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1da177e4 1
5ea626aa 2 Overview of the Linux Virtual File System
1da177e4 3
5ea626aa 4 Original author: Richard Gooch <rgooch@atnf.csiro.au>
1da177e4 5
0746aec3 6 Last updated on June 24, 2007.
1da177e4 7
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8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
1da177e4 10
5ea626aa 11 This file is released under the GPLv2.
1da177e4 12
1da177e4 13
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14Introduction
15============
1da177e4 16
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17The 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.
1da177e4 22
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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.
1da177e4 26
1da177e4 27
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28Directory Entry Cache (dcache)
29------------------------------
1da177e4 30
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31The 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.
44
45
46The Inode Object
47----------------
48
49An 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).
56
57To 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.
65
66
67The File Object
68---------------
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69
70Opening a file requires another operation: allocation of a file
71structure (this is the kernel-side implementation of file
5ea626aa 72descriptors). The freshly allocated file structure is initialized with
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73a 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
a33f3224 75called so the specific filesystem implementation can do its work. You
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76can see that this is another switch performed by the VFS. The file
77structure is placed into the file descriptor table for the process.
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78
79Reading, writing and closing files (and other assorted VFS operations)
80is done by using the userspace file descriptor to grab the appropriate
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81file 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.
1da177e4 84
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85
86Registering and Mounting a Filesystem
cc7d1f8f 87=====================================
1da177e4 88
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89To register and unregister a filesystem, use the following API
90functions:
1da177e4 91
cc7d1f8f 92 #include <linux/fs.h>
1da177e4 93
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94 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
1da177e4 96
cc7d1f8f 97The passed struct file_system_type describes your filesystem. When a
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98request is made to mount a filesystem onto a directory in your namespace,
99the VFS will call the appropriate mount() method for the specific
25985edc 100filesystem. New vfsmount referring to the tree returned by ->mount()
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101will be attached to the mountpoint, so that when pathname resolution
102reaches the mountpoint it will jump into the root of that vfsmount.
1da177e4 103
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104You can see all filesystems that are registered to the kernel in the
105file /proc/filesystems.
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106
107
5ea626aa 108struct file_system_type
cc7d1f8f 109-----------------------
1da177e4 110
1a102ff9 111This describes the filesystem. As of kernel 2.6.39, the following
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112members are defined:
113
114struct file_system_type {
115 const char *name;
116 int fs_flags;
b1349f25 117 struct dentry *(*mount) (struct file_system_type *, int,
1a102ff9 118 const char *, void *);
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119 void (*kill_sb) (struct super_block *);
120 struct module *owner;
121 struct file_system_type * next;
122 struct list_head fs_supers;
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123 struct lock_class_key s_lock_key;
124 struct lock_class_key s_umount_key;
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125};
126
127 name: the name of the filesystem type, such as "ext2", "iso9660",
128 "msdos" and so on
129
130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131
1a102ff9 132 mount: the method to call when a new instance of this
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133 filesystem should be mounted
134
5ea626aa 135 kill_sb: the method to call when an instance of this filesystem
1a102ff9 136 should be shut down
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137
138 owner: for internal VFS use: you should initialize this to THIS_MODULE in
139 most cases.
1da177e4 140
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141 next: for internal VFS use: you should initialize this to NULL
142
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143 s_lock_key, s_umount_key: lockdep-specific
144
1a102ff9 145The mount() method has the following arguments:
1da177e4 146
d9195881 147 struct file_system_type *fs_type: describes the filesystem, partly initialized
0746aec3 148 by the specific filesystem code
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149
150 int flags: mount flags
151
152 const char *dev_name: the device name we are mounting.
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153
154 void *data: arbitrary mount options, usually comes as an ASCII
f84e3f52 155 string (see "Mount Options" section)
1da177e4 156
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157The 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).
1da177e4 160
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161The 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.
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171
172The most interesting member of the superblock structure that the
1a102ff9 173mount() method fills in is the "s_op" field. This is a pointer to
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174a "struct super_operations" which describes the next level of the
175filesystem implementation.
