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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
98request is made to mount a device onto a directory in your filespace,
99the VFS will call the appropriate get_sb() method for the specific
100filesystem. The dentry for the mount point will then be updated to
101point to the root inode for the new filesystem.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700102
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800103You can see all filesystems that are registered to the kernel in the
104file /proc/filesystems.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700105
106
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700107struct file_system_type
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800108-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700109
Borislav Petkov0746aec2007-07-15 23:41:19 -0700110This describes the filesystem. As of kernel 2.6.22, the following
Linus Torvalds1da177e2005-04-16 15:20:36 -0700111members are defined:
112
113struct file_system_type {
114 const char *name;
115 int fs_flags;
Jonathan Corbet5d8b2eb2006-07-10 04:44:07 -0700116 int (*get_sb) (struct file_system_type *, int,
117 const char *, void *, struct vfsmount *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700118 void (*kill_sb) (struct super_block *);
119 struct module *owner;
120 struct file_system_type * next;
121 struct list_head fs_supers;
Borislav Petkov0746aec2007-07-15 23:41:19 -0700122 struct lock_class_key s_lock_key;
123 struct lock_class_key s_umount_key;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700124};
125
126 name: the name of the filesystem type, such as "ext2", "iso9660",
127 "msdos" and so on
128
129 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
130
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700131 get_sb: the method to call when a new instance of this
Linus Torvalds1da177e2005-04-16 15:20:36 -0700132 filesystem should be mounted
133
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700134 kill_sb: the method to call when an instance of this filesystem
135 should be unmounted
Linus Torvalds1da177e2005-04-16 15:20:36 -0700136
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700137 owner: for internal VFS use: you should initialize this to THIS_MODULE in
138 most cases.
139
140 next: for internal VFS use: you should initialize this to NULL
141
Borislav Petkov0746aec2007-07-15 23:41:19 -0700142 s_lock_key, s_umount_key: lockdep-specific
143
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700144The get_sb() method has the following arguments:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700145
Matt LaPlanted9195882008-07-25 19:45:33 -0700146 struct file_system_type *fs_type: describes the filesystem, partly initialized
Borislav Petkov0746aec2007-07-15 23:41:19 -0700147 by the specific filesystem code
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700148
149 int flags: mount flags
150
151 const char *dev_name: the device name we are mounting.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152
153 void *data: arbitrary mount options, usually comes as an ASCII
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800154 string (see "Mount Options" section)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700155
Borislav Petkov0746aec2007-07-15 23:41:19 -0700156 struct vfsmount *mnt: a vfs-internal representation of a mount point
Linus Torvalds1da177e2005-04-16 15:20:36 -0700157
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700158The get_sb() method must determine if the block device specified
Borislav Petkov0746aec2007-07-15 23:41:19 -0700159in the dev_name and fs_type contains a filesystem of the type the method
160supports. If it succeeds in opening the named block device, it initializes a
161struct super_block descriptor for the filesystem contained by the block device.
162On failure it returns an error.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700163
164The most interesting member of the superblock structure that the
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700165get_sb() method fills in is the "s_op" field. This is a pointer to
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166a "struct super_operations" which describes the next level of the
167filesystem implementation.
168
Jim Cromiee3e1bfe2006-01-03 13:35:41 +0100169Usually, a filesystem uses one of the generic get_sb() implementations
170and provides a fill_super() method instead. The generic methods are:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700171
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700172 get_sb_bdev: mount a filesystem residing on a block device
173
174 get_sb_nodev: mount a filesystem that is not backed by a device
175
176 get_sb_single: mount a filesystem which shares the instance between
177 all mounts
178
179A fill_super() method implementation has the following arguments:
180
181 struct super_block *sb: the superblock structure. The method fill_super()
182 must initialize this properly.
183
184 void *data: arbitrary mount options, usually comes as an ASCII
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800185 string (see "Mount Options" section)
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700186
187 int silent: whether or not to be silent on error
188
189
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800190The Superblock Object
191=====================
192
193A superblock object represents a mounted filesystem.
194
195
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700196struct super_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800197-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700198
199This describes how the VFS can manipulate the superblock of your
Borislav Petkov422b14c2007-07-15 23:41:43 -0700200filesystem. As of kernel 2.6.22, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201
202struct super_operations {
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700203 struct inode *(*alloc_inode)(struct super_block *sb);
204 void (*destroy_inode)(struct inode *);
205
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700206 void (*dirty_inode) (struct inode *);
207 int (*write_inode) (struct inode *, int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700208 void (*drop_inode) (struct inode *);
209 void (*delete_inode) (struct inode *);
210 void (*put_super) (struct super_block *);
211 void (*write_super) (struct super_block *);
212 int (*sync_fs)(struct super_block *sb, int wait);
Takashi Satoc4be0c12009-01-09 16:40:58 -0800213 int (*freeze_fs) (struct super_block *);
214 int (*unfreeze_fs) (struct super_block *);
David Howells726c3342006-06-23 02:02:58 -0700215 int (*statfs) (struct dentry *, struct kstatfs *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700216 int (*remount_fs) (struct super_block *, int *, char *);
217 void (*clear_inode) (struct inode *);
218 void (*umount_begin) (struct super_block *);
219
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700220 int (*show_options)(struct seq_file *, struct vfsmount *);
221
222 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
223 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700224};
225
226All methods are called without any locks being held, unless otherwise
227noted. This means that most methods can block safely. All methods are
228only called from a process context (i.e. not from an interrupt handler
229or bottom half).
