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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	
2		      Overview of the Linux Virtual File System
3	
4		Original author: Richard Gooch <rgooch@atnf.csiro.au>
5	
6			  Last updated on June 24, 2007.
7	
8	  Copyright (C) 1999 Richard Gooch
9	  Copyright (C) 2005 Pekka Enberg
10	
11	  This file is released under the GPLv2.
12	
13	
14	Introduction
15	============
16	
17	The Virtual File System (also known as the Virtual Filesystem Switch)
18	is the software layer in the kernel that provides the filesystem
19	interface to userspace programs. It also provides an abstraction
20	within the kernel which allows different filesystem implementations to
21	coexist.
22	
23	VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24	on are called from a process context. Filesystem locking is described
25	in the document Documentation/filesystems/Locking.
26	
27	
28	Directory Entry Cache (dcache)
29	------------------------------
30	
31	The VFS implements the open(2), stat(2), chmod(2), and similar system
32	calls. The pathname argument that is passed to them is used by the VFS
33	to search through the directory entry cache (also known as the dentry
34	cache or dcache). This provides a very fast look-up mechanism to
35	translate a pathname (filename) into a specific dentry. Dentries live
36	in RAM and are never saved to disc: they exist only for performance.
37	
38	The dentry cache is meant to be a view into your entire filespace. As
39	most computers cannot fit all dentries in the RAM at the same time,
40	some bits of the cache are missing. In order to resolve your pathname
41	into a dentry, the VFS may have to resort to creating dentries along
42	the way, and then loading the inode. This is done by looking up the
43	inode.
44	
45	
46	The Inode Object
47	----------------
48	
49	An individual dentry usually has a pointer to an inode. Inodes are
50	filesystem objects such as regular files, directories, FIFOs and other
51	beasts.  They live either on the disc (for block device filesystems)
52	or in the memory (for pseudo filesystems). Inodes that live on the
53	disc are copied into the memory when required and changes to the inode
54	are written back to disc. A single inode can be pointed to by multiple
55	dentries (hard links, for example, do this).
56	
57	To look up an inode requires that the VFS calls the lookup() method of
58	the parent directory inode. This method is installed by the specific
59	filesystem implementation that the inode lives in. Once the VFS has
60	the required dentry (and hence the inode), we can do all those boring
61	things like open(2) the file, or stat(2) it to peek at the inode
62	data. The stat(2) operation is fairly simple: once the VFS has the
63	dentry, it peeks at the inode data and passes some of it back to
64	userspace.
65	
66	
67	The File Object
68	---------------
69	
70	Opening a file requires another operation: allocation of a file
71	structure (this is the kernel-side implementation of file
72	descriptors). The freshly allocated file structure is initialized with
73	a pointer to the dentry and a set of file operation member functions.
74	These are taken from the inode data. The open() file method is then
75	called so the specific filesystem implementation can do its work. You
76	can see that this is another switch performed by the VFS. The file
77	structure is placed into the file descriptor table for the process.
78	
79	Reading, writing and closing files (and other assorted VFS operations)
80	is done by using the userspace file descriptor to grab the appropriate
81	file structure, and then calling the required file structure method to
82	do whatever is required. For as long as the file is open, it keeps the
83	dentry in use, which in turn means that the VFS inode is still in use.
84	
85	
86	Registering and Mounting a Filesystem
87	=====================================
88	
89	To register and unregister a filesystem, use the following API
90	functions:
91	
92	   #include <linux/fs.h>
93	
94	   extern int register_filesystem(struct file_system_type *);
95	   extern int unregister_filesystem(struct file_system_type *);
96	
97	The passed struct file_system_type describes your filesystem. When a
98	request is made to mount a filesystem onto a directory in your namespace,
99	the VFS will call the appropriate mount() method for the specific
100	filesystem.  New vfsmount referring to the tree returned by ->mount()
101	will be attached to the mountpoint, so that when pathname resolution
102	reaches the mountpoint it will jump into the root of that vfsmount.
103	
104	You can see all filesystems that are registered to the kernel in the
105	file /proc/filesystems.
106	
107	
108	struct file_system_type
109	-----------------------
110	
111	This describes the filesystem. As of kernel 2.6.39, the following
112	members are defined:
113	
114	struct file_system_type {
115		const char *name;
116		int fs_flags;
117	        struct dentry *(*mount) (struct file_system_type *, int,
118	                       const char *, void *);
119	        void (*kill_sb) (struct super_block *);
120	        struct module *owner;
121	        struct file_system_type * next;
122	        struct list_head fs_supers;
123		struct lock_class_key s_lock_key;
124		struct lock_class_key s_umount_key;
125	};
126	
127	  name: the name of the filesystem type, such as "ext2", "iso9660",
128		"msdos" and so on
129	
130	  fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131	
132	  mount: the method to call when a new instance of this
133		filesystem should be mounted
134	
135	  kill_sb: the method to call when an instance of this filesystem
136		should be shut down
137	
138	  owner: for internal VFS use: you should initialize this to THIS_MODULE in
139	  	most cases.
140	
141	  next: for internal VFS use: you should initialize this to NULL
142	
143	  s_lock_key, s_umount_key: lockdep-specific
144	
145	The mount() method has the following arguments:
146	
147	  struct file_system_type *fs_type: describes the filesystem, partly initialized
148	  	by the specific filesystem code
149	
150	  int flags: mount flags
151	
152	  const char *dev_name: the device name we are mounting.
153	
154	  void *data: arbitrary mount options, usually comes as an ASCII
155		string (see "Mount Options" section)
156	
157	The mount() method must return the root dentry of the tree requested by
158	caller.  An active reference to its superblock must be grabbed and the
159	superblock must be locked.  On failure it should return ERR_PTR(error).
160	
161	The arguments match those of mount(2) and their interpretation
162	depends on filesystem type.  E.g. for block filesystems, dev_name is
163	interpreted as block device name, that device is opened and if it
164	contains a suitable filesystem image the method creates and initializes
165	struct super_block accordingly, returning its root dentry to caller.
166	
167	->mount() may choose to return a subtree of existing filesystem - it
168	doesn't have to create a new one.  The main result from the caller's
169	point of view is a reference to dentry at the root of (sub)tree to
170	be attached; creation of new superblock is a common side effect.
