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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
Pekka J Enberg5ea626a2005-09-09 13:10:19 -07002 Overview of the Linux Virtual File System
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
Pekka J Enberg5ea626a2005-09-09 13:10:19 -07004 Original author: Richard Gooch <rgooch@atnf.csiro.au>
Linus Torvalds1da177e2005-04-16 15:20:36 -07005
Borislav Petkov0746aec2007-07-15 23:41:19 -07006 Last updated on June 24, 2007.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -07007
8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
10
11 This file is released under the GPLv2.
Linus Torvalds1da177e2005-04-16 15:20:36 -070012
13
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080014Introduction
15============
Linus Torvalds1da177e2005-04-16 15:20:36 -070016
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080017The Virtual File System (also known as the Virtual Filesystem Switch)
18is the software layer in the kernel that provides the filesystem
19interface to userspace programs. It also provides an abstraction
20within the kernel which allows different filesystem implementations to
21coexist.
22
23VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24on are called from a process context. Filesystem locking is described
25in the document Documentation/filesystems/Locking.
Linus Torvalds1da177e2005-04-16 15:20:36 -070026
27
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080028Directory Entry Cache (dcache)
29------------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070030
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080031The VFS implements the open(2), stat(2), chmod(2), and similar system
32calls. The pathname argument that is passed to them is used by the VFS
33to search through the directory entry cache (also known as the dentry
34cache or dcache). This provides a very fast look-up mechanism to
35translate a pathname (filename) into a specific dentry. Dentries live
36in RAM and are never saved to disc: they exist only for performance.
37
38The dentry cache is meant to be a view into your entire filespace. As
39most computers cannot fit all dentries in the RAM at the same time,
40some bits of the cache are missing. In order to resolve your pathname
41into a dentry, the VFS may have to resort to creating dentries along
42the way, and then loading the inode. This is done by looking up the
43inode.
Linus Torvalds1da177e2005-04-16 15:20:36 -070044
Pekka J Enberg5ea626a2005-09-09 13:10:19 -070045
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080046The Inode Object
47----------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070048
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080049An individual dentry usually has a pointer to an inode. Inodes are
50filesystem objects such as regular files, directories, FIFOs and other
51beasts. They live either on the disc (for block device filesystems)
52or in the memory (for pseudo filesystems). Inodes that live on the
53disc are copied into the memory when required and changes to the inode
54are written back to disc. A single inode can be pointed to by multiple
55dentries (hard links, for example, do this).
Linus Torvalds1da177e2005-04-16 15:20:36 -070056
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080057To look up an inode requires that the VFS calls the lookup() method of
58the parent directory inode. This method is installed by the specific
59filesystem implementation that the inode lives in. Once the VFS has
60the required dentry (and hence the inode), we can do all those boring
61things like open(2) the file, or stat(2) it to peek at the inode
62data. The stat(2) operation is fairly simple: once the VFS has the
63dentry, it peeks at the inode data and passes some of it back to
64userspace.
Linus Torvalds1da177e2005-04-16 15:20:36 -070065
Linus Torvalds1da177e2005-04-16 15:20:36 -070066
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080067The File Object
68---------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070069
70Opening a file requires another operation: allocation of a file
71structure (this is the kernel-side implementation of file
Pekka J Enberg5ea626a2005-09-09 13:10:19 -070072descriptors). The freshly allocated file structure is initialized with
Linus Torvalds1da177e2005-04-16 15:20:36 -070073a pointer to the dentry and a set of file operation member functions.
74These are taken from the inode data. The open() file method is then
Francis Galieguea33f3222010-04-23 00:08:02 +020075called so the specific filesystem implementation can do its work. You
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080076can see that this is another switch performed by the VFS. The file
77structure is placed into the file descriptor table for the process.
Linus Torvalds1da177e2005-04-16 15:20:36 -070078
79Reading, writing and closing files (and other assorted VFS operations)
80is done by using the userspace file descriptor to grab the appropriate
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080081file structure, and then calling the required file structure method to
82do whatever is required. For as long as the file is open, it keeps the
83dentry in use, which in turn means that the VFS inode is still in use.
Linus Torvalds1da177e2005-04-16 15:20:36 -070084
Pekka J Enberg5ea626a2005-09-09 13:10:19 -070085
86Registering and Mounting a Filesystem
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080087=====================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070088
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080089To register and unregister a filesystem, use the following API
90functions:
Linus Torvalds1da177e2005-04-16 15:20:36 -070091
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080092 #include <linux/fs.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -070093
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080094 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
Linus Torvalds1da177e2005-04-16 15:20:36 -070096
Pekka Enbergcc7d1f82005-11-07 01:01:08 -080097The passed struct file_system_type describes your filesystem. When a
Al Viro1a102ff2011-03-16 09:07:58 -040098request is made to mount a filesystem onto a directory in your namespace,
99the VFS will call the appropriate mount() method for the specific
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300100filesystem. New vfsmount referring to the tree returned by ->mount()
Al Viro1a102ff2011-03-16 09:07:58 -0400101will be attached to the mountpoint, so that when pathname resolution
102reaches the mountpoint it will jump into the root of that vfsmount.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700103
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800104You can see all filesystems that are registered to the kernel in the
105file /proc/filesystems.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700106
107
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700108struct file_system_type
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800109-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110
Al Viro1a102ff2011-03-16 09:07:58 -0400111This describes the filesystem. As of kernel 2.6.39, the following
Linus Torvalds1da177e2005-04-16 15:20:36 -0700112members are defined:
113
114struct file_system_type {
115 const char *name;
116 int fs_flags;
Al Virob1349f22012-04-02 19:02:48 -0400117 struct dentry *(*mount) (struct file_system_type *, int,
Al Viro1a102ff2011-03-16 09:07:58 -0400118 const char *, void *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700119 void (*kill_sb) (struct super_block *);
120 struct module *owner;
121 struct file_system_type * next;
122 struct list_head fs_supers;
Borislav Petkov0746aec2007-07-15 23:41:19 -0700123 struct lock_class_key s_lock_key;
124 struct lock_class_key s_umount_key;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700125};
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
Al Viro1a102ff2011-03-16 09:07:58 -0400132 mount: the method to call when a new instance of this
Linus Torvalds1da177e2005-04-16 15:20:36 -0700133 filesystem should be mounted
134
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700135 kill_sb: the method to call when an instance of this filesystem
Al Viro1a102ff2011-03-16 09:07:58 -0400136 should be shut down
Linus Torvalds1da177e2005-04-16 15:20:36 -0700137
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700138 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
Borislav Petkov0746aec2007-07-15 23:41:19 -0700143 s_lock_key, s_umount_key: lockdep-specific
144
Al Viro1a102ff2011-03-16 09:07:58 -0400145The mount() method has the following arguments:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700146
Matt LaPlanted9195882008-07-25 19:45:33 -0700147 struct file_system_type *fs_type: describes the filesystem, partly initialized
Borislav Petkov0746aec2007-07-15 23:41:19 -0700148 by the specific filesystem code
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700149
150 int flags: mount flags
151
152 const char *dev_name: the device name we are mounting.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700153
154 void *data: arbitrary mount options, usually comes as an ASCII
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800155 string (see "Mount Options" section)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700156
Al Viro1a102ff2011-03-16 09:07:58 -0400157The mount() method must return the root dentry of the tree requested by
158caller. An active reference to its superblock must be grabbed and the
159superblock must be locked. On failure it should return ERR_PTR(error).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700160
Al Viro1a102ff2011-03-16 09:07:58 -0400161The arguments match those of mount(2) and their interpretation
162depends on filesystem type. E.g. for block filesystems, dev_name is
163interpreted as block device name, that device is opened and if it
164contains a suitable filesystem image the method creates and initializes
165struct super_block accordingly, returning its root dentry to caller.
