFL command
891	  flock: called by the flock(2) system call
893	  splice_write: called by the VFS to splice data from a pipe to a file. This
894			method is used by the splice(2) system call
896	  splice_read: called by the VFS to splice data from file to a pipe. This
897		       method is used by the splice(2) system call
899	  setlease: called by the VFS to set or release a file lock lease. setlease
900		    implementations should call generic_setlease to record or remove
901		    the lease in the inode after setting it.
903	  fallocate: called by the VFS to preallocate blocks or punch a hole.
905	Note that the file operations are implemented by the specific
906	filesystem in which the inode resides. When opening a device node
907	(character or block special) most filesystems will call special
908	support routines in the VFS which will locate the required device
909	driver information. These support routines replace the filesystem file
910	operations with those for the device driver, and then proceed to call
911	the new open() method for the file. This is how opening a device file
912	in the filesystem eventually ends up calling the device driver open()
913	method.
916	Directory Entry Cache (dcache)
917	==============================
920	struct dentry_operations
921	------------------------
923	This describes how a filesystem can overload the standard dentry
924	operations. Dentries and the dcache are the domain of the VFS and the
925	individual filesystem implementations. Device drivers have no business
926	here. These methods may be set to NULL, as they are either optional or
927	the VFS uses a default. As of kernel 2.6.22, the following members are
928	defined:
930	struct dentry_operations {
931		int (*d_revalidate)(struct dentry *, unsigned int);
932		int (*d_weak_revalidate)(struct dentry *, unsigned int);
933		int (*d_hash)(const struct dentry *, struct qstr *);
934		int (*d_compare)(const struct dentry *,
935				unsigned int, const char *, const struct qstr *);
936		int (*d_delete)(const struct dentry *);
937		int (*d_init)(struct dentry *);
938		void (*d_release)(struct dentry *);
939		void (*d_iput)(struct dentry *, struct inode *);
940		char *(*d_dname)(struct dentry *, char *, int);
941		struct vfsmount *(*d_automount)(struct path *);
942		int (*d_manage)(struct dentry *, bool);
943		struct dentry *(*d_real)(struct dentry *, const struct inode *,
944					 unsigned int);
945	};
947	  d_revalidate: called when the VFS needs to revalidate a dentry. This
948		is called whenever a name look-up finds a dentry in the
949		dcache. Most local filesystems leave this as NULL, because all their
950		dentries in the dcache are valid. Network filesystems are different
951		since things can change on the server without the client necessarily
952		being aware of it.
954		This function should return a positive value if the dentry is still
955		valid, and zero or a negative error code if it isn't.
957		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
958		If in rcu-walk mode, the filesystem must revalidate the dentry without
959		blocking or storing to the dentry, d_parent and d_inode should not be
960		used without care (because they can change and, in d_inode case, even
961		become NULL under us).
963		If a situation is encountered that rcu-walk cannot handle, return
964		-ECHILD and it will be called again in ref-walk mode.
966	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
967		This is called when a path-walk ends at dentry that was not acquired by
968		doing a lookup in the parent directory. This includes "/", "." and "..",
969		as well as procfs-style symlinks and mountpoint traversal.
971		In this case, we are less concerned with whether the dentry is still
972		fully correct, but rather that the inode is still valid. As with
973		d_revalidate, most local filesystems will set this to NULL since their
974		dcache entries are always valid.
976		This function has the same return code semantics as d_revalidate.
978		d_weak_revalidate is only called after leaving rcu-walk mode.
980	  d_hash: called when the VFS adds a dentry to the hash table. The first
981		dentry passed to d_hash is the parent directory that the name is
982		to be hashed into.
984		Same locking and synchronisation rules as d_compare regarding
985		what is safe to dereference etc.
987	  d_compare: called to compare a dentry name with a given name. The first
988		dentry is the parent of the dentry to be compared, the second is
989		the child dentry. len and name string are properties of the dentry
990		to be compared. qstr is the name to compare it with.