230
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700231 alloc_inode: this method is called by inode_alloc() to allocate memory
NeilBrown341546f2006-03-25 03:07:56 -0800232 for struct inode and initialize it. If this function is not
233 defined, a simple 'struct inode' is allocated. Normally
234 alloc_inode will be used to allocate a larger structure which
235 contains a 'struct inode' embedded within it.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700236
237 destroy_inode: this method is called by destroy_inode() to release
NeilBrown341546f2006-03-25 03:07:56 -0800238 resources allocated for struct inode. It is only required if
239 ->alloc_inode was defined and simply undoes anything done by
240 ->alloc_inode.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700241
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700242 dirty_inode: this method is called by the VFS to mark an inode dirty.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243
244 write_inode: this method is called when the VFS needs to write an
245 inode to disc. The second parameter indicates whether the write
246 should be synchronous or not, not all filesystems check this flag.
247
Linus Torvalds1da177e2005-04-16 15:20:36 -0700248 drop_inode: called when the last access to the inode is dropped,
249 with the inode_lock spinlock held.
250
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700251 This method should be either NULL (normal UNIX filesystem
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252 semantics) or "generic_delete_inode" (for filesystems that do not
253 want to cache inodes - causing "delete_inode" to always be
254 called regardless of the value of i_nlink)
255
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700256 The "generic_delete_inode()" behavior is equivalent to the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700257 old practice of using "force_delete" in the put_inode() case,
258 but does not have the races that the "force_delete()" approach
259 had.
260
261 delete_inode: called when the VFS wants to delete an inode
262
Linus Torvalds1da177e2005-04-16 15:20:36 -0700263 put_super: called when the VFS wishes to free the superblock
264 (i.e. unmount). This is called with the superblock lock held
265
266 write_super: called when the VFS superblock needs to be written to
267 disc. This method is optional
268
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700269 sync_fs: called when VFS is writing out all dirty data associated with
270 a superblock. The second parameter indicates whether the method
271 should wait until the write out has been completed. Optional.
272
Takashi Satoc4be0c12009-01-09 16:40:58 -0800273 freeze_fs: called when VFS is locking a filesystem and
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800274 forcing it into a consistent state. This method is currently
275 used by the Logical Volume Manager (LVM).
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700276
Takashi Satoc4be0c12009-01-09 16:40:58 -0800277 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700278 again.
279
Adrian McMenamin66672fe2009-04-20 18:38:28 -0700280 statfs: called when the VFS needs to get filesystem statistics.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700281
282 remount_fs: called when the filesystem is remounted. This is called
283 with the kernel lock held
284
285 clear_inode: called then the VFS clears the inode. Optional
286
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700287 umount_begin: called when the VFS is unmounting a filesystem.
288
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800289 show_options: called by the VFS to show mount options for
290 /proc/<pid>/mounts. (see "Mount Options" section)
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700291
292 quota_read: called by the VFS to read from filesystem quota file.
293
294 quota_write: called by the VFS to write to filesystem quota file.
295
David Howells12debc42008-02-07 00:15:52 -0800296Whoever sets up the inode is responsible for filling in the "i_op" field. This
297is a pointer to a "struct inode_operations" which describes the methods that
298can be performed on individual inodes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700299
300
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800301The Inode Object
302================
303
304An inode object represents an object within the filesystem.
305
306
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700307struct inode_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800308-----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700309
310This describes how the VFS can manipulate an inode in your
Borislav Petkov422b14c2007-07-15 23:41:43 -0700311filesystem. As of kernel 2.6.22, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700312
313struct inode_operations {
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700314 int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
315 struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700316 int (*link) (struct dentry *,struct inode *,struct dentry *);
317 int (*unlink) (struct inode *,struct dentry *);
318 int (*symlink) (struct inode *,struct dentry *,const char *);
319 int (*mkdir) (struct inode *,struct dentry *,int);
320 int (*rmdir) (struct inode *,struct dentry *);
321 int (*mknod) (struct inode *,struct dentry *,int,dev_t);
322 int (*rename) (struct inode *, struct dentry *,
323 struct inode *, struct dentry *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700324 int (*readlink) (struct dentry *, char __user *,int);
325 void * (*follow_link) (struct dentry *, struct nameidata *);
326 void (*put_link) (struct dentry *, struct nameidata *, void *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700327 void (*truncate) (struct inode *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700328 int (*permission) (struct inode *, int, struct nameidata *);
329 int (*setattr) (struct dentry *, struct iattr *);
330 int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
331 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
332 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
333 ssize_t (*listxattr) (struct dentry *, char *, size_t);
334 int (*removexattr) (struct dentry *, const char *);
Borislav Petkov422b14c2007-07-15 23:41:43 -0700335 void (*truncate_range)(struct inode *, loff_t, loff_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700336};
337
338Again, all methods are called without any locks being held, unless
339otherwise noted.