176
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177Usually, a filesystem uses one of the generic mount() implementations
178and provides a fill_super() callback instead. The generic variants are:
5ea626aa 179
1a102ff9 180 mount_bdev: mount a filesystem residing on a block device
1da177e4 181
1a102ff9 182 mount_nodev: mount a filesystem that is not backed by a device
5ea626aa 183
1a102ff9 184 mount_single: mount a filesystem which shares the instance between
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185 all mounts
186
1a102ff9 187A fill_super() callback implementation has the following arguments:
5ea626aa 188
1a102ff9 189 struct super_block *sb: the superblock structure. The callback
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190 must initialize this properly.
191
192 void *data: arbitrary mount options, usually comes as an ASCII
f84e3f52 193 string (see "Mount Options" section)
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194
195 int silent: whether or not to be silent on error
196
197
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198The Superblock Object
199=====================
200
201A superblock object represents a mounted filesystem.
202
203
5ea626aa 204struct super_operations
cc7d1f8f 205-----------------------
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206
207This describes how the VFS can manipulate the superblock of your
422b14c2 208filesystem. As of kernel 2.6.22, the following members are defined:
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209
210struct super_operations {
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211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
213
aa385729 214 void (*dirty_inode) (struct inode *, int flags);
5ea626aa 215 int (*write_inode) (struct inode *, int);
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216 void (*drop_inode) (struct inode *);
217 void (*delete_inode) (struct inode *);
218 void (*put_super) (struct super_block *);
5ea626aa 219 int (*sync_fs)(struct super_block *sb, int wait);
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220 int (*freeze_fs) (struct super_block *);
221 int (*unfreeze_fs) (struct super_block *);
726c3342 222 int (*statfs) (struct dentry *, struct kstatfs *);
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223 int (*remount_fs) (struct super_block *, int *, char *);
224 void (*clear_inode) (struct inode *);
225 void (*umount_begin) (struct super_block *);
226
34c80b1d 227 int (*show_options)(struct seq_file *, struct dentry *);
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228
229 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
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231 int (*nr_cached_objects)(struct super_block *);
232 void (*free_cached_objects)(struct super_block *, int);
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233};
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
4e07ad64 240 alloc_inode: this method is called by alloc_inode() to allocate memory
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241 for struct inode and initialize it. If this function is not
242 defined, a simple 'struct inode' is allocated. Normally
243 alloc_inode will be used to allocate a larger structure which
244 contains a 'struct inode' embedded within it.
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245
246 destroy_inode: this method is called by destroy_inode() to release
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247 resources allocated for struct inode. It is only required if
248 ->alloc_inode was defined and simply undoes anything done by
249 ->alloc_inode.
5ea626aa 250
5ea626aa 251 dirty_inode: this method is called by the VFS to mark an inode dirty.
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252
253 write_inode: this method is called when the VFS needs to write an
254 inode to disc. The second parameter indicates whether the write
255 should be synchronous or not, not all filesystems check this flag.
256
1da177e4 257 drop_inode: called when the last access to the inode is dropped,
f283c86a 258 with the inode->i_lock spinlock held.
1da177e4 259
5ea626aa 260 This method should be either NULL (normal UNIX filesystem
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261 semantics) or "generic_delete_inode" (for filesystems that do not
262 want to cache inodes - causing "delete_inode" to always be
263 called regardless of the value of i_nlink)
264
5ea626aa 265 The "generic_delete_inode()" behavior is equivalent to the
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266 old practice of using "force_delete" in the put_inode() case,
267 but does not have the races that the "force_delete()" approach
268 had.
269
270 delete_inode: called when the VFS wants to delete an inode
271
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272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
274
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275 sync_fs: called when VFS is writing out all dirty data associated with
276 a superblock. The second parameter indicates whether the method
277 should wait until the write out has been completed. Optional.
278
c4be0c1d 279 freeze_fs: called when VFS is locking a filesystem and
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280 forcing it into a consistent state. This method is currently
281 used by the Logical Volume Manager (LVM).
5ea626aa 282
c4be0c1d 283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
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284 again.
285
66672fef 286 statfs: called when the VFS needs to get filesystem statistics.
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287
288 remount_fs: called when the filesystem is remounted. This is called
289 with the kernel lock held
290
291 clear_inode: called then the VFS clears the inode. Optional
292
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293 umount_begin: called when the VFS is unmounting a filesystem.
294
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295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
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297
298 quota_read: called by the VFS to read from filesystem quota file.