992		Must be constant and idempotent, and should not take locks if
993		possible, and should not or store into the dentry.
994		Should not dereference pointers outside the dentry without
995		lots of care (eg.  d_parent, d_inode, d_name should not be used).
997		However, our vfsmount is pinned, and RCU held, so the dentries and
998		inodes won't disappear, neither will our sb or filesystem module.
999		->d_sb may be used.
1001		It is a tricky calling convention because it needs to be called under
1002		"rcu-walk", ie. without any locks or references on things.
1004	  d_delete: called when the last reference to a dentry is dropped and the
1005		dcache is deciding whether or not to cache it. Return 1 to delete
1006		immediately, or 0 to cache the dentry. Default is NULL which means to
1007		always cache a reachable dentry. d_delete must be constant and
1008		idempotent.
1010	  d_init: called when a dentry is allocated
1012	  d_release: called when a dentry is really deallocated
1014	  d_iput: called when a dentry loses its inode (just prior to its
1015		being deallocated). The default when this is NULL is that the
1016		VFS calls iput(). If you define this method, you must call
1017		iput() yourself
1019	  d_dname: called when the pathname of a dentry should be generated.
1020		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1021		pathname generation. (Instead of doing it when dentry is created,
1022		it's done only when the path is needed.). Real filesystems probably
1023		dont want to use it, because their dentries are present in global
1024		dcache hash, so their hash should be an invariant. As no lock is
1025		held, d_dname() should not try to modify the dentry itself, unless
1026		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1027		tricky. The correct way to return for example "Hello" is to put it
1028		at the end of the buffer, and returns a pointer to the first char.
1029		dynamic_dname() helper function is provided to take care of this.
1031		Example :
1033		static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1034		{
1035			return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1036					dentry->d_inode->i_ino);
1037		}
1039	  d_automount: called when an automount dentry is to be traversed (optional).
1040		This should create a new VFS mount record and return the record to the
1041		caller.  The caller is supplied with a path parameter giving the
1042		automount directory to describe the automount target and the parent
1043		VFS mount record to provide inheritable mount parameters.  NULL should
1044		be returned if someone else managed to make the automount first.  If
1045		the vfsmount creation failed, then an error code should be returned.
1046		If -EISDIR is returned, then the directory will be treated as an
1047		ordinary directory and returned to pathwalk to continue walking.
1049		If a vfsmount is returned, the caller will attempt to mount it on the
1050		mountpoint and will remove the vfsmount from its expiration list in
1051		the case of failure.  The vfsmount should be returned with 2 refs on
1052		it to prevent automatic expiration - the caller will clean up the
1053		additional ref.
1055		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1056		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1057		inode being added.
1059	  d_manage: called to allow the filesystem to manage the transition from a
1060		dentry (optional).  This allows autofs, for example, to hold up clients
1061		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1062		past and construct the subtree there.  0 should be returned to let the
1063		calling process continue.  -EISDIR can be returned to tell pathwalk to
1064		use this directory as an ordinary directory and to ignore anything
1065		mounted on it and not to check the automount flag.  Any other error
1066		code will abort pathwalk completely.
1068		If the 'rcu_walk' parameter is true, then the caller is doing a
1069		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1070		and the caller can be asked to leave it and call again by returning
1071		-ECHILD.  -EISDIR may also be returned to tell pathwalk to
1072		ignore d_automount or any mounts.
1074		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1075		dentry being transited from.
1077	  d_real: overlay/union type filesystems implement this method to return one of
1078		the underlying dentries hidden by the overlay.  It is used in three
1079		different modes:
1081		Called from open it may need to copy-up the file depending on the
1082		supplied open flags.  This mode is selected with a non-zero flags
1083		argument.  In this mode the d_real method can return an error.