340
Linus Torvalds1da177e2005-04-16 15:20:36 -0700341 create: called by the open(2) and creat(2) system calls. Only
342 required if you want to support regular files. The dentry you
343 get should not have an inode (i.e. it should be a negative
344 dentry). Here you will probably call d_instantiate() with the
345 dentry and the newly created inode
346
347 lookup: called when the VFS needs to look up an inode in a parent
348 directory. The name to look for is found in the dentry. This
349 method must call d_add() to insert the found inode into the
350 dentry. The "i_count" field in the inode structure should be
351 incremented. If the named inode does not exist a NULL inode
352 should be inserted into the dentry (this is called a negative
353 dentry). Returning an error code from this routine must only
354 be done on a real error, otherwise creating inodes with system
355 calls like create(2), mknod(2), mkdir(2) and so on will fail.
356 If you wish to overload the dentry methods then you should
357 initialise the "d_dop" field in the dentry; this is a pointer
358 to a struct "dentry_operations".
359 This method is called with the directory inode semaphore held
360
361 link: called by the link(2) system call. Only required if you want
362 to support hard links. You will probably need to call
363 d_instantiate() just as you would in the create() method
364
365 unlink: called by the unlink(2) system call. Only required if you
366 want to support deleting inodes
367
368 symlink: called by the symlink(2) system call. Only required if you
369 want to support symlinks. You will probably need to call
370 d_instantiate() just as you would in the create() method
371
372 mkdir: called by the mkdir(2) system call. Only required if you want
373 to support creating subdirectories. You will probably need to
374 call d_instantiate() just as you would in the create() method
375
376 rmdir: called by the rmdir(2) system call. Only required if you want
377 to support deleting subdirectories
378
379 mknod: called by the mknod(2) system call to create a device (char,
380 block) inode or a named pipe (FIFO) or socket. Only required
381 if you want to support creating these types of inodes. You
382 will probably need to call d_instantiate() just as you would
383 in the create() method
384
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800385 rename: called by the rename(2) system call to rename the object to
386 have the parent and name given by the second inode and dentry.
387
Linus Torvalds1da177e2005-04-16 15:20:36 -0700388 readlink: called by the readlink(2) system call. Only required if
389 you want to support reading symbolic links
390
391 follow_link: called by the VFS to follow a symbolic link to the
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700392 inode it points to. Only required if you want to support
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800393 symbolic links. This method returns a void pointer cookie
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700394 that is passed to put_link().
395
396 put_link: called by the VFS to release resources allocated by
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800397 follow_link(). The cookie returned by follow_link() is passed
Paolo Ornati670e9f32006-10-03 22:57:56 +0200398 to this method as the last parameter. It is used by
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800399 filesystems such as NFS where page cache is not stable
400 (i.e. page that was installed when the symbolic link walk
401 started might not be in the page cache at the end of the
402 walk).
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700403
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800404 truncate: called by the VFS to change the size of a file. The
405 i_size field of the inode is set to the desired size by the
406 VFS before this method is called. This method is called by
407 the truncate(2) system call and related functionality.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700408
409 permission: called by the VFS to check for access rights on a POSIX-like
410 filesystem.
411
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800412 setattr: called by the VFS to set attributes for a file. This method
413 is called by chmod(2) and related system calls.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700414
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800415 getattr: called by the VFS to get attributes of a file. This method
416 is called by stat(2) and related system calls.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700417
418 setxattr: called by the VFS to set an extended attribute for a file.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800419 Extended attribute is a name:value pair associated with an
420 inode. This method is called by setxattr(2) system call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700421
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800422 getxattr: called by the VFS to retrieve the value of an extended
423 attribute name. This method is called by getxattr(2) function
424 call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700425
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800426 listxattr: called by the VFS to list all extended attributes for a
427 given file. This method is called by listxattr(2) system call.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700428
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800429 removexattr: called by the VFS to remove an extended attribute from
430 a file. This method is called by removexattr(2) system call.
431
Borislav Petkov422b14c2007-07-15 23:41:43 -0700432 truncate_range: a method provided by the underlying filesystem to truncate a
433 range of blocks , i.e. punch a hole somewhere in a file.
434
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800435
436The Address Space Object
437========================
438
NeilBrown341546f2006-03-25 03:07:56 -0800439The address space object is used to group and manage pages in the page
440cache. It can be used to keep track of the pages in a file (or
441anything else) and also track the mapping of sections of the file into
442process address spaces.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700443
NeilBrown341546f2006-03-25 03:07:56 -0800444There are a number of distinct yet related services that an
445address-space can provide. These include communicating memory
446pressure, page lookup by address, and keeping track of pages tagged as
447Dirty or Writeback.
448
NeilBrowna9e102b2006-03-25 03:08:29 -0800449The first can be used independently to the others. The VM can try to
NeilBrown341546f2006-03-25 03:07:56 -0800450either write dirty pages in order to clean them, or release clean
451pages in order to reuse them. To do this it can call the ->writepage
452method on dirty pages, and ->releasepage on clean pages with
453PagePrivate set. Clean pages without PagePrivate and with no external
454references will be released without notice being given to the
455address_space.