299
300 quota_write: called by the VFS to write to filesystem quota file.
301
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302 nr_cached_objects: called by the sb cache shrinking function for the
303 filesystem to return the number of freeable cached objects it contains.
304 Optional.
305
306 free_cache_objects: called by the sb cache shrinking function for the
307 filesystem to scan the number of objects indicated to try to free them.
308 Optional, but any filesystem implementing this method needs to also
309 implement ->nr_cached_objects for it to be called correctly.
310
311 We can't do anything with any errors that the filesystem might
312 encountered, hence the void return type. This will never be called if
313 the VM is trying to reclaim under GFP_NOFS conditions, hence this
314 method does not need to handle that situation itself.
315
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316 Implementations must include conditional reschedule calls inside any
317 scanning loop that is done. This allows the VFS to determine
318 appropriate scan batch sizes without having to worry about whether
319 implementations will cause holdoff problems due to large scan batch
320 sizes.
321
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322Whoever 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.
1da177e4 325
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326struct xattr_handlers
327---------------------
328
329On filesystems that support extended attributes (xattrs), the s_xattr
330superblock field points to a NULL-terminated array of xattr handlers. Extended
331attributes are name:value pairs.
332
333 name: Indicates that the handler matches attributes with the specified name
334 (such as "system.posix_acl_access"); the prefix field must be NULL.
335
336 prefix: Indicates that the handler matches all attributes with the specified
337 name prefix (such as "user."); the name field must be NULL.
338
339 list: Determine if attributes matching this xattr handler should be listed
340 for a particular dentry. Used by some listxattr implementations like
341 generic_listxattr.
342
343 get: Called by the VFS to get the value of a particular extended attribute.
344 This method is called by the getxattr(2) system call.
345
346 set: Called by the VFS to set the value of a particular extended attribute.
347 When the new value is NULL, called to remove a particular extended
348 attribute. This method is called by the the setxattr(2) and
349 removexattr(2) system calls.
350
351When none of the xattr handlers of a filesystem match the specified attribute
352name or when a filesystem doesn't support extended attributes, the various
353*xattr(2) system calls return -EOPNOTSUPP.
354
1da177e4 355
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356The Inode Object
357================
358
359An inode object represents an object within the filesystem.
360
361
5ea626aa 362struct inode_operations
cc7d1f8f 363-----------------------
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364
365This describes how the VFS can manipulate an inode in your
422b14c2 366filesystem. As of kernel 2.6.22, the following members are defined:
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367
368struct inode_operations {
ebfc3b49 369 int (*create) (struct inode *,struct dentry *, umode_t, bool);
00cd8dd3 370 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
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371 int (*link) (struct dentry *,struct inode *,struct dentry *);
372 int (*unlink) (struct inode *,struct dentry *);
373 int (*symlink) (struct inode *,struct dentry *,const char *);
18bb1db3 374 int (*mkdir) (struct inode *,struct dentry *,umode_t);
1da177e4 375 int (*rmdir) (struct inode *,struct dentry *);
1a67aafb 376 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
1da177e4 377 int (*rename) (struct inode *, struct dentry *,
520c8b16 378 struct inode *, struct dentry *, unsigned int);
5ea626aa 379 int (*readlink) (struct dentry *, char __user *,int);
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380 const char *(*get_link) (struct dentry *, struct inode *,
381 struct delayed_call *);
10556cb2 382 int (*permission) (struct inode *, int);
4e34e719 383 int (*get_acl)(struct inode *, int);
5ea626aa 384 int (*setattr) (struct dentry *, struct iattr *);
75dd7e4b 385 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
5ea626aa 386 ssize_t (*listxattr) (struct dentry *, char *, size_t);
c3b2da31 387 void (*update_time)(struct inode *, struct timespec *, int);
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388 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
389 unsigned open_flag, umode_t create_mode, int *opened);
48bde8d3 390 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
1da177e4
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391};
392
393Again, all methods are called without any locks being held, unless
394otherwise noted.