166
167->mount() may choose to return a subtree of existing filesystem - it
168doesn't have to create a new one. The main result from the caller's
169point of view is a reference to dentry at the root of (sub)tree to
170be attached; creation of new superblock is a common side effect.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700171
172The most interesting member of the superblock structure that the
Al Viro1a102ff2011-03-16 09:07:58 -0400173mount() method fills in is the "s_op" field. This is a pointer to
Linus Torvalds1da177e2005-04-16 15:20:36 -0700174a "struct super_operations" which describes the next level of the
175filesystem implementation.
176
Al Viro1a102ff2011-03-16 09:07:58 -0400177Usually, a filesystem uses one of the generic mount() implementations
178and provides a fill_super() callback instead. The generic variants are:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700179
Al Viro1a102ff2011-03-16 09:07:58 -0400180 mount_bdev: mount a filesystem residing on a block device
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700181
Al Viro1a102ff2011-03-16 09:07:58 -0400182 mount_nodev: mount a filesystem that is not backed by a device
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700183
Al Viro1a102ff2011-03-16 09:07:58 -0400184 mount_single: mount a filesystem which shares the instance between
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700185 all mounts
186
Al Viro1a102ff2011-03-16 09:07:58 -0400187A fill_super() callback implementation has the following arguments:
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700188
Al Viro1a102ff2011-03-16 09:07:58 -0400189 struct super_block *sb: the superblock structure. The callback
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700190 must initialize this properly.
191
192 void *data: arbitrary mount options, usually comes as an ASCII
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800193 string (see "Mount Options" section)
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700194
195 int silent: whether or not to be silent on error
196
197
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800198The Superblock Object
199=====================
200
201A superblock object represents a mounted filesystem.
202
203
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700204struct super_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800205-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206
207This describes how the VFS can manipulate the superblock of your
Borislav Petkov422b14c2007-07-15 23:41:43 -0700208filesystem. As of kernel 2.6.22, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700209
210struct super_operations {
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
213
Christoph Hellwigaa385722011-05-27 06:53:02 -0400214 void (*dirty_inode) (struct inode *, int flags);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700215 int (*write_inode) (struct inode *, int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700216 void (*drop_inode) (struct inode *);
217 void (*delete_inode) (struct inode *);
218 void (*put_super) (struct super_block *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700219 int (*sync_fs)(struct super_block *sb, int wait);
Takashi Satoc4be0c12009-01-09 16:40:58 -0800220 int (*freeze_fs) (struct super_block *);
221 int (*unfreeze_fs) (struct super_block *);
David Howells726c3342006-06-23 02:02:58 -0700222 int (*statfs) (struct dentry *, struct kstatfs *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700223 int (*remount_fs) (struct super_block *, int *, char *);
224 void (*clear_inode) (struct inode *);
225 void (*umount_begin) (struct super_block *);
226
Al Viro34c80b12011-12-08 21:32:45 -0500227 int (*show_options)(struct seq_file *, struct dentry *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700228
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);
Dave Chinner0e1fdaf2011-07-08 14:14:44 +1000231 int (*nr_cached_objects)(struct super_block *);
232 void (*free_cached_objects)(struct super_block *, int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700233};
234
235All methods are called without any locks being held, unless otherwise
236noted. This means that most methods can block safely. All methods are
237only called from a process context (i.e. not from an interrupt handler
238or bottom half).
239
Kirill Smelkov4e07ad62014-08-14 15:25:10 +0400240 alloc_inode: this method is called by alloc_inode() to allocate memory
NeilBrown341546f2006-03-25 03:07:56 -0800241 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.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700245
246 destroy_inode: this method is called by destroy_inode() to release
NeilBrown341546f2006-03-25 03:07:56 -0800247 resources allocated for struct inode. It is only required if
248 ->alloc_inode was defined and simply undoes anything done by
249 ->alloc_inode.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700250
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700251 dirty_inode: this method is called by the VFS to mark an inode dirty.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252
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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700257 drop_inode: called when the last access to the inode is dropped,
Dave Chinnerf283c862011-03-22 22:23:39 +1100258 with the inode->i_lock spinlock held.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700259
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700260 This method should be either NULL (normal UNIX filesystem
Linus Torvalds1da177e2005-04-16 15:20:36 -0700261 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
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700265 The "generic_delete_inode()" behavior is equivalent to the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700266 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
Linus Torvalds1da177e2005-04-16 15:20:36 -0700272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
274
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700275 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
Takashi Satoc4be0c12009-01-09 16:40:58 -0800279 freeze_fs: called when VFS is locking a filesystem and
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800280 forcing it into a consistent state. This method is currently
281 used by the Logical Volume Manager (LVM).
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700282
Takashi Satoc4be0c12009-01-09 16:40:58 -0800283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700284 again.
285
Adrian McMenamin66672fe2009-04-20 18:38:28 -0700286 statfs: called when the VFS needs to get filesystem statistics.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700287
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
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700293 umount_begin: called when the VFS is unmounting a filesystem.
294
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700297
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
Dave Chinner0e1fdaf2011-07-08 14:14:44 +1000302 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
Dave Chinner8ab47662011-07-08 14:14:45 +1000316 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
David Howells12debc42008-02-07 00:15:52 -0800322Whoever sets up the inode is responsible for filling in the "i_op" field. This
323is a pointer to a "struct inode_operations" which describes the methods that
324can be performed on individual inodes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700325
326
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800327The Inode Object
328================
329
330An inode object represents an object within the filesystem.
331
332
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700333struct inode_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800334-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700335
336This describes how the VFS can manipulate an inode in your
Borislav Petkov422b14c2007-07-15 23:41:43 -0700337filesystem. As of kernel 2.6.22, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700338
339struct inode_operations {
Al Viroebfc3b42012-06-10 18:05:36 -0400340 int (*create) (struct inode *,struct dentry *, umode_t, bool);
Al Viro00cd8dd2012-06-10 17:13:09 -0400341 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700342 int (*link) (struct dentry *,struct inode *,struct dentry *);
343 int (*unlink) (struct inode *,struct dentry *);
344 int (*symlink) (struct inode *,struct dentry *,const char *);
Al Viro18bb1db2011-07-26 01:41:39 -0400345 int (*mkdir) (struct inode *,struct dentry *,umode_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700346 int (*rmdir) (struct inode *,struct dentry *);
Al Viro1a67aaf2011-07-26 01:52:52 -0400347 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700348 int (*rename) (struct inode *, struct dentry *,
349 struct inode *, struct dentry *);
Miklos Szeredi520c8b12014-04-01 17:08:42 +0200350 int (*rename2) (struct inode *, struct dentry *,
351 struct inode *, struct dentry *, unsigned int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700352 int (*readlink) (struct dentry *, char __user *,int);
353 void * (*follow_link) (struct dentry *, struct nameidata *);
354 void (*put_link) (struct dentry *, struct nameidata *, void *);
Al Viro10556cb2011-06-20 19:28:19 -0400355 int (*permission) (struct inode *, int);
Christoph Hellwig4e34e712011-07-23 17:37:31 +0200356 int (*get_acl)(struct inode *, int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700357 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 *);
Josef Bacikc3b2da32012-03-26 09:59:21 -0400363 void (*update_time)(struct inode *, struct timespec *, int);
Miklos Szeredi0854d452013-09-16 14:51:55 +0200364 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
365 unsigned open_flag, umode_t create_mode, int *opened);
Al Viro48bde8d2013-07-03 16:19:23 +0400366 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700367};
368
369Again, all methods are called without any locks being held, unless
370otherwise noted.