456
NeilBrowna9e102b2006-03-25 03:08:29 -0800457To achieve this functionality, pages need to be placed on an LRU with
NeilBrown341546f2006-03-25 03:07:56 -0800458lru_cache_add and mark_page_active needs to be called whenever the
459page is used.
460
461Pages are normally kept in a radix tree index by ->index. This tree
462maintains information about the PG_Dirty and PG_Writeback status of
463each page, so that pages with either of these flags can be found
464quickly.
465
466The Dirty tag is primarily used by mpage_writepages - the default
467->writepages method. It uses the tag to find dirty pages to call
468->writepage on. If mpage_writepages is not used (i.e. the address
NeilBrowna9e102b2006-03-25 03:08:29 -0800469provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
NeilBrown341546f2006-03-25 03:07:56 -0800470almost unused. write_inode_now and sync_inode do use it (through
471__sync_single_inode) to check if ->writepages has been successful in
472writing out the whole address_space.
473
474The Writeback tag is used by filemap*wait* and sync_page* functions,
Christoph Hellwig94004ed2009-09-30 22:16:33 +0200475via filemap_fdatawait_range, to wait for all writeback to
NeilBrown341546f2006-03-25 03:07:56 -0800476complete. While waiting ->sync_page (if defined) will be called on
NeilBrowna9e102b2006-03-25 03:08:29 -0800477each page that is found to require writeback.
NeilBrown341546f2006-03-25 03:07:56 -0800478
479An address_space handler may attach extra information to a page,
480typically using the 'private' field in the 'struct page'. If such
481information is attached, the PG_Private flag should be set. This will
NeilBrowna9e102b2006-03-25 03:08:29 -0800482cause various VM routines to make extra calls into the address_space
NeilBrown341546f2006-03-25 03:07:56 -0800483handler to deal with that data.
484
485An address space acts as an intermediate between storage and
486application. Data is read into the address space a whole page at a
487time, and provided to the application either by copying of the page,
488or by memory-mapping the page.
489Data is written into the address space by the application, and then
490written-back to storage typically in whole pages, however the
NeilBrowna9e102b2006-03-25 03:08:29 -0800491address_space has finer control of write sizes.
NeilBrown341546f2006-03-25 03:07:56 -0800492
493The read process essentially only requires 'readpage'. The write
Nick Piggin4e02ed42008-10-29 14:00:55 -0700494process is more complicated and uses write_begin/write_end or
NeilBrown341546f2006-03-25 03:07:56 -0800495set_page_dirty to write data into the address_space, and writepage,
496sync_page, and writepages to writeback data to storage.
497
498Adding and removing pages to/from an address_space is protected by the
499inode's i_mutex.
500
501When data is written to a page, the PG_Dirty flag should be set. It
502typically remains set until writepage asks for it to be written. This
503should clear PG_Dirty and set PG_Writeback. It can be actually
504written at any point after PG_Dirty is clear. Once it is known to be
505safe, PG_Writeback is cleared.
506
507Writeback makes use of a writeback_control structure...
Linus Torvalds1da177e2005-04-16 15:20:36 -0700508
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700509struct address_space_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800510-------------------------------
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700511
512This describes how the VFS can manipulate mapping of a file to page cache in
Borislav Petkov422b14c2007-07-15 23:41:43 -0700513your filesystem. As of kernel 2.6.22, the following members are defined:
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700514
515struct address_space_operations {
516 int (*writepage)(struct page *page, struct writeback_control *wbc);
517 int (*readpage)(struct file *, struct page *);
518 int (*sync_page)(struct page *);
519 int (*writepages)(struct address_space *, struct writeback_control *);
520 int (*set_page_dirty)(struct page *page);
521 int (*readpages)(struct file *filp, struct address_space *mapping,
522 struct list_head *pages, unsigned nr_pages);
Nick Pigginafddba42007-10-16 01:25:01 -0700523 int (*write_begin)(struct file *, struct address_space *mapping,
524 loff_t pos, unsigned len, unsigned flags,
525 struct page **pagep, void **fsdata);
526 int (*write_end)(struct file *, struct address_space *mapping,
527 loff_t pos, unsigned len, unsigned copied,
528 struct page *page, void *fsdata);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700529 sector_t (*bmap)(struct address_space *, sector_t);
530 int (*invalidatepage) (struct page *, unsigned long);
531 int (*releasepage) (struct page *, int);
532 ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
533 loff_t offset, unsigned long nr_segs);
534 struct page* (*get_xip_page)(struct address_space *, sector_t,
535 int);
NeilBrown341546f2006-03-25 03:07:56 -0800536 /* migrate the contents of a page to the specified target */
537 int (*migratepage) (struct page *, struct page *);
Borislav Petkov422b14c2007-07-15 23:41:43 -0700538 int (*launder_page) (struct page *);
Andi Kleen25718732009-09-16 11:50:13 +0200539 int (*error_remove_page) (struct mapping *mapping, struct page *page);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700540};
541
NeilBrown341546f2006-03-25 03:07:56 -0800542 writepage: called by the VM to write a dirty page to backing store.