171	
172	The most interesting member of the superblock structure that the
173	mount() method fills in is the "s_op" field. This is a pointer to
174	a "struct super_operations" which describes the next level of the
175	filesystem implementation.
176	
177	Usually, a filesystem uses one of the generic mount() implementations
178	and provides a fill_super() callback instead. The generic variants are:
179	
180	  mount_bdev: mount a filesystem residing on a block device
181	
182	  mount_nodev: mount a filesystem that is not backed by a device
183	
184	  mount_single: mount a filesystem which shares the instance between
185	  	all mounts
186	
187	A fill_super() callback implementation has the following arguments:
188	
189	  struct super_block *sb: the superblock structure. The callback
190	  	must initialize this properly.
191	
192	  void *data: arbitrary mount options, usually comes as an ASCII
193		string (see "Mount Options" section)
194	
195	  int silent: whether or not to be silent on error
196	
197	
198	The Superblock Object
199	=====================
200	
201	A superblock object represents a mounted filesystem.
202	
203	
204	struct super_operations
205	-----------------------
206	
207	This describes how the VFS can manipulate the superblock of your
208	filesystem. As of kernel 2.6.22, the following members are defined:
209	
210	struct super_operations {
211	        struct inode *(*alloc_inode)(struct super_block *sb);
212	        void (*destroy_inode)(struct inode *);
213	
214	        void (*dirty_inode) (struct inode *, int flags);
215	        int (*write_inode) (struct inode *, int);
216	        void (*drop_inode) (struct inode *);
217	        void (*delete_inode) (struct inode *);
218	        void (*put_super) (struct super_block *);
219	        int (*sync_fs)(struct super_block *sb, int wait);
220	        int (*freeze_fs) (struct super_block *);
221	        int (*unfreeze_fs) (struct super_block *);
222	        int (*statfs) (struct dentry *, struct kstatfs *);
223	        int (*remount_fs) (struct super_block *, int *, char *);
224	        void (*clear_inode) (struct inode *);
225	        void (*umount_begin) (struct super_block *);
226	
227	        int (*show_options)(struct seq_file *, struct dentry *);
228	
229	        ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230	        ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
231		int (*nr_cached_objects)(struct super_block *);
232		void (*free_cached_objects)(struct super_block *, int);
233	};
234	
235	All methods are called without any locks being held, unless otherwise
236	noted. This means that most methods can block safely. All methods are
237	only called from a process context (i.e. not from an interrupt handler
238	or bottom half).
239	
240	  alloc_inode: this method is called by alloc_inode() to allocate memory
241	 	for struct inode and initialize it.  If this function is not
242	 	defined, a simple 'struct inode' is allocated.  Normally
243	 	alloc_inode will be used to allocate a larger structure which
244	 	contains a 'struct inode' embedded within it.
245	
246	  destroy_inode: this method is called by destroy_inode() to release
247	  	resources allocated for struct inode.  It is only required if
248	  	->alloc_inode was defined and simply undoes anything done by
249		->alloc_inode.
250	
251	  dirty_inode: this method is called by the VFS to mark an inode dirty.
252	
253	  write_inode: this method is called when the VFS needs to write an
254		inode to disc.  The second parameter indicates whether the write
255		should be synchronous or not, not all filesystems check this flag.
256	
257	  drop_inode: called when the last access to the inode is dropped,
258		with the inode->i_lock spinlock held.
259	
260		This method should be either NULL (normal UNIX filesystem
261		semantics) or "generic_delete_inode" (for filesystems that do not
262		want to cache inodes - causing "delete_inode" to always be
263		called regardless of the value of i_nlink)
264	
265		The "generic_delete_inode()" behavior is equivalent to the
266		old practice of using "force_delete" in the put_inode() case,
267		but does not have the races that the "force_delete()" approach
268		had. 
269	
270	  delete_inode: called when the VFS wants to delete an inode
271	
272	  put_super: called when the VFS wishes to free the superblock
273		(i.e. unmount). This is called with the superblock lock held
274	
275	  sync_fs: called when VFS is writing out all dirty data associated with
276	  	a superblock. The second parameter indicates whether the method
277		should wait until the write out has been completed. Optional.
278	
279	  freeze_fs: called when VFS is locking a filesystem and
280	  	forcing it into a consistent state.  This method is currently
281	  	used by the Logical Volume Manager (LVM).
282	
283	  unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284	  	again.
285	
286	  statfs: called when the VFS needs to get filesystem statistics.
287	
288	  remount_fs: called when the filesystem is remounted. This is called
289		with the kernel lock held
290	
291	  clear_inode: called then the VFS clears the inode. Optional
292	
293	  umount_begin: called when the VFS is unmounting a filesystem.
294	
295	  show_options: called by the VFS to show mount options for
296		/proc/<pid>/mounts.  (see "Mount Options" section)
297	
298	  quota_read: called by the VFS to read from filesystem quota file.
299	
300	  quota_write: called by the VFS to write to filesystem quota file.
301	
302	  nr_cached_objects: called by the sb cache shrinking function for the
303		filesystem to return the number of freeable cached objects it contains.
304		Optional.
305	
306	  free_cache_objects: called by the sb cache shrinking function for the
307		filesystem to scan the number of objects indicated to try to free them.
308		Optional, but any filesystem implementing this method needs to also
309		implement ->nr_cached_objects for it to be called correctly.
310	
311		We can't do anything with any errors that the filesystem might
312		encountered, hence the void return type. This will never be called if
313		the VM is trying to reclaim under GFP_NOFS conditions, hence this
314		method does not need to handle that situation itself.
315	
316		Implementations must include conditional reschedule calls inside any
317		scanning loop that is done. This allows the VFS to determine
318		appropriate scan batch sizes without having to worry about whether
319		implementations will cause holdoff problems due to large scan batch
320		sizes.
321	
322	Whoever sets up the inode is responsible for filling in the "i_op" field. This
323	is a pointer to a "struct inode_operations" which describes the methods that
324	can be performed on individual inodes.
325	
326	struct xattr_handlers
327	---------------------
328	
329	On filesystems that support extended attributes (xattrs), the s_xattr
330	superblock field points to a NULL-terminated array of xattr handlers.  Extended
331	attributes are name:value pairs.