371
Linus Torvalds1da177e2005-04-16 15:20:36 -0700372 create: called by the open(2) and creat(2) system calls. Only
373 required if you want to support regular files. The dentry you
374 get should not have an inode (i.e. it should be a negative
375 dentry). Here you will probably call d_instantiate() with the
376 dentry and the newly created inode
377
378 lookup: called when the VFS needs to look up an inode in a parent
379 directory. The name to look for is found in the dentry. This
380 method must call d_add() to insert the found inode into the
381 dentry. The "i_count" field in the inode structure should be
382 incremented. If the named inode does not exist a NULL inode
383 should be inserted into the dentry (this is called a negative
384 dentry). Returning an error code from this routine must only
385 be done on a real error, otherwise creating inodes with system
386 calls like create(2), mknod(2), mkdir(2) and so on will fail.
387 If you wish to overload the dentry methods then you should
388 initialise the "d_dop" field in the dentry; this is a pointer
389 to a struct "dentry_operations".
390 This method is called with the directory inode semaphore held
391
392 link: called by the link(2) system call. Only required if you want
393 to support hard links. You will probably need to call
394 d_instantiate() just as you would in the create() method
395
396 unlink: called by the unlink(2) system call. Only required if you
397 want to support deleting inodes
398
399 symlink: called by the symlink(2) system call. Only required if you
400 want to support symlinks. You will probably need to call
401 d_instantiate() just as you would in the create() method
402
403 mkdir: called by the mkdir(2) system call. Only required if you want
404 to support creating subdirectories. You will probably need to
405 call d_instantiate() just as you would in the create() method
406
407 rmdir: called by the rmdir(2) system call. Only required if you want
408 to support deleting subdirectories
409
410 mknod: called by the mknod(2) system call to create a device (char,
411 block) inode or a named pipe (FIFO) or socket. Only required
412 if you want to support creating these types of inodes. You
413 will probably need to call d_instantiate() just as you would
414 in the create() method
415
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800416 rename: called by the rename(2) system call to rename the object to
417 have the parent and name given by the second inode and dentry.
418
Miklos Szeredi520c8b12014-04-01 17:08:42 +0200419 rename2: this has an additional flags argument compared to rename.
420 If no flags are supported by the filesystem then this method
421 need not be implemented. If some flags are supported then the
422 filesystem must return -EINVAL for any unsupported or unknown
423 flags. Currently the following flags are implemented:
424 (1) RENAME_NOREPLACE: this flag indicates that if the target
425 of the rename exists the rename should fail with -EEXIST
426 instead of replacing the target. The VFS already checks for
427 existence, so for local filesystems the RENAME_NOREPLACE
428 implementation is equivalent to plain rename.
429 (2) RENAME_EXCHANGE: exchange source and target. Both must
430 exist; this is checked by the VFS. Unlike plain rename,
431 source and target may be of different type.
432
Linus Torvalds1da177e2005-04-16 15:20:36 -0700433 readlink: called by the readlink(2) system call. Only required if
434 you want to support reading symbolic links
435
436 follow_link: called by the VFS to follow a symbolic link to the
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700437 inode it points to. Only required if you want to support
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800438 symbolic links. This method returns a void pointer cookie
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700439 that is passed to put_link().
440
441 put_link: called by the VFS to release resources allocated by
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800442 follow_link(). The cookie returned by follow_link() is passed
Paolo Ornati670e9f32006-10-03 22:57:56 +0200443 to this method as the last parameter. It is used by
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800444 filesystems such as NFS where page cache is not stable
445 (i.e. page that was installed when the symbolic link walk
446 started might not be in the page cache at the end of the
447 walk).
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700448
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700449 permission: called by the VFS to check for access rights on a POSIX-like
450 filesystem.
451
Al Viro10556cb2011-06-20 19:28:19 -0400452 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
Nick Piggina82416d2011-01-14 02:26:53 +0000453 mode, the filesystem must check the permission without blocking or
Nick Pigginb74c79e2011-01-07 17:49:58 +1100454 storing to the inode.
455
456 If a situation is encountered that rcu-walk cannot handle, return
457 -ECHILD and it will be called again in ref-walk mode.
458
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800459 setattr: called by the VFS to set attributes for a file. This method
460 is called by chmod(2) and related system calls.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700461
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800462 getattr: called by the VFS to get attributes of a file. This method
463 is called by stat(2) and related system calls.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700464
465 setxattr: called by the VFS to set an extended attribute for a file.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800466 Extended attribute is a name:value pair associated with an
467 inode. This method is called by setxattr(2) system call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700468
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800469 getxattr: called by the VFS to retrieve the value of an extended
470 attribute name. This method is called by getxattr(2) function
471 call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700472
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800473 listxattr: called by the VFS to list all extended attributes for a
474 given file. This method is called by listxattr(2) system call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700475
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800476 removexattr: called by the VFS to remove an extended attribute from
477 a file. This method is called by removexattr(2) system call.
478
Josef Bacikc3b2da32012-03-26 09:59:21 -0400479 update_time: called by the VFS to update a specific time or the i_version of
480 an inode. If this is not defined the VFS will update the inode itself
481 and call mark_inode_dirty_sync.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800482
Miklos Szeredid18e9002012-06-05 15:10:17 +0200483 atomic_open: called on the last component of an open. Using this optional
484 method the filesystem can look up, possibly create and open the file in
485 one atomic operation. If it cannot perform this (e.g. the file type
Al Virod9585272012-06-22 12:39:14 +0400486 turned out to be wrong) it may signal this by returning 1 instead of
Miklos Szeredi0854d452013-09-16 14:51:55 +0200487 usual 0 or -ve . This method is only called if the last component is
488 negative or needs lookup. Cached positive dentries are still handled by
489 f_op->open(). If the file was created, the FILE_CREATED flag should be
490 set in "opened". In case of O_EXCL the method must only succeed if the
491 file didn't exist and hence FILE_CREATED shall always be set on success.
Miklos Szeredid18e9002012-06-05 15:10:17 +0200492
Al Viro48bde8d2013-07-03 16:19:23 +0400493 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
494 atomically creating, opening and unlinking a file in given directory.