NeilBrowna9e102b2006-03-25 03:08:29 -0800543 This may happen for data integrity reasons (i.e. 'sync'), or
NeilBrown341546f2006-03-25 03:07:56 -0800544 to free up memory (flush). The difference can be seen in
545 wbc->sync_mode.
546 The PG_Dirty flag has been cleared and PageLocked is true.
547 writepage should start writeout, should set PG_Writeback,
548 and should make sure the page is unlocked, either synchronously
549 or asynchronously when the write operation completes.
550
551 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
NeilBrowna9e102b2006-03-25 03:08:29 -0800552 try too hard if there are problems, and may choose to write out
553 other pages from the mapping if that is easier (e.g. due to
554 internal dependencies). If it chooses not to start writeout, it
555 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
NeilBrown341546f2006-03-25 03:07:56 -0800556 calling ->writepage on that page.
557
558 See the file "Locking" for more details.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700559
560 readpage: called by the VM to read a page from backing store.
NeilBrown341546f2006-03-25 03:07:56 -0800561 The page will be Locked when readpage is called, and should be
562 unlocked and marked uptodate once the read completes.
563 If ->readpage discovers that it needs to unlock the page for
564 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
NeilBrowna9e102b2006-03-25 03:08:29 -0800565 In this case, the page will be relocated, relocked and if
NeilBrown341546f2006-03-25 03:07:56 -0800566 that all succeeds, ->readpage will be called again.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700567
568 sync_page: called by the VM to notify the backing store to perform all
569 queued I/O operations for a page. I/O operations for other pages
570 associated with this address_space object may also be performed.
571
NeilBrown341546f2006-03-25 03:07:56 -0800572 This function is optional and is called only for pages with
573 PG_Writeback set while waiting for the writeback to complete.
574
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700575 writepages: called by the VM to write out pages associated with the
NeilBrowna9e102b2006-03-25 03:08:29 -0800576 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
577 the writeback_control will specify a range of pages that must be
578 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
NeilBrown341546f2006-03-25 03:07:56 -0800579 and that many pages should be written if possible.
580 If no ->writepages is given, then mpage_writepages is used
NeilBrowna9e102b2006-03-25 03:08:29 -0800581 instead. This will choose pages from the address space that are
NeilBrown341546f2006-03-25 03:07:56 -0800582 tagged as DIRTY and will pass them to ->writepage.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700583
584 set_page_dirty: called by the VM to set a page dirty.
NeilBrown341546f2006-03-25 03:07:56 -0800585 This is particularly needed if an address space attaches
586 private data to a page, and that data needs to be updated when
587 a page is dirtied. This is called, for example, when a memory
588 mapped page gets modified.
589 If defined, it should set the PageDirty flag, and the
590 PAGECACHE_TAG_DIRTY tag in the radix tree.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700591
592 readpages: called by the VM to read pages associated with the address_space
NeilBrown341546f2006-03-25 03:07:56 -0800593 object. This is essentially just a vector version of
594 readpage. Instead of just one page, several pages are
595 requested.
NeilBrowna9e102b2006-03-25 03:08:29 -0800596 readpages is only used for read-ahead, so read errors are
NeilBrown341546f2006-03-25 03:07:56 -0800597 ignored. If anything goes wrong, feel free to give up.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700598
Nick Piggin4e02ed42008-10-29 14:00:55 -0700599 write_begin:
Nick Pigginafddba42007-10-16 01:25:01 -0700600 Called by the generic buffered write code to ask the filesystem to
601 prepare to write len bytes at the given offset in the file. The
602 address_space should check that the write will be able to complete,
603 by allocating space if necessary and doing any other internal
604 housekeeping. If the write will update parts of any basic-blocks on
605 storage, then those blocks should be pre-read (if they haven't been
606 read already) so that the updated blocks can be written out properly.
607
608 The filesystem must return the locked pagecache page for the specified
609 offset, in *pagep, for the caller to write into.
610
Nick Piggin4e02ed42008-10-29 14:00:55 -0700611 It must be able to cope with short writes (where the length passed to
612 write_begin is greater than the number of bytes copied into the page).
613
Nick Pigginafddba42007-10-16 01:25:01 -0700614 flags is a field for AOP_FLAG_xxx flags, described in
615 include/linux/fs.h.
616
617 A void * may be returned in fsdata, which then gets passed into
618 write_end.
619
620 Returns 0 on success; < 0 on failure (which is the error code), in
621 which case write_end is not called.
622
623 write_end: After a successful write_begin, and data copy, write_end must
624 be called. len is the original len passed to write_begin, and copied
625 is the amount that was able to be copied (copied == len is always true
626 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
627
628 The filesystem must take care of unlocking the page and releasing it
629 refcount, and updating i_size.
630
631 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
632 that were able to be copied into pagecache.