332	
333	  name: Indicates that the handler matches attributes with the specified name
334		(such as "system.posix_acl_access"); the prefix field must be NULL.
335	
336	  prefix: Indicates that the handler matches all attributes with the specified
337		name prefix (such as "user."); the name field must be NULL.
338	
339	  list: Determine if attributes matching this xattr handler should be listed
340		for a particular dentry.  Used by some listxattr implementations like
341		generic_listxattr.
342	
343	  get: Called by the VFS to get the value of a particular extended attribute.
344		This method is called by the getxattr(2) system call.
345	
346	  set: Called by the VFS to set the value of a particular extended attribute.
347		When the new value is NULL, called to remove a particular extended
348		attribute.  This method is called by the the setxattr(2) and
349		removexattr(2) system calls.
350	
351	When none of the xattr handlers of a filesystem match the specified attribute
352	name or when a filesystem doesn't support extended attributes, the various
353	*xattr(2) system calls return -EOPNOTSUPP.
354	
355	
356	The Inode Object
357	================
358	
359	An inode object represents an object within the filesystem.
360	
361	
362	struct inode_operations
363	-----------------------
364	
365	This describes how the VFS can manipulate an inode in your
366	filesystem. As of kernel 2.6.22, the following members are defined:
367	
368	struct inode_operations {
369		int (*create) (struct inode *,struct dentry *, umode_t, bool);
370		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
371		int (*link) (struct dentry *,struct inode *,struct dentry *);
372		int (*unlink) (struct inode *,struct dentry *);
373		int (*symlink) (struct inode *,struct dentry *,const char *);
374		int (*mkdir) (struct inode *,struct dentry *,umode_t);
375		int (*rmdir) (struct inode *,struct dentry *);
376		int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
377		int (*rename) (struct inode *, struct dentry *,
378				struct inode *, struct dentry *, unsigned int);
379		int (*readlink) (struct dentry *, char __user *,int);
380		const char *(*get_link) (struct dentry *, struct inode *,
381					 struct delayed_call *);
382		int (*permission) (struct inode *, int);
383		int (*get_acl)(struct inode *, int);
384		int (*setattr) (struct dentry *, struct iattr *);
385		int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
386		ssize_t (*listxattr) (struct dentry *, char *, size_t);
387		void (*update_time)(struct inode *, struct timespec *, int);
388		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
389				unsigned open_flag, umode_t create_mode, int *opened);
390		int (*tmpfile) (struct inode *, struct dentry *, umode_t);
391	};
392	
393	Again, all methods are called without any locks being held, unless
394	otherwise noted.
395	
396	  create: called by the open(2) and creat(2) system calls. Only
397		required if you want to support regular files. The dentry you
398		get should not have an inode (i.e. it should be a negative
399		dentry). Here you will probably call d_instantiate() with the
400		dentry and the newly created inode
401	
402	  lookup: called when the VFS needs to look up an inode in a parent
403		directory. The name to look for is found in the dentry. This
404		method must call d_add() to insert the found inode into the
405		dentry. The "i_count" field in the inode structure should be
406		incremented. If the named inode does not exist a NULL inode
407		should be inserted into the dentry (this is called a negative
408		dentry). Returning an error code from this routine must only
409		be done on a real error, otherwise creating inodes with system
410		calls like create(2), mknod(2), mkdir(2) and so on will fail.
411		If you wish to overload the dentry methods then you should
412		initialise the "d_dop" field in the dentry; this is a pointer
413		to a struct "dentry_operations".
414		This method is called with the directory inode semaphore held
415	
416	  link: called by the link(2) system call. Only required if you want
417		to support hard links. You will probably need to call
418		d_instantiate() just as you would in the create() method
419	
420	  unlink: called by the unlink(2) system call. Only required if you
421		want to support deleting inodes
422	
423	  symlink: called by the symlink(2) system call. Only required if you
424		want to support symlinks. You will probably need to call
425		d_instantiate() just as you would in the create() method
426	
427	  mkdir: called by the mkdir(2) system call. Only required if you want
428		to support creating subdirectories. You will probably need to
429		call d_instantiate() just as you would in the create() method
430	
431	  rmdir: called by the rmdir(2) system call. Only required if you want
432		to support deleting subdirectories
433	
434	  mknod: called by the mknod(2) system call to create a device (char,
435		block) inode or a named pipe (FIFO) or socket. Only required
436		if you want to support creating these types of inodes. You
437		will probably need to call d_instantiate() just as you would
438		in the create() method
439	
440	  rename: called by the rename(2) system call to rename the object to
441		have the parent and name given by the second inode and dentry.
442	
443		The filesystem must return -EINVAL for any unsupported or
444		unknown	flags.  Currently the following flags are implemented:
445		(1) RENAME_NOREPLACE: this flag indicates that if the target
446		of the rename exists the rename should fail with -EEXIST
447		instead of replacing the target.  The VFS already checks for
448		existence, so for local filesystems the RENAME_NOREPLACE
449		implementation is equivalent to plain rename.
450		(2) RENAME_EXCHANGE: exchange source and target.  Both must
451		exist; this is checked by the VFS.  Unlike plain rename,
452		source and target may be of different type.
453	
454	  get_link: called by the VFS to follow a symbolic link to the
455		inode it points to.  Only required if you want to support
456		symbolic links.  This method returns the symlink body
457		to traverse (and possibly resets the current position with
458		nd_jump_link()).  If the body won't go away until the inode
459		is gone, nothing else is needed; if it needs to be otherwise
460		pinned, arrange for its release by having get_link(..., ..., done)
461		do set_delayed_call(done, destructor, argument).
462		In that case destructor(argument) will be called once VFS is
463		done with the body you've returned.
464		May be called in RCU mode; that is indicated by NULL dentry
465		argument.  If request can't be handled without leaving RCU mode,
466		have it return ERR_PTR(-ECHILD).
467	
468	  readlink: this is now just an override for use by readlink(2) for the
469		cases when ->get_link uses nd_jump_link() or object is not in
470		fact a symlink.  Normally filesystems should only implement
471		->get_link for symlinks and readlink(2) will automatically use
472		that.
473	
474	  permission: called by the VFS to check for access rights on a POSIX-like
475	  	filesystem.
476	
477		May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
478	        mode, the filesystem must check the permission without blocking or
479		storing to the inode.