495
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800496The Address Space Object
497========================
498
NeilBrown341546f2006-03-25 03:07:56 -0800499The address space object is used to group and manage pages in the page
500cache. It can be used to keep track of the pages in a file (or
501anything else) and also track the mapping of sections of the file into
502process address spaces.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700503
NeilBrown341546f2006-03-25 03:07:56 -0800504There are a number of distinct yet related services that an
505address-space can provide. These include communicating memory
506pressure, page lookup by address, and keeping track of pages tagged as
507Dirty or Writeback.
508
NeilBrowna9e102b2006-03-25 03:08:29 -0800509The first can be used independently to the others. The VM can try to
NeilBrown341546f2006-03-25 03:07:56 -0800510either write dirty pages in order to clean them, or release clean
511pages in order to reuse them. To do this it can call the ->writepage
512method on dirty pages, and ->releasepage on clean pages with
513PagePrivate set. Clean pages without PagePrivate and with no external
514references will be released without notice being given to the
515address_space.
516
NeilBrowna9e102b2006-03-25 03:08:29 -0800517To achieve this functionality, pages need to be placed on an LRU with
NeilBrown341546f2006-03-25 03:07:56 -0800518lru_cache_add and mark_page_active needs to be called whenever the
519page is used.
520
521Pages are normally kept in a radix tree index by ->index. This tree
522maintains information about the PG_Dirty and PG_Writeback status of
523each page, so that pages with either of these flags can be found
524quickly.
525
526The Dirty tag is primarily used by mpage_writepages - the default
527->writepages method. It uses the tag to find dirty pages to call
528->writepage on. If mpage_writepages is not used (i.e. the address
NeilBrowna9e102b2006-03-25 03:08:29 -0800529provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
NeilBrown341546f2006-03-25 03:07:56 -0800530almost unused. write_inode_now and sync_inode do use it (through
531__sync_single_inode) to check if ->writepages has been successful in
532writing out the whole address_space.
533
534The Writeback tag is used by filemap*wait* and sync_page* functions,
Christoph Hellwig94004ed2009-09-30 22:16:33 +0200535via filemap_fdatawait_range, to wait for all writeback to
NeilBrown341546f2006-03-25 03:07:56 -0800536complete. While waiting ->sync_page (if defined) will be called on
NeilBrowna9e102b2006-03-25 03:08:29 -0800537each page that is found to require writeback.
NeilBrown341546f2006-03-25 03:07:56 -0800538
539An address_space handler may attach extra information to a page,
540typically using the 'private' field in the 'struct page'. If such
541information is attached, the PG_Private flag should be set. This will
NeilBrowna9e102b2006-03-25 03:08:29 -0800542cause various VM routines to make extra calls into the address_space
NeilBrown341546f2006-03-25 03:07:56 -0800543handler to deal with that data.
544
545An address space acts as an intermediate between storage and
546application. Data is read into the address space a whole page at a
547time, and provided to the application either by copying of the page,
548or by memory-mapping the page.
549Data is written into the address space by the application, and then
550written-back to storage typically in whole pages, however the
NeilBrowna9e102b2006-03-25 03:08:29 -0800551address_space has finer control of write sizes.
NeilBrown341546f2006-03-25 03:07:56 -0800552
553The read process essentially only requires 'readpage'. The write
Nick Piggin4e02ed42008-10-29 14:00:55 -0700554process is more complicated and uses write_begin/write_end or
NeilBrown341546f2006-03-25 03:07:56 -0800555set_page_dirty to write data into the address_space, and writepage,
556sync_page, and writepages to writeback data to storage.
557
558Adding and removing pages to/from an address_space is protected by the
559inode's i_mutex.
560
561When data is written to a page, the PG_Dirty flag should be set. It
562typically remains set until writepage asks for it to be written. This
563should clear PG_Dirty and set PG_Writeback. It can be actually
564written at any point after PG_Dirty is clear. Once it is known to be
565safe, PG_Writeback is cleared.
566
567Writeback makes use of a writeback_control structure...
Linus Torvalds1da177e2005-04-16 15:20:36 -0700568
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700569struct address_space_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800570-------------------------------
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700571
572This describes how the VFS can manipulate mapping of a file to page cache in
Lukas Czernerd47992f2013-05-21 23:17:23 -0400573your filesystem. The following members are defined:
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700574
575struct address_space_operations {
576 int (*writepage)(struct page *page, struct writeback_control *wbc);
577 int (*readpage)(struct file *, struct page *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700578 int (*writepages)(struct address_space *, struct writeback_control *);
579 int (*set_page_dirty)(struct page *page);
580 int (*readpages)(struct file *filp, struct address_space *mapping,
581 struct list_head *pages, unsigned nr_pages);
Nick Pigginafddba42007-10-16 01:25:01 -0700582 int (*write_begin)(struct file *, struct address_space *mapping,
583 loff_t pos, unsigned len, unsigned flags,
584 struct page **pagep, void **fsdata);
585 int (*write_end)(struct file *, struct address_space *mapping,
586 loff_t pos, unsigned len, unsigned copied,
587 struct page *page, void *fsdata);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700588 sector_t (*bmap)(struct address_space *, sector_t);
Lukas Czernerd47992f2013-05-21 23:17:23 -0400589 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700590 int (*releasepage) (struct page *, int);
Linus Torvalds6072d132010-12-01 13:35:19 -0500591 void (*freepage)(struct page *);
Al Virod8d3d942014-03-04 21:27:34 -0500592 ssize_t (*direct_IO)(int, struct kiocb *, struct iov_iter *iter, loff_t offset);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700593 struct page* (*get_xip_page)(struct address_space *, sector_t,
594 int);
NeilBrown341546f2006-03-25 03:07:56 -0800595 /* migrate the contents of a page to the specified target */
596 int (*migratepage) (struct page *, struct page *);
Borislav Petkov422b14c2007-07-15 23:41:43 -0700597 int (*launder_page) (struct page *);
Al Viroc186afb42014-02-02 21:16:54 -0500598 int (*is_partially_uptodate) (struct page *, unsigned long,
Mel Gorman26c0c5b2013-07-03 15:04:45 -0700599 unsigned long);
Mel Gorman543cc112013-07-03 15:04:46 -0700600 void (*is_dirty_writeback) (struct page *, bool *, bool *);
Andi Kleen25718732009-09-16 11:50:13 +0200601 int (*error_remove_page) (struct mapping *mapping, struct page *page);
Mel Gorman62c230b2012-07-31 16:44:55 -0700602 int (*swap_activate)(struct file *);
603 int (*swap_deactivate)(struct file *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700604};
605
NeilBrown341546f2006-03-25 03:07:56 -0800606 writepage: called by the VM to write a dirty page to backing store.
NeilBrowna9e102b2006-03-25 03:08:29 -0800607 This may happen for data integrity reasons (i.e. 'sync'), or
NeilBrown341546f2006-03-25 03:07:56 -0800608 to free up memory (flush). The difference can be seen in
609 wbc->sync_mode.
610 The PG_Dirty flag has been cleared and PageLocked is true.
611 writepage should start writeout, should set PG_Writeback,
612 and should make sure the page is unlocked, either synchronously
613 or asynchronously when the write operation completes.