395
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396 create: called by the open(2) and creat(2) system calls. Only
397 required if you want to support regular files. The dentry you
398 get should not have an inode (i.e. it should be a negative
399 dentry). Here you will probably call d_instantiate() with the
400 dentry and the newly created inode
401
402 lookup: called when the VFS needs to look up an inode in a parent
403 directory. The name to look for is found in the dentry. This
404 method must call d_add() to insert the found inode into the
405 dentry. The "i_count" field in the inode structure should be
406 incremented. If the named inode does not exist a NULL inode
407 should be inserted into the dentry (this is called a negative
408 dentry). Returning an error code from this routine must only
409 be done on a real error, otherwise creating inodes with system
410 calls like create(2), mknod(2), mkdir(2) and so on will fail.
411 If you wish to overload the dentry methods then you should
412 initialise the "d_dop" field in the dentry; this is a pointer
413 to a struct "dentry_operations".
414 This method is called with the directory inode semaphore held
415
416 link: called by the link(2) system call. Only required if you want
417 to support hard links. You will probably need to call
418 d_instantiate() just as you would in the create() method
419
420 unlink: called by the unlink(2) system call. Only required if you
421 want to support deleting inodes
422
423 symlink: called by the symlink(2) system call. Only required if you
424 want to support symlinks. You will probably need to call
425 d_instantiate() just as you would in the create() method
426
427 mkdir: called by the mkdir(2) system call. Only required if you want
428 to support creating subdirectories. You will probably need to
429 call d_instantiate() just as you would in the create() method
430
431 rmdir: called by the rmdir(2) system call. Only required if you want
432 to support deleting subdirectories
433
434 mknod: called by the mknod(2) system call to create a device (char,
435 block) inode or a named pipe (FIFO) or socket. Only required
436 if you want to support creating these types of inodes. You
437 will probably need to call d_instantiate() just as you would
438 in the create() method
439
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440 rename: called by the rename(2) system call to rename the object to
441 have the parent and name given by the second inode and dentry.
442
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443 The filesystem must return -EINVAL for any unsupported or
444 unknown flags. Currently the following flags are implemented:
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445 (1) RENAME_NOREPLACE: this flag indicates that if the target
446 of the rename exists the rename should fail with -EEXIST
447 instead of replacing the target. The VFS already checks for
448 existence, so for local filesystems the RENAME_NOREPLACE
449 implementation is equivalent to plain rename.
450 (2) RENAME_EXCHANGE: exchange source and target. Both must
451 exist; this is checked by the VFS. Unlike plain rename,
452 source and target may be of different type.
453
fceef393 454 get_link: called by the VFS to follow a symbolic link to the
5ea626aa 455 inode it points to. Only required if you want to support
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AV
456 symbolic links. This method returns the symlink body
457 to traverse (and possibly resets the current position with
458 nd_jump_link()). If the body won't go away until the inode
459 is gone, nothing else is needed; if it needs to be otherwise
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460 pinned, arrange for its release by having get_link(..., ..., done)
461 do set_delayed_call(done, destructor, argument).
462 In that case destructor(argument) will be called once VFS is
463 done with the body you've returned.
464 May be called in RCU mode; that is indicated by NULL dentry
465 argument. If request can't be handled without leaving RCU mode,
466 have it return ERR_PTR(-ECHILD).
cc7d1f8f 467
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468 readlink: this is now just an override for use by readlink(2) for the
469 cases when ->get_link uses nd_jump_link() or object is not in
470 fact a symlink. Normally filesystems should only implement
471 ->get_link for symlinks and readlink(2) will automatically use
472 that.
473
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474 permission: called by the VFS to check for access rights on a POSIX-like
475 filesystem.
476
10556cb2 477 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
a82416da 478 mode, the filesystem must check the permission without blocking or
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479 storing to the inode.
480
481 If a situation is encountered that rcu-walk cannot handle, return
482 -ECHILD and it will be called again in ref-walk mode.
483
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484 setattr: called by the VFS to set attributes for a file. This method
485 is called by chmod(2) and related system calls.
5ea626aa 486
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487 getattr: called by the VFS to get attributes of a file. This method
488 is called by stat(2) and related system calls.
5ea626aa 489
cc7d1f8f 490 listxattr: called by the VFS to list all extended attributes for a
6c6ef9f2 491 given file. This method is called by the listxattr(2) system call.
5ea626aa 492
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JB
493 update_time: called by the VFS to update a specific time or the i_version of
494 an inode. If this is not defined the VFS will update the inode itself
495 and call mark_inode_dirty_sync.