480	
481		If a situation is encountered that rcu-walk cannot handle, return
482		-ECHILD and it will be called again in ref-walk mode.
483	
484	  setattr: called by the VFS to set attributes for a file. This method
485	  	is called by chmod(2) and related system calls.
486	
487	  getattr: called by the VFS to get attributes of a file. This method
488	  	is called by stat(2) and related system calls.
489	
490	  listxattr: called by the VFS to list all extended attributes for a
491		given file. This method is called by the listxattr(2) system call.
492	
493	  update_time: called by the VFS to update a specific time or the i_version of
494	  	an inode.  If this is not defined the VFS will update the inode itself
495	  	and call mark_inode_dirty_sync.
496	
497	  atomic_open: called on the last component of an open.  Using this optional
498	  	method the filesystem can look up, possibly create and open the file in
499	  	one atomic operation.  If it cannot perform this (e.g. the file type
500	  	turned out to be wrong) it may signal this by returning 1 instead of
501		usual 0 or -ve .  This method is only called if the last component is
502		negative or needs lookup.  Cached positive dentries are still handled by
503		f_op->open().  If the file was created, the FILE_CREATED flag should be
504		set in "opened".  In case of O_EXCL the method must only succeed if the
505		file didn't exist and hence FILE_CREATED shall always be set on success.
506	
507	  tmpfile: called in the end of O_TMPFILE open().  Optional, equivalent to
508		atomically creating, opening and unlinking a file in given directory.
509	
510	The Address Space Object
511	========================
512	
513	The address space object is used to group and manage pages in the page
514	cache.  It can be used to keep track of the pages in a file (or
515	anything else) and also track the mapping of sections of the file into
516	process address spaces.
517	
518	There are a number of distinct yet related services that an
519	address-space can provide.  These include communicating memory
520	pressure, page lookup by address, and keeping track of pages tagged as
521	Dirty or Writeback.
522	
523	The first can be used independently to the others.  The VM can try to
524	either write dirty pages in order to clean them, or release clean
525	pages in order to reuse them.  To do this it can call the ->writepage
526	method on dirty pages, and ->releasepage on clean pages with
527	PagePrivate set. Clean pages without PagePrivate and with no external
528	references will be released without notice being given to the
529	address_space.
530	
531	To achieve this functionality, pages need to be placed on an LRU with
532	lru_cache_add and mark_page_active needs to be called whenever the
533	page is used.
534	
535	Pages are normally kept in a radix tree index by ->index. This tree
536	maintains information about the PG_Dirty and PG_Writeback status of
537	each page, so that pages with either of these flags can be found
538	quickly.
539	
540	The Dirty tag is primarily used by mpage_writepages - the default
541	->writepages method.  It uses the tag to find dirty pages to call
542	->writepage on.  If mpage_writepages is not used (i.e. the address
543	provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
544	almost unused.  write_inode_now and sync_inode do use it (through
545	__sync_single_inode) to check if ->writepages has been successful in
546	writing out the whole address_space.
547	
548	The Writeback tag is used by filemap*wait* and sync_page* functions,
549	via filemap_fdatawait_range, to wait for all writeback to complete.
550	
551	An address_space handler may attach extra information to a page,
552	typically using the 'private' field in the 'struct page'.  If such
553	information is attached, the PG_Private flag should be set.  This will
554	cause various VM routines to make extra calls into the address_space
555	handler to deal with that data.
556	
557	An address space acts as an intermediate between storage and
558	application.  Data is read into the address space a whole page at a
559	time, and provided to the application either by copying of the page,
560	or by memory-mapping the page.
561	Data is written into the address space by the application, and then
562	written-back to storage typically in whole pages, however the
563	address_space has finer control of write sizes.
564	
565	The read process essentially only requires 'readpage'.  The write
566	process is more complicated and uses write_begin/write_end or
567	set_page_dirty to write data into the address_space, and writepage
568	and writepages to writeback data to storage.
569	
570	Adding and removing pages to/from an address_space is protected by the
571	inode's i_mutex.
572	
573	When data is written to a page, the PG_Dirty flag should be set.  It
574	typically remains set until writepage asks for it to be written.  This
575	should clear PG_Dirty and set PG_Writeback.  It can be actually
576	written at any point after PG_Dirty is clear.  Once it is known to be
577	safe, PG_Writeback is cleared.
578	
579	Writeback makes use of a writeback_control structure to direct the
580	operations.  This gives the the writepage and writepages operations some
581	information about the nature of and reason for the writeback request,
582	and the constraints under which it is being done.  It is also used to
583	return information back to the caller about the result of a writepage or
584	writepages request.
585	
586	Handling errors during writeback
587	--------------------------------
588	Most applications that do buffered I/O will periodically call a file
589	synchronization call (fsync, fdatasync, msync or sync_file_range) to
590	ensure that data written has made it to the backing store.  When there
591	is an error during writeback, they expect that error to be reported when
592	a file sync request is made.  After an error has been reported on one
593	request, subsequent requests on the same file descriptor should return
594	0, unless further writeback errors have occurred since the previous file
595	syncronization.
596	
597	Ideally, the kernel would report errors only on file descriptions on
598	which writes were done that subsequently failed to be written back.  The
599	generic pagecache infrastructure does not track the file descriptions
600	that have dirtied each individual page however, so determining which
601	file descriptors should get back an error is not possible.
602	
603	Instead, the generic writeback error tracking infrastructure in the
604	kernel settles for reporting errors to fsync on all file descriptions
605	that were open at the time that the error occurred.  In a situation with
606	multiple writers, all of them will get back an error on a subsequent fsync,
607	even if all of the writes done through that particular file descriptor
608	succeeded (or even if there were no writes on that file descriptor at all).
609	
610	Filesystems that wish to use this infrastructure should call
611	mapping_set_error to record the error in the address_space when it
612	occurs.  Then, after writing back data from the pagecache in their
613	file->fsync operation, they should call file_check_and_advance_wb_err to
614	ensure that the struct file's error cursor has advanced to the correct
615	point in the stream of errors emitted by the backing device(s).