614
615 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
NeilBrowna9e102b2006-03-25 03:08:29 -0800616 try too hard if there are problems, and may choose to write out
617 other pages from the mapping if that is easier (e.g. due to
618 internal dependencies). If it chooses not to start writeout, it
619 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
NeilBrown341546f2006-03-25 03:07:56 -0800620 calling ->writepage on that page.
621
622 See the file "Locking" for more details.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700623
624 readpage: called by the VM to read a page from backing store.
NeilBrown341546f2006-03-25 03:07:56 -0800625 The page will be Locked when readpage is called, and should be
626 unlocked and marked uptodate once the read completes.
627 If ->readpage discovers that it needs to unlock the page for
628 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
NeilBrowna9e102b2006-03-25 03:08:29 -0800629 In this case, the page will be relocated, relocked and if
NeilBrown341546f2006-03-25 03:07:56 -0800630 that all succeeds, ->readpage will be called again.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700631
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700632 writepages: called by the VM to write out pages associated with the
NeilBrowna9e102b2006-03-25 03:08:29 -0800633 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
634 the writeback_control will specify a range of pages that must be
635 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
NeilBrown341546f2006-03-25 03:07:56 -0800636 and that many pages should be written if possible.
637 If no ->writepages is given, then mpage_writepages is used
NeilBrowna9e102b2006-03-25 03:08:29 -0800638 instead. This will choose pages from the address space that are
NeilBrown341546f2006-03-25 03:07:56 -0800639 tagged as DIRTY and will pass them to ->writepage.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700640
641 set_page_dirty: called by the VM to set a page dirty.
NeilBrown341546f2006-03-25 03:07:56 -0800642 This is particularly needed if an address space attaches
643 private data to a page, and that data needs to be updated when
644 a page is dirtied. This is called, for example, when a memory
645 mapped page gets modified.
646 If defined, it should set the PageDirty flag, and the
647 PAGECACHE_TAG_DIRTY tag in the radix tree.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700648
649 readpages: called by the VM to read pages associated with the address_space
NeilBrown341546f2006-03-25 03:07:56 -0800650 object. This is essentially just a vector version of
651 readpage. Instead of just one page, several pages are
652 requested.
NeilBrowna9e102b2006-03-25 03:08:29 -0800653 readpages is only used for read-ahead, so read errors are
NeilBrown341546f2006-03-25 03:07:56 -0800654 ignored. If anything goes wrong, feel free to give up.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700655
Nick Piggin4e02ed42008-10-29 14:00:55 -0700656 write_begin:
Nick Pigginafddba42007-10-16 01:25:01 -0700657 Called by the generic buffered write code to ask the filesystem to
658 prepare to write len bytes at the given offset in the file. The
659 address_space should check that the write will be able to complete,
660 by allocating space if necessary and doing any other internal
661 housekeeping. If the write will update parts of any basic-blocks on
662 storage, then those blocks should be pre-read (if they haven't been
663 read already) so that the updated blocks can be written out properly.
664
665 The filesystem must return the locked pagecache page for the specified
666 offset, in *pagep, for the caller to write into.
667
Nick Piggin4e02ed42008-10-29 14:00:55 -0700668 It must be able to cope with short writes (where the length passed to
669 write_begin is greater than the number of bytes copied into the page).
670
Nick Pigginafddba42007-10-16 01:25:01 -0700671 flags is a field for AOP_FLAG_xxx flags, described in
672 include/linux/fs.h.
673
674 A void * may be returned in fsdata, which then gets passed into
675 write_end.
676
677 Returns 0 on success; < 0 on failure (which is the error code), in
678 which case write_end is not called.
679
680 write_end: After a successful write_begin, and data copy, write_end must
681 be called. len is the original len passed to write_begin, and copied
682 is the amount that was able to be copied (copied == len is always true
683 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
684
685 The filesystem must take care of unlocking the page and releasing it
686 refcount, and updating i_size.
687
688 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
689 that were able to be copied into pagecache.
690
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700691 bmap: called by the VFS to map a logical block offset within object to
NeilBrowna9e102b2006-03-25 03:08:29 -0800692 physical block number. This method is used by the FIBMAP
NeilBrown341546f2006-03-25 03:07:56 -0800693 ioctl and for working with swap-files. To be able to swap to
NeilBrowna9e102b2006-03-25 03:08:29 -0800694 a file, the file must have a stable mapping to a block
NeilBrown341546f2006-03-25 03:07:56 -0800695 device. The swap system does not go through the filesystem
696 but instead uses bmap to find out where the blocks in the file
697 are and uses those addresses directly.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700698
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700699
NeilBrown341546f2006-03-25 03:07:56 -0800700 invalidatepage: If a page has PagePrivate set, then invalidatepage
701 will be called when part or all of the page is to be removed
NeilBrowna9e102b2006-03-25 03:08:29 -0800702 from the address space. This generally corresponds to either a
Lukas Czernerd47992f2013-05-21 23:17:23 -0400703 truncation, punch hole or a complete invalidation of the address
704 space (in the latter case 'offset' will always be 0 and 'length'
705 will be PAGE_CACHE_SIZE). Any private data associated with the page
706 should be updated to reflect this truncation. If offset is 0 and
707 length is PAGE_CACHE_SIZE, then the private data should be released,
708 because the page must be able to be completely discarded. This may
709 be done by calling the ->releasepage function, but in this case the
710 release MUST succeed.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700711
NeilBrown341546f2006-03-25 03:07:56 -0800712 releasepage: releasepage is called on PagePrivate pages to indicate
713 that the page should be freed if possible. ->releasepage
714 should remove any private data from the page and clear the
Andrew Morton4fe65ca2010-12-02 14:31:19 -0800715 PagePrivate flag. If releasepage() fails for some reason, it must
716 indicate failure with a 0 return value.
717 releasepage() is used in two distinct though related cases. The
718 first is when the VM finds a clean page with no active users and
NeilBrown341546f2006-03-25 03:07:56 -0800719 wants to make it a free page. If ->releasepage succeeds, the
720 page will be removed from the address_space and become free.
721
Shaun Zinckbc5b1d52007-10-20 02:35:36 +0200722 The second case is when a request has been made to invalidate
NeilBrown341546f2006-03-25 03:07:56 -0800723 some or all pages in an address_space. This can happen
724 through the fadvice(POSIX_FADV_DONTNEED) system call or by the
725 filesystem explicitly requesting it as nfs and 9fs do (when
726 they believe the cache may be out of date with storage) by
727 calling invalidate_inode_pages2().
728 If the filesystem makes such a call, and needs to be certain
NeilBrowna9e102b2006-03-25 03:08:29 -0800729 that all pages are invalidated, then its releasepage will
NeilBrown341546f2006-03-25 03:07:56 -0800730 need to ensure this. Possibly it can clear the PageUptodate
731 bit if it cannot free private data yet.
732
Linus Torvalds6072d132010-12-01 13:35:19 -0500733 freepage: freepage is called once the page is no longer visible in
734 the page cache in order to allow the cleanup of any private
735 data. Since it may be called by the memory reclaimer, it
736 should not assume that the original address_space mapping still
737 exists, and it should not block.