633
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700634 bmap: called by the VFS to map a logical block offset within object to
NeilBrowna9e102b2006-03-25 03:08:29 -0800635 physical block number. This method is used by the FIBMAP
NeilBrown341546f2006-03-25 03:07:56 -0800636 ioctl and for working with swap-files. To be able to swap to
NeilBrowna9e102b2006-03-25 03:08:29 -0800637 a file, the file must have a stable mapping to a block
NeilBrown341546f2006-03-25 03:07:56 -0800638 device. The swap system does not go through the filesystem
639 but instead uses bmap to find out where the blocks in the file
640 are and uses those addresses directly.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700641
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700642
NeilBrown341546f2006-03-25 03:07:56 -0800643 invalidatepage: If a page has PagePrivate set, then invalidatepage
644 will be called when part or all of the page is to be removed
NeilBrowna9e102b2006-03-25 03:08:29 -0800645 from the address space. This generally corresponds to either a
NeilBrown341546f2006-03-25 03:07:56 -0800646 truncation or a complete invalidation of the address space
647 (in the latter case 'offset' will always be 0).
648 Any private data associated with the page should be updated
649 to reflect this truncation. If offset is 0, then
650 the private data should be released, because the page
651 must be able to be completely discarded. This may be done by
652 calling the ->releasepage function, but in this case the
653 release MUST succeed.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700654
NeilBrown341546f2006-03-25 03:07:56 -0800655 releasepage: releasepage is called on PagePrivate pages to indicate
656 that the page should be freed if possible. ->releasepage
657 should remove any private data from the page and clear the
658 PagePrivate flag. It may also remove the page from the
659 address_space. If this fails for some reason, it may indicate
660 failure with a 0 return value.
661 This is used in two distinct though related cases. The first
662 is when the VM finds a clean page with no active users and
663 wants to make it a free page. If ->releasepage succeeds, the
664 page will be removed from the address_space and become free.
665
Shaun Zinckbc5b1d52007-10-20 02:35:36 +0200666 The second case is when a request has been made to invalidate
NeilBrown341546f2006-03-25 03:07:56 -0800667 some or all pages in an address_space. This can happen
668 through the fadvice(POSIX_FADV_DONTNEED) system call or by the
669 filesystem explicitly requesting it as nfs and 9fs do (when
670 they believe the cache may be out of date with storage) by
671 calling invalidate_inode_pages2().
672 If the filesystem makes such a call, and needs to be certain
NeilBrowna9e102b2006-03-25 03:08:29 -0800673 that all pages are invalidated, then its releasepage will
NeilBrown341546f2006-03-25 03:07:56 -0800674 need to ensure this. Possibly it can clear the PageUptodate
675 bit if it cannot free private data yet.
676
677 direct_IO: called by the generic read/write routines to perform
678 direct_IO - that is IO requests which bypass the page cache
NeilBrowna9e102b2006-03-25 03:08:29 -0800679 and transfer data directly between the storage and the
NeilBrown341546f2006-03-25 03:07:56 -0800680 application's address space.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700681
682 get_xip_page: called by the VM to translate a block number to a page.
683 The page is valid until the corresponding filesystem is unmounted.
684 Filesystems that want to use execute-in-place (XIP) need to implement
685 it. An example implementation can be found in fs/ext2/xip.c.
686
NeilBrown341546f2006-03-25 03:07:56 -0800687 migrate_page: This is used to compact the physical memory usage.
688 If the VM wants to relocate a page (maybe off a memory card
689 that is signalling imminent failure) it will pass a new page
690 and an old page to this function. migrate_page should
691 transfer any private data across and update any references
692 that it has to the page.
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700693
Borislav Petkov422b14c2007-07-15 23:41:43 -0700694 launder_page: Called before freeing a page - it writes back the dirty page. To
695 prevent redirtying the page, it is kept locked during the whole
696 operation.
697
Andi Kleen25718732009-09-16 11:50:13 +0200698 error_remove_page: normally set to generic_error_remove_page if truncation
699 is ok for this address space. Used for memory failure handling.
700 Setting this implies you deal with pages going away under you,
701 unless you have them locked or reference counts increased.
702
703
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800704The File Object
705===============
706
707A file object represents a file opened by a process.
708
709
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700710struct file_operations
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800711----------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700712
713This describes how the VFS can manipulate an open file. As of kernel
Borislav Petkov422b14c2007-07-15 23:41:43 -07007142.6.22, the following members are defined:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700715
716struct file_operations {
Borislav Petkov422b14c2007-07-15 23:41:43 -0700717 struct module *owner;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700718 loff_t (*llseek) (struct file *, loff_t, int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700719 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700720 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
Badari Pulavarty027445c2006-09-30 23:28:46 -0700721 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
722 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700723 int (*readdir) (struct file *, void *, filldir_t);
724 unsigned int (*poll) (struct file *, struct poll_table_struct *);
725 int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700726 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
727 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700728 int (*mmap) (struct file *, struct vm_area_struct *);
729 int (*open) (struct inode *, struct file *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700730 int (*flush) (struct file *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700731 int (*release) (struct inode *, struct file *);
Christoph Hellwig7ea80852010-05-26 17:53:25 +0200732 int (*fsync) (struct file *, int datasync);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700733 int (*aio_fsync) (struct kiocb *, int datasync);
734 int (*fasync) (int, struct file *, int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700735 int (*lock) (struct file *, int, struct file_lock *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700736 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
737 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
738 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
739 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
740 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
741 int (*check_flags)(int);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700742 int (*flock) (struct file *, int, struct file_lock *);
Borislav Petkov422b14c2007-07-15 23:41:43 -0700743 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
744 ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700745};
746
747Again, all methods are called without any locks being held, unless
748otherwise noted.