616	
617	struct address_space_operations
618	-------------------------------
619	
620	This describes how the VFS can manipulate mapping of a file to page cache in
621	your filesystem. The following members are defined:
622	
623	struct address_space_operations {
624		int (*writepage)(struct page *page, struct writeback_control *wbc);
625		int (*readpage)(struct file *, struct page *);
626		int (*writepages)(struct address_space *, struct writeback_control *);
627		int (*set_page_dirty)(struct page *page);
628		int (*readpages)(struct file *filp, struct address_space *mapping,
629				struct list_head *pages, unsigned nr_pages);
630		int (*write_begin)(struct file *, struct address_space *mapping,
631					loff_t pos, unsigned len, unsigned flags,
632					struct page **pagep, void **fsdata);
633		int (*write_end)(struct file *, struct address_space *mapping,
634					loff_t pos, unsigned len, unsigned copied,
635					struct page *page, void *fsdata);
636		sector_t (*bmap)(struct address_space *, sector_t);
637		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
638		int (*releasepage) (struct page *, int);
639		void (*freepage)(struct page *);
640		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
641		/* isolate a page for migration */
642		bool (*isolate_page) (struct page *, isolate_mode_t);
643		/* migrate the contents of a page to the specified target */
644		int (*migratepage) (struct page *, struct page *);
645		/* put migration-failed page back to right list */
646		void (*putback_page) (struct page *);
647		int (*launder_page) (struct page *);
648	
649		int (*is_partially_uptodate) (struct page *, unsigned long,
650						unsigned long);
651		void (*is_dirty_writeback) (struct page *, bool *, bool *);
652		int (*error_remove_page) (struct mapping *mapping, struct page *page);
653		int (*swap_activate)(struct file *);
654		int (*swap_deactivate)(struct file *);
655	};
656	
657	  writepage: called by the VM to write a dirty page to backing store.
658	      This may happen for data integrity reasons (i.e. 'sync'), or
659	      to free up memory (flush).  The difference can be seen in
660	      wbc->sync_mode.
661	      The PG_Dirty flag has been cleared and PageLocked is true.
662	      writepage should start writeout, should set PG_Writeback,
663	      and should make sure the page is unlocked, either synchronously
664	      or asynchronously when the write operation completes.
665	
666	      If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
667	      try too hard if there are problems, and may choose to write out
668	      other pages from the mapping if that is easier (e.g. due to
669	      internal dependencies).  If it chooses not to start writeout, it
670	      should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
671	      calling ->writepage on that page.
672	
673	      See the file "Locking" for more details.
674	
675	  readpage: called by the VM to read a page from backing store.
676	       The page will be Locked when readpage is called, and should be
677	       unlocked and marked uptodate once the read completes.
678	       If ->readpage discovers that it needs to unlock the page for
679	       some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
680	       In this case, the page will be relocated, relocked and if
681	       that all succeeds, ->readpage will be called again.
682	
683	  writepages: called by the VM to write out pages associated with the
684	  	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
685	  	the writeback_control will specify a range of pages that must be
686	  	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is given
687		and that many pages should be written if possible.
688		If no ->writepages is given, then mpage_writepages is used
689	  	instead.  This will choose pages from the address space that are
690	  	tagged as DIRTY and will pass them to ->writepage.
691	
692	  set_page_dirty: called by the VM to set a page dirty.
693	        This is particularly needed if an address space attaches
694	        private data to a page, and that data needs to be updated when
695	        a page is dirtied.  This is called, for example, when a memory
696		mapped page gets modified.
697		If defined, it should set the PageDirty flag, and the
698	        PAGECACHE_TAG_DIRTY tag in the radix tree.
699	
700	  readpages: called by the VM to read pages associated with the address_space
701	  	object. This is essentially just a vector version of
702	  	readpage.  Instead of just one page, several pages are
703	  	requested.
704		readpages is only used for read-ahead, so read errors are
705	  	ignored.  If anything goes wrong, feel free to give up.
706	
707	  write_begin:
708		Called by the generic buffered write code to ask the filesystem to
709		prepare to write len bytes at the given offset in the file. The
710		address_space should check that the write will be able to complete,
711		by allocating space if necessary and doing any other internal
712		housekeeping.  If the write will update parts of any basic-blocks on
713		storage, then those blocks should be pre-read (if they haven't been
714		read already) so that the updated blocks can be written out properly.
715	
716	        The filesystem must return the locked pagecache page for the specified
717		offset, in *pagep, for the caller to write into.
718	
719		It must be able to cope with short writes (where the length passed to
720		write_begin is greater than the number of bytes copied into the page).
721	
722		flags is a field for AOP_FLAG_xxx flags, described in
723		include/linux/fs.h.
724	
725	        A void * may be returned in fsdata, which then gets passed into
726	        write_end.
727	
728	        Returns 0 on success; < 0 on failure (which is the error code), in
729		which case write_end is not called.
730	
731	  write_end: After a successful write_begin, and data copy, write_end must
732	        be called. len is the original len passed to write_begin, and copied
733	        is the amount that was able to be copied.
734	
735	        The filesystem must take care of unlocking the page and releasing it
736	        refcount, and updating i_size.
737	
738	        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
739	        that were able to be copied into pagecache.
740	
741	  bmap: called by the VFS to map a logical block offset within object to
742	  	physical block number. This method is used by the FIBMAP
743	  	ioctl and for working with swap-files.  To be able to swap to
744	  	a file, the file must have a stable mapping to a block
745	  	device.  The swap system does not go through the filesystem
746	  	but instead uses bmap to find out where the blocks in the file
747	  	are and uses those addresses directly.
748	
749	  invalidatepage: If a page has PagePrivate set, then invalidatepage
750	        will be called when part or all of the page is to be removed
751		from the address space.  This generally corresponds to either a
752		truncation, punch hole  or a complete invalidation of the address
753		space (in the latter case 'offset' will always be 0 and 'length'
754		will be PAGE_SIZE). Any private data associated with the page
755		should be updated to reflect this truncation.  If offset is 0 and
756		length is PAGE_SIZE, then the private data should be released,
757		because the page must be able to be completely discarded.  This may
758		be done by calling the ->releasepage function, but in this case the
759		release MUST succeed.
760	
761	  releasepage: releasepage is called on PagePrivate pages to indicate
762	        that the page should be freed if possible.  ->releasepage
763	        should remove any private data from the page and clear the
764	        PagePrivate flag. If releasepage() fails for some reason, it must
765		indicate failure with a 0 return value.