738
NeilBrown341546f2006-03-25 03:07:56 -0800739 direct_IO: called by the generic read/write routines to perform
740 direct_IO - that is IO requests which bypass the page cache
NeilBrowna9e102b2006-03-25 03:08:29 -0800741 and transfer data directly between the storage and the
NeilBrown341546f2006-03-25 03:07:56 -0800742 application's address space.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700743
744 get_xip_page: called by the VM to translate a block number to a page.
745 The page is valid until the corresponding filesystem is unmounted.
746 Filesystems that want to use execute-in-place (XIP) need to implement
747 it. An example implementation can be found in fs/ext2/xip.c.
748
NeilBrown341546f2006-03-25 03:07:56 -0800749 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.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700755
Borislav Petkov422b14c2007-07-15 23:41:43 -0700756 launder_page: Called before freeing a page - it writes back the dirty page. To
757 prevent redirtying the page, it is kept locked during the whole
758 operation.
759
Mel Gorman26c0c5b2013-07-03 15:04:45 -0700760 is_partially_uptodate: Called by the VM when reading a file through the
761 pagecache when the underlying blocksize != pagesize. If the required
762 block is up to date then the read can complete without needing the IO
763 to bring the whole page up to date.
764
Mel Gorman543cc112013-07-03 15:04:46 -0700765 is_dirty_writeback: Called by the VM when attempting to reclaim a page.
766 The VM uses dirty and writeback information to determine if it needs
767 to stall to allow flushers a chance to complete some IO. Ordinarily
768 it can use PageDirty and PageWriteback but some filesystems have
769 more complex state (unstable pages in NFS prevent reclaim) or
770 do not set those flags due to locking problems (jbd). This callback
771 allows a filesystem to indicate to the VM if a page should be
772 treated as dirty or writeback for the purposes of stalling.
773
Andi Kleen25718732009-09-16 11:50:13 +0200774 error_remove_page: normally set to generic_error_remove_page if truncation
775 is ok for this address space. Used for memory failure handling.
776 Setting this implies you deal with pages going away under you,
777 unless you have them locked or reference counts increased.
778
Mel Gorman62c230b2012-07-31 16:44:55 -0700779 swap_activate: Called when swapon is used on a file to allocate
780 space if necessary and pin the block lookup information in
781 memory. A return value of zero indicates success,
782 in which case this file can be used to back swapspace. The
783 swapspace operations will be proxied to this address space's
784 ->swap_{out,in} methods.
785
786 swap_deactivate: Called during swapoff on files where swap_activate
787 was successful.
788
Andi Kleen25718732009-09-16 11:50:13 +0200789
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800790The File Object
791===============
792
793A file object represents a file opened by a process.
794
795
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700796struct file_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800797----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700798
799This describes how the VFS can manipulate an open file. As of kernel
Richard Yao46bf16c2014-01-30 15:46:12 -08008003.12, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700801
802struct file_operations {
Borislav Petkov422b14c2007-07-15 23:41:43 -0700803 struct module *owner;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700804 loff_t (*llseek) (struct file *, loff_t, int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700805 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700806 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
Badari Pulavarty027445c2006-09-30 23:28:46 -0700807 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
808 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
Al Viro293bc982014-02-11 18:37:41 -0500809 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
810 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
Al Viro2233f312013-05-22 21:44:23 -0400811 int (*iterate) (struct file *, struct dir_context *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700812 unsigned int (*poll) (struct file *, struct poll_table_struct *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700813 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
814 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700815 int (*mmap) (struct file *, struct vm_area_struct *);
816 int (*open) (struct inode *, struct file *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700817 int (*flush) (struct file *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700818 int (*release) (struct inode *, struct file *);
Josef Bacik02c24a82011-07-16 20:44:56 -0400819 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700820 int (*aio_fsync) (struct kiocb *, int datasync);
821 int (*fasync) (int, struct file *, int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700822 int (*lock) (struct file *, int, struct file_lock *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700823 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
824 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
825 int (*check_flags)(int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700826 int (*flock) (struct file *, int, struct file_lock *);
Borislav Petkov422b14c2007-07-15 23:41:43 -0700827 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
828 ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
Jeff Laytone6f5c782014-08-22 10:40:25 -0400829 int (*setlease)(struct file *, long arg, struct file_lock **, void **);
Hugh Dickins17cf28a2012-05-29 15:06:41 -0700830 long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
Richard Yao46bf16c2014-01-30 15:46:12 -0800831 int (*show_fdinfo)(struct seq_file *m, struct file *f);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700832};
833
834Again, all methods are called without any locks being held, unless
835otherwise noted.
836
837 llseek: called when the VFS needs to move the file position index
838
839 read: called by read(2) and related system calls
840
Al Viro293bc982014-02-11 18:37:41 -0500841 aio_read: vectored, possibly asynchronous read
842
843 read_iter: possibly asynchronous read with iov_iter as destination
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700844
Linus Torvalds1da177e2005-04-16 15:20:36 -0700845 write: called by write(2) and related system calls
846
Al Viro293bc982014-02-11 18:37:41 -0500847 aio_write: vectored, possibly asynchronous write
848
849 write_iter: possibly asynchronous write with iov_iter as source
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700850
Al Viro2233f312013-05-22 21:44:23 -0400851 iterate: called when the VFS needs to read the directory contents
Linus Torvalds1da177e2005-04-16 15:20:36 -0700852
853 poll: called by the VFS when a process wants to check if there is
854 activity on this file and (optionally) go to sleep until there
855 is activity. Called by the select(2) and poll(2) system calls
856
Arnd Bergmannb19dd422010-07-04 00:15:10 +0200857 unlocked_ioctl: called by the ioctl(2) system call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700858
859 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
860 are used on 64 bit kernels.
861
Linus Torvalds1da177e2005-04-16 15:20:36 -0700862 mmap: called by the mmap(2) system call
863
864 open: called by the VFS when an inode should be opened. When the VFS
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700865 opens a file, it creates a new "struct file". It then calls the
866 open method for the newly allocated file structure. You might
867 think that the open method really belongs in
868 "struct inode_operations", and you may be right. I think it's
869 done the way it is because it makes filesystems simpler to
870 implement. The open() method is a good place to initialize the
871 "private_data" member in the file structure if you want to point
872 to a device structure
873
874 flush: called by the close(2) system call to flush a file
Linus Torvalds1da177e2005-04-16 15:20:36 -0700875
876 release: called when the last reference to an open file is closed
877
878 fsync: called by the fsync(2) system call
879
880 fasync: called by the fcntl(2) system call when asynchronous
881 (non-blocking) mode is enabled for a file
882
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700883 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
884 commands
885
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700886 get_unmapped_area: called by the mmap(2) system call
887
888 check_flags: called by the fcntl(2) system call for F_SETFL command
889
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700890 flock: called by the flock(2) system call
891
Pekka J Enbergd1195c52006-04-11 14:21:59 +0200892 splice_write: called by the VFS to splice data from a pipe to a file. This
893 method is used by the splice(2) system call
894
895 splice_read: called by the VFS to splice data from file to a pipe. This
896 method is used by the splice(2) system call
897
Jeff Laytonf82b4b62014-08-22 18:50:48 -0400898 setlease: called by the VFS to set or release a file lock lease. setlease
899 implementations should call generic_setlease to record or remove
900 the lease in the inode after setting it.