1085		Called from file_dentry() it returns the real dentry matching the inode
1086		argument.  The real dentry may be from a lower layer already copied up,
1087		but still referenced from the file.  This mode is selected with a
1088		non-NULL inode argument.  This will always succeed.
1090		With NULL inode and zero flags the topmost real underlying dentry is
1091		returned.  This will always succeed.
1093		This method is never called with both non-NULL inode and non-zero flags.
1095	Each dentry has a pointer to its parent dentry, as well as a hash list
1096	of child dentries. Child dentries are basically like files in a
1097	directory.
1100	Directory Entry Cache API
1101	--------------------------
1103	There are a number of functions defined which permit a filesystem to
1104	manipulate dentries:
1106	  dget: open a new handle for an existing dentry (this just increments
1107		the usage count)
1109	  dput: close a handle for a dentry (decrements the usage count). If
1110		the usage count drops to 0, and the dentry is still in its
1111		parent's hash, the "d_delete" method is called to check whether
1112		it should be cached. If it should not be cached, or if the dentry
1113		is not hashed, it is deleted. Otherwise cached dentries are put
1114		into an LRU list to be reclaimed on memory shortage.
1116	  d_drop: this unhashes a dentry from its parents hash list. A
1117		subsequent call to dput() will deallocate the dentry if its
1118		usage count drops to 0
1120	  d_delete: delete a dentry. If there are no other open references to
1121		the dentry then the dentry is turned into a negative dentry
1122		(the d_iput() method is called). If there are other
1123		references, then d_drop() is called instead
1125	  d_add: add a dentry to its parents hash list and then calls
1126		d_instantiate()
1128	  d_instantiate: add a dentry to the alias hash list for the inode and
1129		updates the "d_inode" member. The "i_count" member in the
1130		inode structure should be set/incremented. If the inode
1131		pointer is NULL, the dentry is called a "negative
1132		dentry". This function is commonly called when an inode is
1133		created for an existing negative dentry
1135	  d_lookup: look up a dentry given its parent and path name component
1136		It looks up the child of that given name from the dcache
1137		hash table. If it is found, the reference count is incremented
1138		and the dentry is returned. The caller must use dput()
1139		to free the dentry when it finishes using it.
1141	Mount Options
1142	=============
1144	Parsing options
1145	---------------
1147	On mount and remount the filesystem is passed a string containing a
1148	comma separated list of mount options.  The options can have either of
1149	these forms:
1151	  option
1152	  option=value
1154	The <linux/parser.h> header defines an API that helps parse these
1155	options.  There are plenty of examples on how to use it in existing
1156	filesystems.
1158	Showing options
1159	---------------
1161	If a filesystem accepts mount options, it must define show_options()
1162	to show all the currently active options.  The rules are:
1164	  - options MUST be shown which are not default or their values differ
1165	    from the default
1167	  - options MAY be shown which are enabled by default or have their
1168	    default value
1170	Options used only internally between a mount helper and the kernel
1171	(such as file descriptors), or which only have an effect during the
1172	mounting (such as ones controlling the creation of a journal) are exempt
1173	from the above rules.
1175	The underlying reason for the above rules is to make sure, that a
1176	mount can be accurately replicated (e.g. umounting and mounting again)
1177	based on the information found in /proc/mounts.
1179	A simple method of saving options at mount/remount time and showing
1180	them is provided with the save_mount_options() and
1181	generic_show_options() helper functions.  Please note, that using
1182	these may have drawbacks.  For more info see header comments for these
1183	functions in fs/namespace.c.
1185	Resources
1186	=========
1188	(Note some of these resources are not up-to-date with the latest kernel
1189	 version.)
1191	Creating Linux virtual filesystems. 2002
1192	    <http://lwn.net/Articles/13325/>
1194	The Linux Virtual File-system Layer by Neil Brown. 1999
1195	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1197	A tour of the Linux VFS by Michael K. Johnson. 1996
1198	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1200	A small trail through the Linux kernel by Andries Brouwer. 2001
1201	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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