766		releasepage() is used in two distinct though related cases.  The
767		first is when the VM finds a clean page with no active users and
768	        wants to make it a free page.  If ->releasepage succeeds, the
769	        page will be removed from the address_space and become free.
770	
771		The second case is when a request has been made to invalidate
772	        some or all pages in an address_space.  This can happen
773	        through the fadvise(POSIX_FADV_DONTNEED) system call or by the
774	        filesystem explicitly requesting it as nfs and 9fs do (when
775	        they believe the cache may be out of date with storage) by
776	        calling invalidate_inode_pages2().
777		If the filesystem makes such a call, and needs to be certain
778	        that all pages are invalidated, then its releasepage will
779	        need to ensure this.  Possibly it can clear the PageUptodate
780	        bit if it cannot free private data yet.
781	
782	  freepage: freepage is called once the page is no longer visible in
783	        the page cache in order to allow the cleanup of any private
784		data. Since it may be called by the memory reclaimer, it
785		should not assume that the original address_space mapping still
786		exists, and it should not block.
787	
788	  direct_IO: called by the generic read/write routines to perform
789	        direct_IO - that is IO requests which bypass the page cache
790	        and transfer data directly between the storage and the
791	        application's address space.
792	
793	  isolate_page: Called by the VM when isolating a movable non-lru page.
794		If page is successfully isolated, VM marks the page as PG_isolated
795		via __SetPageIsolated.
796	
797	  migrate_page:  This is used to compact the physical memory usage.
798	        If the VM wants to relocate a page (maybe off a memory card
799	        that is signalling imminent failure) it will pass a new page
800		and an old page to this function.  migrate_page should
801		transfer any private data across and update any references
802	        that it has to the page.
803	
804	  putback_page: Called by the VM when isolated page's migration fails.
805	
806	  launder_page: Called before freeing a page - it writes back the dirty page. To
807	  	prevent redirtying the page, it is kept locked during the whole
808		operation.
809	
810	  is_partially_uptodate: Called by the VM when reading a file through the
811		pagecache when the underlying blocksize != pagesize. If the required
812		block is up to date then the read can complete without needing the IO
813		to bring the whole page up to date.
814	
815	  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
816		The VM uses dirty and writeback information to determine if it needs
817		to stall to allow flushers a chance to complete some IO. Ordinarily
818		it can use PageDirty and PageWriteback but some filesystems have
819		more complex state (unstable pages in NFS prevent reclaim) or
820		do not set those flags due to locking problems. This callback
821		allows a filesystem to indicate to the VM if a page should be
822		treated as dirty or writeback for the purposes of stalling.
823	
824	  error_remove_page: normally set to generic_error_remove_page if truncation
825		is ok for this address space. Used for memory failure handling.
826		Setting this implies you deal with pages going away under you,
827		unless you have them locked or reference counts increased.
828	
829	  swap_activate: Called when swapon is used on a file to allocate
830		space if necessary and pin the block lookup information in
831		memory. A return value of zero indicates success,
832		in which case this file can be used to back swapspace. The
833		swapspace operations will be proxied to this address space's
834		->swap_{out,in} methods.
835	
836	  swap_deactivate: Called during swapoff on files where swap_activate
837		was successful.
838	
839	
840	The File Object
841	===============
842	
843	A file object represents a file opened by a process. This is also known
844	as an "open file description" in POSIX parlance.
845	
846	
847	struct file_operations
848	----------------------
849	
850	This describes how the VFS can manipulate an open file. As of kernel
851	4.1, the following members are defined:
852	
853	struct file_operations {
854		struct module *owner;
855		loff_t (*llseek) (struct file *, loff_t, int);
856		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
857		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
858		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
859		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
860		int (*iterate) (struct file *, struct dir_context *);
861		unsigned int (*poll) (struct file *, struct poll_table_struct *);
862		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
863		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
864		int (*mmap) (struct file *, struct vm_area_struct *);
865		int (*mremap)(struct file *, struct vm_area_struct *);
866		int (*open) (struct inode *, struct file *);
867		int (*flush) (struct file *, fl_owner_t id);
868		int (*release) (struct inode *, struct file *);
869		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
870		int (*fasync) (int, struct file *, int);
871		int (*lock) (struct file *, int, struct file_lock *);
872		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
873		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
874		int (*check_flags)(int);
875		int (*flock) (struct file *, int, struct file_lock *);
876		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
877		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
878		int (*setlease)(struct file *, long, struct file_lock **, void **);
879		long (*fallocate)(struct file *file, int mode, loff_t offset,
880				  loff_t len);
881		void (*show_fdinfo)(struct seq_file *m, struct file *f);
882	#ifndef CONFIG_MMU
883		unsigned (*mmap_capabilities)(struct file *);
884	#endif
885	};
886	
887	Again, all methods are called without any locks being held, unless
888	otherwise noted.
889	
890	  llseek: called when the VFS needs to move the file position index
891	
892	  read: called by read(2) and related system calls
893	
894	  read_iter: possibly asynchronous read with iov_iter as destination
895	
896	  write: called by write(2) and related system calls
897	
898	  write_iter: possibly asynchronous write with iov_iter as source
899	
900	  iterate: called when the VFS needs to read the directory contents
901	
902	  poll: called by the VFS when a process wants to check if there is
903		activity on this file and (optionally) go to sleep until there
904		is activity. Called by the select(2) and poll(2) system calls
905	
906	  unlocked_ioctl: called by the ioctl(2) system call.
907	
908	  compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
909	 	 are used on 64 bit kernels.
910	
911	  mmap: called by the mmap(2) system call
912	
913	  open: called by the VFS when an inode should be opened. When the VFS
914		opens a file, it creates a new "struct file". It then calls the
915		open method for the newly allocated file structure. You might
916		think that the open method really belongs in
917		"struct inode_operations", and you may be right. I think it's
918		done the way it is because it makes filesystems simpler to
919		implement. The open() method is a good place to initialize the
920		"private_data" member in the file structure if you want to point
921		to a device structure
922	
923	  flush: called by the close(2) system call to flush a file
924	
925	  release: called when the last reference to an open file is closed
926	
927	  fsync: called by the fsync(2) system call. Also see the section above
928		 entitled "Handling errors during writeback".