Hugh Dickins17cf28a2012-05-29 15:06:41 -0700901
902 fallocate: called by the VFS to preallocate blocks or punch a hole.
903
Linus Torvalds1da177e2005-04-16 15:20:36 -0700904Note that the file operations are implemented by the specific
905filesystem in which the inode resides. When opening a device node
906(character or block special) most filesystems will call special
907support routines in the VFS which will locate the required device
908driver information. These support routines replace the filesystem file
909operations with those for the device driver, and then proceed to call
910the new open() method for the file. This is how opening a device file
911in the filesystem eventually ends up calling the device driver open()
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700912method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700913
914
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700915Directory Entry Cache (dcache)
916==============================
917
Linus Torvalds1da177e2005-04-16 15:20:36 -0700918
919struct dentry_operations
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700920------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700921
922This describes how a filesystem can overload the standard dentry
923operations. Dentries and the dcache are the domain of the VFS and the
924individual filesystem implementations. Device drivers have no business
925here. These methods may be set to NULL, as they are either optional or
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700926the VFS uses a default. As of kernel 2.6.22, the following members are
Linus Torvalds1da177e2005-04-16 15:20:36 -0700927defined:
928
929struct dentry_operations {
Al Viro0b728e12012-06-10 16:03:43 -0400930 int (*d_revalidate)(struct dentry *, unsigned int);
Jeff Laytonecf3d1f2013-02-20 11:19:05 -0500931 int (*d_weak_revalidate)(struct dentry *, unsigned int);
Linus Torvaldsda53be12013-05-21 15:22:44 -0700932 int (*d_hash)(const struct dentry *, struct qstr *);
933 int (*d_compare)(const struct dentry *, const struct dentry *,
Nick Piggin621e1552011-01-07 17:49:27 +1100934 unsigned int, const char *, const struct qstr *);
Nick Pigginfe15ce42011-01-07 17:49:23 +1100935 int (*d_delete)(const struct dentry *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700936 void (*d_release)(struct dentry *);
937 void (*d_iput)(struct dentry *, struct inode *);
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700938 char *(*d_dname)(struct dentry *, char *, int);
David Howells9875cf82011-01-14 18:45:21 +0000939 struct vfsmount *(*d_automount)(struct path *);
Al Viro1aed3e42011-03-18 09:09:02 -0400940 int (*d_manage)(struct dentry *, bool);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700941};
942
943 d_revalidate: called when the VFS needs to revalidate a dentry. This
944 is called whenever a name look-up finds a dentry in the
Jeff Laytonecf3d1f2013-02-20 11:19:05 -0500945 dcache. Most local filesystems leave this as NULL, because all their
946 dentries in the dcache are valid. Network filesystems are different
947 since things can change on the server without the client necessarily
948 being aware of it.
949
950 This function should return a positive value if the dentry is still
951 valid, and zero or a negative error code if it isn't.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700952
Al Viro0b728e12012-06-10 16:03:43 -0400953 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
Nick Piggin34286d62011-01-07 17:49:57 +1100954 If in rcu-walk mode, the filesystem must revalidate the dentry without
955 blocking or storing to the dentry, d_parent and d_inode should not be
Al Viro0b728e12012-06-10 16:03:43 -0400956 used without care (because they can change and, in d_inode case, even
957 become NULL under us).
Nick Piggin34286d62011-01-07 17:49:57 +1100958
959 If a situation is encountered that rcu-walk cannot handle, return
960 -ECHILD and it will be called again in ref-walk mode.
961
Jeff Laytonecf3d1f2013-02-20 11:19:05 -0500962 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
963 This is called when a path-walk ends at dentry that was not acquired by
964 doing a lookup in the parent directory. This includes "/", "." and "..",
965 as well as procfs-style symlinks and mountpoint traversal.
966
967 In this case, we are less concerned with whether the dentry is still
968 fully correct, but rather that the inode is still valid. As with
969 d_revalidate, most local filesystems will set this to NULL since their
970 dcache entries are always valid.
971
972 This function has the same return code semantics as d_revalidate.
973
974 d_weak_revalidate is only called after leaving rcu-walk mode.
975
Nick Piggin621e1552011-01-07 17:49:27 +1100976 d_hash: called when the VFS adds a dentry to the hash table. The first
977 dentry passed to d_hash is the parent directory that the name is
Linus Torvaldsda53be12013-05-21 15:22:44 -0700978 to be hashed into.
Nick Pigginb1e6a012011-01-07 17:49:28 +1100979
980 Same locking and synchronisation rules as d_compare regarding
981 what is safe to dereference etc.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700982
Nick Piggin621e1552011-01-07 17:49:27 +1100983 d_compare: called to compare a dentry name with a given name. The first
984 dentry is the parent of the dentry to be compared, the second is
Linus Torvaldsda53be12013-05-21 15:22:44 -0700985 the child dentry. len and name string are properties of the dentry
986 to be compared. qstr is the name to compare it with.
Nick Piggin621e1552011-01-07 17:49:27 +1100987
988 Must be constant and idempotent, and should not take locks if
Linus Torvaldsda53be12013-05-21 15:22:44 -0700989 possible, and should not or store into the dentry.
990 Should not dereference pointers outside the dentry without
Nick Piggin621e1552011-01-07 17:49:27 +1100991 lots of care (eg. d_parent, d_inode, d_name should not be used).
992
993 However, our vfsmount is pinned, and RCU held, so the dentries and
994 inodes won't disappear, neither will our sb or filesystem module.
Linus Torvaldsda53be12013-05-21 15:22:44 -0700995 ->d_sb may be used.
Nick Piggin621e1552011-01-07 17:49:27 +1100996
997 It is a tricky calling convention because it needs to be called under
998 "rcu-walk", ie. without any locks or references on things.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700999
Nick Pigginfe15ce42011-01-07 17:49:23 +11001000 d_delete: called when the last reference to a dentry is dropped and the
1001 dcache is deciding whether or not to cache it. Return 1 to delete
1002 immediately, or 0 to cache the dentry. Default is NULL which means to
1003 always cache a reachable dentry. d_delete must be constant and
1004 idempotent.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001005
1006 d_release: called when a dentry is really deallocated
1007
1008 d_iput: called when a dentry loses its inode (just prior to its
1009 being deallocated). The default when this is NULL is that the
1010 VFS calls iput(). If you define this method, you must call
1011 iput() yourself
1012
Eric Dumazetc23fbb62007-05-08 00:26:18 -07001013 d_dname: called when the pathname of a dentry should be generated.
Matt LaPlanted9195882008-07-25 19:45:33 -07001014 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
Eric Dumazetc23fbb62007-05-08 00:26:18 -07001015 pathname generation. (Instead of doing it when dentry is created,
Matt LaPlanted9195882008-07-25 19:45:33 -07001016 it's done only when the path is needed.). Real filesystems probably
Eric Dumazetc23fbb62007-05-08 00:26:18 -07001017 dont want to use it, because their dentries are present in global
1018 dcache hash, so their hash should be an invariant. As no lock is
1019 held, d_dname() should not try to modify the dentry itself, unless
1020 appropriate SMP safety is used. CAUTION : d_path() logic is quite
1021 tricky. The correct way to return for example "Hello" is to put it
1022 at the end of the buffer, and returns a pointer to the first char.