749
750 llseek: called when the VFS needs to move the file position index
751
752 read: called by read(2) and related system calls
753
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700754 aio_read: called by io_submit(2) and other asynchronous I/O operations
755
Linus Torvalds1da177e2005-04-16 15:20:36 -0700756 write: called by write(2) and related system calls
757
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700758 aio_write: called by io_submit(2) and other asynchronous I/O operations
759
Linus Torvalds1da177e2005-04-16 15:20:36 -0700760 readdir: called when the VFS needs to read the directory contents
761
762 poll: called by the VFS when a process wants to check if there is
763 activity on this file and (optionally) go to sleep until there
764 is activity. Called by the select(2) and poll(2) system calls
765
766 ioctl: called by the ioctl(2) system call
767
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700768 unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not
769 require the BKL should use this method instead of the ioctl() above.
770
771 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
772 are used on 64 bit kernels.
773
Linus Torvalds1da177e2005-04-16 15:20:36 -0700774 mmap: called by the mmap(2) system call
775
776 open: called by the VFS when an inode should be opened. When the VFS
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700777 opens a file, it creates a new "struct file". It then calls the
778 open method for the newly allocated file structure. You might
779 think that the open method really belongs in
780 "struct inode_operations", and you may be right. I think it's
781 done the way it is because it makes filesystems simpler to
782 implement. The open() method is a good place to initialize the
783 "private_data" member in the file structure if you want to point
784 to a device structure
785
786 flush: called by the close(2) system call to flush a file
Linus Torvalds1da177e2005-04-16 15:20:36 -0700787
788 release: called when the last reference to an open file is closed
789
790 fsync: called by the fsync(2) system call
791
792 fasync: called by the fcntl(2) system call when asynchronous
793 (non-blocking) mode is enabled for a file
794
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700795 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
796 commands
797
798 readv: called by the readv(2) system call
799
800 writev: called by the writev(2) system call
801
802 sendfile: called by the sendfile(2) system call
803
804 get_unmapped_area: called by the mmap(2) system call
805
806 check_flags: called by the fcntl(2) system call for F_SETFL command
807
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700808 flock: called by the flock(2) system call
809
Pekka J Enbergd1195c52006-04-11 14:21:59 +0200810 splice_write: called by the VFS to splice data from a pipe to a file. This
811 method is used by the splice(2) system call
812
813 splice_read: called by the VFS to splice data from file to a pipe. This
814 method is used by the splice(2) system call
815
Linus Torvalds1da177e2005-04-16 15:20:36 -0700816Note that the file operations are implemented by the specific
817filesystem in which the inode resides. When opening a device node
818(character or block special) most filesystems will call special
819support routines in the VFS which will locate the required device
820driver information. These support routines replace the filesystem file
821operations with those for the device driver, and then proceed to call
822the new open() method for the file. This is how opening a device file
823in the filesystem eventually ends up calling the device driver open()
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700824method.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700825
826
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700827Directory Entry Cache (dcache)
828==============================
829
Linus Torvalds1da177e2005-04-16 15:20:36 -0700830
831struct dentry_operations
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700832------------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -0700833
834This describes how a filesystem can overload the standard dentry
835operations. Dentries and the dcache are the domain of the VFS and the
836individual filesystem implementations. Device drivers have no business
837here. These methods may be set to NULL, as they are either optional or
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700838the VFS uses a default. As of kernel 2.6.22, the following members are
Linus Torvalds1da177e2005-04-16 15:20:36 -0700839defined:
840
841struct dentry_operations {
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700842 int (*d_revalidate)(struct dentry *, struct nameidata *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700843 int (*d_hash) (struct dentry *, struct qstr *);
844 int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700845 int (*d_delete)(struct dentry *);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700846 void (*d_release)(struct dentry *);
847 void (*d_iput)(struct dentry *, struct inode *);
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700848 char *(*d_dname)(struct dentry *, char *, int);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700849};
850
851 d_revalidate: called when the VFS needs to revalidate a dentry. This
852 is called whenever a name look-up finds a dentry in the
853 dcache. Most filesystems leave this as NULL, because all their
854 dentries in the dcache are valid
855
856 d_hash: called when the VFS adds a dentry to the hash table
857
858 d_compare: called when a dentry should be compared with another
859
860 d_delete: called when the last reference to a dentry is
861 deleted. This means no-one is using the dentry, however it is
862 still valid and in the dcache
863
864 d_release: called when a dentry is really deallocated
865
866 d_iput: called when a dentry loses its inode (just prior to its
867 being deallocated). The default when this is NULL is that the
868 VFS calls iput(). If you define this method, you must call
869 iput() yourself
870
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700871 d_dname: called when the pathname of a dentry should be generated.