929	
930	  fasync: called by the fcntl(2) system call when asynchronous
931		(non-blocking) mode is enabled for a file
932	
933	  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
934	  	commands
935	
936	  get_unmapped_area: called by the mmap(2) system call
937	
938	  check_flags: called by the fcntl(2) system call for F_SETFL command
939	
940	  flock: called by the flock(2) system call
941	
942	  splice_write: called by the VFS to splice data from a pipe to a file. This
943			method is used by the splice(2) system call
944	
945	  splice_read: called by the VFS to splice data from file to a pipe. This
946		       method is used by the splice(2) system call
947	
948	  setlease: called by the VFS to set or release a file lock lease. setlease
949		    implementations should call generic_setlease to record or remove
950		    the lease in the inode after setting it.
951	
952	  fallocate: called by the VFS to preallocate blocks or punch a hole.
953	
954	Note that the file operations are implemented by the specific
955	filesystem in which the inode resides. When opening a device node
956	(character or block special) most filesystems will call special
957	support routines in the VFS which will locate the required device
958	driver information. These support routines replace the filesystem file
959	operations with those for the device driver, and then proceed to call
960	the new open() method for the file. This is how opening a device file
961	in the filesystem eventually ends up calling the device driver open()
962	method.
963	
964	
965	Directory Entry Cache (dcache)
966	==============================
967	
968	
969	struct dentry_operations
970	------------------------
971	
972	This describes how a filesystem can overload the standard dentry
973	operations. Dentries and the dcache are the domain of the VFS and the
974	individual filesystem implementations. Device drivers have no business
975	here. These methods may be set to NULL, as they are either optional or
976	the VFS uses a default. As of kernel 2.6.22, the following members are
977	defined:
978	
979	struct dentry_operations {
980		int (*d_revalidate)(struct dentry *, unsigned int);
981		int (*d_weak_revalidate)(struct dentry *, unsigned int);
982		int (*d_hash)(const struct dentry *, struct qstr *);
983		int (*d_compare)(const struct dentry *,
984				unsigned int, const char *, const struct qstr *);
985		int (*d_delete)(const struct dentry *);
986		int (*d_init)(struct dentry *);
987		void (*d_release)(struct dentry *);
988		void (*d_iput)(struct dentry *, struct inode *);
989		char *(*d_dname)(struct dentry *, char *, int);
990		struct vfsmount *(*d_automount)(struct path *);
991		int (*d_manage)(const struct path *, bool);
992		struct dentry *(*d_real)(struct dentry *, const struct inode *,
993					 unsigned int);
994	};
995	
996	  d_revalidate: called when the VFS needs to revalidate a dentry. This
997		is called whenever a name look-up finds a dentry in the
998		dcache. Most local filesystems leave this as NULL, because all their
999		dentries in the dcache are valid. Network filesystems are different
1000		since things can change on the server without the client necessarily
1001		being aware of it.
1002	
1003		This function should return a positive value if the dentry is still
1004		valid, and zero or a negative error code if it isn't.
1005	
1006		d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
1007		If in rcu-walk mode, the filesystem must revalidate the dentry without
1008		blocking or storing to the dentry, d_parent and d_inode should not be
1009		used without care (because they can change and, in d_inode case, even
1010		become NULL under us).
1011	
1012		If a situation is encountered that rcu-walk cannot handle, return
1013		-ECHILD and it will be called again in ref-walk mode.
1014	
1015	 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
1016		This is called when a path-walk ends at dentry that was not acquired by
1017		doing a lookup in the parent directory. This includes "/", "." and "..",
1018		as well as procfs-style symlinks and mountpoint traversal.
1019	
1020		In this case, we are less concerned with whether the dentry is still
1021		fully correct, but rather that the inode is still valid. As with
1022		d_revalidate, most local filesystems will set this to NULL since their
1023		dcache entries are always valid.
1024	
1025		This function has the same return code semantics as d_revalidate.
1026	
1027		d_weak_revalidate is only called after leaving rcu-walk mode.
1028	
1029	  d_hash: called when the VFS adds a dentry to the hash table. The first
1030		dentry passed to d_hash is the parent directory that the name is
1031		to be hashed into.
1032	
1033		Same locking and synchronisation rules as d_compare regarding
1034		what is safe to dereference etc.
1035	
1036	  d_compare: called to compare a dentry name with a given name. The first
1037		dentry is the parent of the dentry to be compared, the second is
1038		the child dentry. len and name string are properties of the dentry
1039		to be compared. qstr is the name to compare it with.
1040	
1041		Must be constant and idempotent, and should not take locks if
1042		possible, and should not or store into the dentry.
1043		Should not dereference pointers outside the dentry without
1044		lots of care (eg.  d_parent, d_inode, d_name should not be used).
1045	
1046		However, our vfsmount is pinned, and RCU held, so the dentries and
1047		inodes won't disappear, neither will our sb or filesystem module.
1048		->d_sb may be used.
1049	
1050		It is a tricky calling convention because it needs to be called under
1051		"rcu-walk", ie. without any locks or references on things.
1052	
1053	  d_delete: called when the last reference to a dentry is dropped and the
1054		dcache is deciding whether or not to cache it. Return 1 to delete
1055		immediately, or 0 to cache the dentry. Default is NULL which means to
1056		always cache a reachable dentry. d_delete must be constant and
1057		idempotent.
1058	
1059	  d_init: called when a dentry is allocated
1060	
1061	  d_release: called when a dentry is really deallocated
1062	
1063	  d_iput: called when a dentry loses its inode (just prior to its
1064		being deallocated). The default when this is NULL is that the
1065		VFS calls iput(). If you define this method, you must call
1066		iput() yourself
1067	
1068	  d_dname: called when the pathname of a dentry should be generated.
1069		Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1070		pathname generation. (Instead of doing it when dentry is created,
1071		it's done only when the path is needed.). Real filesystems probably
1072		dont want to use it, because their dentries are present in global
1073		dcache hash, so their hash should be an invariant. As no lock is
1074		held, d_dname() should not try to modify the dentry itself, unless
1075		appropriate SMP safety is used. CAUTION : d_path() logic is quite
1076		tricky. The correct way to return for example "Hello" is to put it
1077		at the end of the buffer, and returns a pointer to the first char.
1078		dynamic_dname() helper function is provided to take care of this.