1023 dynamic_dname() helper function is provided to take care of this.
1024
David Howells9875cf82011-01-14 18:45:21 +00001025 d_automount: called when an automount dentry is to be traversed (optional).
David Howellsea5b7782011-01-14 19:10:03 +00001026 This should create a new VFS mount record and return the record to the
1027 caller. The caller is supplied with a path parameter giving the
1028 automount directory to describe the automount target and the parent
1029 VFS mount record to provide inheritable mount parameters. NULL should
1030 be returned if someone else managed to make the automount first. If
1031 the vfsmount creation failed, then an error code should be returned.
1032 If -EISDIR is returned, then the directory will be treated as an
1033 ordinary directory and returned to pathwalk to continue walking.
1034
1035 If a vfsmount is returned, the caller will attempt to mount it on the
1036 mountpoint and will remove the vfsmount from its expiration list in
1037 the case of failure. The vfsmount should be returned with 2 refs on
1038 it to prevent automatic expiration - the caller will clean up the
1039 additional ref.
David Howells9875cf82011-01-14 18:45:21 +00001040
1041 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1042 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1043 inode being added.
1044
David Howellscc53ce52011-01-14 18:45:26 +00001045 d_manage: called to allow the filesystem to manage the transition from a
1046 dentry (optional). This allows autofs, for example, to hold up clients
1047 waiting to explore behind a 'mountpoint' whilst letting the daemon go
1048 past and construct the subtree there. 0 should be returned to let the
1049 calling process continue. -EISDIR can be returned to tell pathwalk to
1050 use this directory as an ordinary directory and to ignore anything
1051 mounted on it and not to check the automount flag. Any other error
1052 code will abort pathwalk completely.
1053
David Howellsab909112011-01-14 18:46:51 +00001054 If the 'rcu_walk' parameter is true, then the caller is doing a
1055 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
Masanari Iida40e47122012-03-04 23:16:11 +09001056 and the caller can be asked to leave it and call again by returning
NeilBrownb8faf032014-08-04 17:06:29 +10001057 -ECHILD. -EISDIR may also be returned to tell pathwalk to
1058 ignore d_automount or any mounts.
David Howellsab909112011-01-14 18:46:51 +00001059
David Howellscc53ce52011-01-14 18:45:26 +00001060 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1061 dentry being transited from.
1062
Eric Dumazetc23fbb62007-05-08 00:26:18 -07001063Example :
1064
1065static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1066{
1067 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1068 dentry->d_inode->i_ino);
1069}
1070
Linus Torvalds1da177e2005-04-16 15:20:36 -07001071Each dentry has a pointer to its parent dentry, as well as a hash list
1072of child dentries. Child dentries are basically like files in a
1073directory.
1074
Pekka J Enberg5ea626a2005-09-09 13:10:19 -07001075
Pekka Enbergcc7d1f82005-11-07 01:01:08 -08001076Directory Entry Cache API
Linus Torvalds1da177e2005-04-16 15:20:36 -07001077--------------------------
1078
1079There are a number of functions defined which permit a filesystem to
1080manipulate dentries:
1081
1082 dget: open a new handle for an existing dentry (this just increments
1083 the usage count)
1084
1085 dput: close a handle for a dentry (decrements the usage count). If
Nick Pigginfe15ce42011-01-07 17:49:23 +11001086 the usage count drops to 0, and the dentry is still in its
1087 parent's hash, the "d_delete" method is called to check whether
1088 it should be cached. If it should not be cached, or if the dentry
1089 is not hashed, it is deleted. Otherwise cached dentries are put
1090 into an LRU list to be reclaimed on memory shortage.
Linus Torvalds1da177e2005-04-16 15:20:36 -07001091
1092 d_drop: this unhashes a dentry from its parents hash list. A
Pekka J Enberg5ea626a2005-09-09 13:10:19 -07001093 subsequent call to dput() will deallocate the dentry if its
Linus Torvalds1da177e2005-04-16 15:20:36 -07001094 usage count drops to 0
1095
1096 d_delete: delete a dentry. If there are no other open references to
1097 the dentry then the dentry is turned into a negative dentry
1098 (the d_iput() method is called). If there are other
1099 references, then d_drop() is called instead
1100
1101 d_add: add a dentry to its parents hash list and then calls
1102 d_instantiate()
1103
1104 d_instantiate: add a dentry to the alias hash list for the inode and
1105 updates the "d_inode" member. The "i_count" member in the
1106 inode structure should be set/incremented. If the inode
1107 pointer is NULL, the dentry is called a "negative
1108 dentry". This function is commonly called when an inode is
1109 created for an existing negative dentry
1110
1111 d_lookup: look up a dentry given its parent and path name component
1112 It looks up the child of that given name from the dcache
1113 hash table. If it is found, the reference count is incremented
Zhaoleibe42c4c2008-12-01 14:34:58 -08001114 and the dentry is returned. The caller must use dput()
Linus Torvalds1da177e2005-04-16 15:20:36 -07001115 to free the dentry when it finishes using it.
1116
Miklos Szeredif84e3f52008-02-08 04:21:34 -08001117Mount Options
1118=============
1119
1120Parsing options
1121---------------
1122
1123On mount and remount the filesystem is passed a string containing a
1124comma separated list of mount options. The options can have either of
1125these forms:
1126
1127 option
1128 option=value
1129
1130The <linux/parser.h> header defines an API that helps parse these
1131options. There are plenty of examples on how to use it in existing
1132filesystems.
1133
1134Showing options
1135---------------
1136
1137If a filesystem accepts mount options, it must define show_options()
1138to show all the currently active options. The rules are:
1139
1140 - options MUST be shown which are not default or their values differ
1141 from the default
1142
1143 - options MAY be shown which are enabled by default or have their
1144 default value
1145
1146Options used only internally between a mount helper and the kernel
1147(such as file descriptors), or which only have an effect during the
1148mounting (such as ones controlling the creation of a journal) are exempt
1149from the above rules.
1150
1151The underlying reason for the above rules is to make sure, that a
1152mount can be accurately replicated (e.g. umounting and mounting again)
1153based on the information found in /proc/mounts.
1154
1155A simple method of saving options at mount/remount time and showing
1156them is provided with the save_mount_options() and
1157generic_show_options() helper functions. Please note, that using
1158these may have drawbacks. For more info see header comments for these
1159functions in fs/namespace.c.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -08001160
1161Resources
1162=========
1163
1164(Note some of these resources are not up-to-date with the latest kernel
1165 version.)
1166
1167Creating Linux virtual filesystems. 2002
1168 <http://lwn.net/Articles/13325/>
1169
1170The Linux Virtual File-system Layer by Neil Brown. 1999
1171 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1172
1173A tour of the Linux VFS by Michael K. Johnson. 1996
1174 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1175
1176A small trail through the Linux kernel by Andries Brouwer. 2001
1177 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>