Matt LaPlanted9195882008-07-25 19:45:33 -0700872 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700873 pathname generation. (Instead of doing it when dentry is created,
Matt LaPlanted9195882008-07-25 19:45:33 -0700874 it's done only when the path is needed.). Real filesystems probably
Eric Dumazetc23fbb62007-05-08 00:26:18 -0700875 dont want to use it, because their dentries are present in global
876 dcache hash, so their hash should be an invariant. As no lock is
877 held, d_dname() should not try to modify the dentry itself, unless
878 appropriate SMP safety is used. CAUTION : d_path() logic is quite
879 tricky. The correct way to return for example "Hello" is to put it
880 at the end of the buffer, and returns a pointer to the first char.
881 dynamic_dname() helper function is provided to take care of this.
882
883Example :
884
885static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
886{
887 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
888 dentry->d_inode->i_ino);
889}
890
Linus Torvalds1da177e2005-04-16 15:20:36 -0700891Each dentry has a pointer to its parent dentry, as well as a hash list
892of child dentries. Child dentries are basically like files in a
893directory.
894
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700895
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800896Directory Entry Cache API
Linus Torvalds1da177e2005-04-16 15:20:36 -0700897--------------------------
898
899There are a number of functions defined which permit a filesystem to
900manipulate dentries:
901
902 dget: open a new handle for an existing dentry (this just increments
903 the usage count)
904
905 dput: close a handle for a dentry (decrements the usage count). If
906 the usage count drops to 0, the "d_delete" method is called
907 and the dentry is placed on the unused list if the dentry is
908 still in its parents hash list. Putting the dentry on the
909 unused list just means that if the system needs some RAM, it
910 goes through the unused list of dentries and deallocates them.
911 If the dentry has already been unhashed and the usage count
912 drops to 0, in this case the dentry is deallocated after the
913 "d_delete" method is called
914
915 d_drop: this unhashes a dentry from its parents hash list. A
Pekka J Enberg5ea626a2005-09-09 13:10:19 -0700916 subsequent call to dput() will deallocate the dentry if its
Linus Torvalds1da177e2005-04-16 15:20:36 -0700917 usage count drops to 0
918
919 d_delete: delete a dentry. If there are no other open references to
920 the dentry then the dentry is turned into a negative dentry
921 (the d_iput() method is called). If there are other
922 references, then d_drop() is called instead
923
924 d_add: add a dentry to its parents hash list and then calls
925 d_instantiate()
926
927 d_instantiate: add a dentry to the alias hash list for the inode and
928 updates the "d_inode" member. The "i_count" member in the
929 inode structure should be set/incremented. If the inode
930 pointer is NULL, the dentry is called a "negative
931 dentry". This function is commonly called when an inode is
932 created for an existing negative dentry
933
934 d_lookup: look up a dentry given its parent and path name component
935 It looks up the child of that given name from the dcache
936 hash table. If it is found, the reference count is incremented
Zhaoleibe42c4c2008-12-01 14:34:58 -0800937 and the dentry is returned. The caller must use dput()
Linus Torvalds1da177e2005-04-16 15:20:36 -0700938 to free the dentry when it finishes using it.
939
Pekka Enbergcbf8f0f2005-11-07 01:01:09 -0800940For further information on dentry locking, please refer to the document
941Documentation/filesystems/dentry-locking.txt.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800942
Miklos Szeredif84e3f52008-02-08 04:21:34 -0800943Mount Options
944=============
945
946Parsing options
947---------------
948
949On mount and remount the filesystem is passed a string containing a
950comma separated list of mount options. The options can have either of
951these forms:
952
953 option
954 option=value
955
956The <linux/parser.h> header defines an API that helps parse these
957options. There are plenty of examples on how to use it in existing
958filesystems.
959
960Showing options
961---------------
962
963If a filesystem accepts mount options, it must define show_options()
964to show all the currently active options. The rules are:
965
966 - options MUST be shown which are not default or their values differ
967 from the default
968
969 - options MAY be shown which are enabled by default or have their
970 default value
971
972Options used only internally between a mount helper and the kernel
973(such as file descriptors), or which only have an effect during the
974mounting (such as ones controlling the creation of a journal) are exempt
975from the above rules.
976
977The underlying reason for the above rules is to make sure, that a
978mount can be accurately replicated (e.g. umounting and mounting again)
979based on the information found in /proc/mounts.
980
981A simple method of saving options at mount/remount time and showing
982them is provided with the save_mount_options() and
983generic_show_options() helper functions. Please note, that using
984these may have drawbacks. For more info see header comments for these
985functions in fs/namespace.c.
Pekka Enbergcc7d1f82005-11-07 01:01:08 -0800986
987Resources
988=========
989
990(Note some of these resources are not up-to-date with the latest kernel
991 version.)
992
993Creating Linux virtual filesystems. 2002
994 <http://lwn.net/Articles/13325/>
995
996The Linux Virtual File-system Layer by Neil Brown. 1999
997 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
998
999A tour of the Linux VFS by Michael K. Johnson. 1996
1000 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1001
1002A small trail through the Linux kernel by Andries Brouwer. 2001
1003 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>