1079	
1080		Example :
1081	
1082		static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1083		{
1084			return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1085					dentry->d_inode->i_ino);
1086		}
1087	
1088	  d_automount: called when an automount dentry is to be traversed (optional).
1089		This should create a new VFS mount record and return the record to the
1090		caller.  The caller is supplied with a path parameter giving the
1091		automount directory to describe the automount target and the parent
1092		VFS mount record to provide inheritable mount parameters.  NULL should
1093		be returned if someone else managed to make the automount first.  If
1094		the vfsmount creation failed, then an error code should be returned.
1095		If -EISDIR is returned, then the directory will be treated as an
1096		ordinary directory and returned to pathwalk to continue walking.
1097	
1098		If a vfsmount is returned, the caller will attempt to mount it on the
1099		mountpoint and will remove the vfsmount from its expiration list in
1100		the case of failure.  The vfsmount should be returned with 2 refs on
1101		it to prevent automatic expiration - the caller will clean up the
1102		additional ref.
1103	
1104		This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1105		dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1106		inode being added.
1107	
1108	  d_manage: called to allow the filesystem to manage the transition from a
1109		dentry (optional).  This allows autofs, for example, to hold up clients
1110		waiting to explore behind a 'mountpoint' whilst letting the daemon go
1111		past and construct the subtree there.  0 should be returned to let the
1112		calling process continue.  -EISDIR can be returned to tell pathwalk to
1113		use this directory as an ordinary directory and to ignore anything
1114		mounted on it and not to check the automount flag.  Any other error
1115		code will abort pathwalk completely.
1116	
1117		If the 'rcu_walk' parameter is true, then the caller is doing a
1118		pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
1119		and the caller can be asked to leave it and call again by returning
1120		-ECHILD.  -EISDIR may also be returned to tell pathwalk to
1121		ignore d_automount or any mounts.
1122	
1123		This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1124		dentry being transited from.
1125	
1126	  d_real: overlay/union type filesystems implement this method to return one of
1127		the underlying dentries hidden by the overlay.  It is used in three
1128		different modes:
1129	
1130		Called from open it may need to copy-up the file depending on the
1131		supplied open flags.  This mode is selected with a non-zero flags
1132		argument.  In this mode the d_real method can return an error.
1133	
1134		Called from file_dentry() it returns the real dentry matching the inode
1135		argument.  The real dentry may be from a lower layer already copied up,
1136		but still referenced from the file.  This mode is selected with a
1137		non-NULL inode argument.  This will always succeed.
1138	
1139		With NULL inode and zero flags the topmost real underlying dentry is
1140		returned.  This will always succeed.
1141	
1142		This method is never called with both non-NULL inode and non-zero flags.
1143	
1144	Each dentry has a pointer to its parent dentry, as well as a hash list
1145	of child dentries. Child dentries are basically like files in a
1146	directory.
1147	
1148	
1149	Directory Entry Cache API
1150	--------------------------
1151	
1152	There are a number of functions defined which permit a filesystem to
1153	manipulate dentries:
1154	
1155	  dget: open a new handle for an existing dentry (this just increments
1156		the usage count)
1157	
1158	  dput: close a handle for a dentry (decrements the usage count). If
1159		the usage count drops to 0, and the dentry is still in its
1160		parent's hash, the "d_delete" method is called to check whether
1161		it should be cached. If it should not be cached, or if the dentry
1162		is not hashed, it is deleted. Otherwise cached dentries are put
1163		into an LRU list to be reclaimed on memory shortage.
1164	
1165	  d_drop: this unhashes a dentry from its parents hash list. A
1166		subsequent call to dput() will deallocate the dentry if its
1167		usage count drops to 0
1168	
1169	  d_delete: delete a dentry. If there are no other open references to
1170		the dentry then the dentry is turned into a negative dentry
1171		(the d_iput() method is called). If there are other
1172		references, then d_drop() is called instead
1173	
1174	  d_add: add a dentry to its parents hash list and then calls
1175		d_instantiate()
1176	
1177	  d_instantiate: add a dentry to the alias hash list for the inode and
1178		updates the "d_inode" member. The "i_count" member in the
1179		inode structure should be set/incremented. If the inode
1180		pointer is NULL, the dentry is called a "negative
1181		dentry". This function is commonly called when an inode is
1182		created for an existing negative dentry
1183	
1184	  d_lookup: look up a dentry given its parent and path name component
1185		It looks up the child of that given name from the dcache
1186		hash table. If it is found, the reference count is incremented
1187		and the dentry is returned. The caller must use dput()
1188		to free the dentry when it finishes using it.
1189	
1190	Mount Options
1191	=============
1192	
1193	Parsing options
1194	---------------
1195	
1196	On mount and remount the filesystem is passed a string containing a
1197	comma separated list of mount options.  The options can have either of
1198	these forms:
1199	
1200	  option
1201	  option=value
1202	
1203	The <linux/parser.h> header defines an API that helps parse these
1204	options.  There are plenty of examples on how to use it in existing
1205	filesystems.
1206	
1207	Showing options
1208	---------------
1209	
1210	If a filesystem accepts mount options, it must define show_options()
1211	to show all the currently active options.  The rules are:
1212	
1213	  - options MUST be shown which are not default or their values differ
1214	    from the default
1215	
1216	  - options MAY be shown which are enabled by default or have their
1217	    default value
1218	
1219	Options used only internally between a mount helper and the kernel
1220	(such as file descriptors), or which only have an effect during the
1221	mounting (such as ones controlling the creation of a journal) are exempt
1222	from the above rules.
1223	
1224	The underlying reason for the above rules is to make sure, that a
1225	mount can be accurately replicated (e.g. umounting and mounting again)
1226	based on the information found in /proc/mounts.
1227	
1228	Resources
1229	=========
1230	
1231	(Note some of these resources are not up-to-date with the latest kernel
1232	 version.)
1233	
1234	Creating Linux virtual filesystems. 2002
1235	    <http://lwn.net/Articles/13325/>
1236	
1237	The Linux Virtual File-system Layer by Neil Brown. 1999
1238	    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1239	
1240	A tour of the Linux VFS by Michael K. Johnson. 1996
1241	    <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1242	
1243	A small trail through the Linux kernel by Andries Brouwer. 2001
1244	    <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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