5ea626aa 496
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MS
497 atomic_open: called on the last component of an open. Using this optional
498 method the filesystem can look up, possibly create and open the file in
499 one atomic operation. If it cannot perform this (e.g. the file type
d9585277 500 turned out to be wrong) it may signal this by returning 1 instead of
0854d450
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501 usual 0 or -ve . This method is only called if the last component is
502 negative or needs lookup. Cached positive dentries are still handled by
503 f_op->open(). If the file was created, the FILE_CREATED flag should be
504 set in "opened". In case of O_EXCL the method must only succeed if the
505 file didn't exist and hence FILE_CREATED shall always be set on success.
d18e9008 506
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507 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
508 atomically creating, opening and unlinking a file in given directory.
509
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510The Address Space Object
511========================
512
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513The address space object is used to group and manage pages in the page
514cache. It can be used to keep track of the pages in a file (or
515anything else) and also track the mapping of sections of the file into
516process address spaces.
517
518There are a number of distinct yet related services that an
519address-space can provide. These include communicating memory
520pressure, page lookup by address, and keeping track of pages tagged as
521Dirty or Writeback.
522
a9e102b6 523The first can be used independently to the others. The VM can try to
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N
524either write dirty pages in order to clean them, or release clean
525pages in order to reuse them. To do this it can call the ->writepage
526method on dirty pages, and ->releasepage on clean pages with
527PagePrivate set. Clean pages without PagePrivate and with no external
528references will be released without notice being given to the
529address_space.
530
a9e102b6 531To achieve this functionality, pages need to be placed on an LRU with
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N
532lru_cache_add and mark_page_active needs to be called whenever the
533page is used.
534
535Pages are normally kept in a radix tree index by ->index. This tree
536maintains information about the PG_Dirty and PG_Writeback status of
537each page, so that pages with either of these flags can be found
538quickly.
539
540The Dirty tag is primarily used by mpage_writepages - the default
541->writepages method. It uses the tag to find dirty pages to call
542->writepage on. If mpage_writepages is not used (i.e. the address
a9e102b6 543provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
341546f5
N
544almost unused. write_inode_now and sync_inode do use it (through
545__sync_single_inode) to check if ->writepages has been successful in
546writing out the whole address_space.
547
548The Writeback tag is used by filemap*wait* and sync_page* functions,
f4e6d844 549via filemap_fdatawait_range, to wait for all writeback to complete.
341546f5
N
550
551An address_space handler may attach extra information to a page,
552typically using the 'private' field in the 'struct page'. If such
553information is attached, the PG_Private flag should be set. This will
a9e102b6 554cause various VM routines to make extra calls into the address_space
341546f5
N
555handler to deal with that data.
556
557An address space acts as an intermediate between storage and
558application. Data is read into the address space a whole page at a
559time, and provided to the application either by copying of the page,
560or by memory-mapping the page.
561Data is written into the address space by the application, and then
562written-back to storage typically in whole pages, however the
a9e102b6 563address_space has finer control of write sizes.
341546f5
N
564
565The read process essentially only requires 'readpage'. The write
4e02ed4b 566process is more complicated and uses write_begin/write_end or
f4e6d844
MW
567set_page_dirty to write data into the address_space, and writepage
568and writepages to writeback data to storage.
341546f5
N
569
570Adding and removing pages to/from an address_space is protected by the
571inode's i_mutex.
572
573When data is written to a page, the PG_Dirty flag should be set. It
574typically remains set until writepage asks for it to be written. This
575should clear PG_Dirty and set PG_Writeback. It can be actually
576written at any point after PG_Dirty is clear. Once it is known to be
577safe, PG_Writeback is cleared.
578
acbf3c34
JL
579Writeback makes use of a writeback_control structure to direct the
580operations. This gives the the writepage and writepages operations some
581information about the nature of and reason for the writeback request,
582and the constraints under which it is being done. It is also used to
583return information back to the caller about the result of a writepage or
584writepages request.
585
586Handling errors during writeback
587--------------------------------
588Most applications that do buffered I/O will periodically call a file
589synchronization call (fsync, fdatasync, msync or sync_file_range) to
590ensure that data written has made it to the backing store. When there
591is an error during writeback, they expect that error to be reported when
592a file sync request is made. After an error has been reported on one
593request, subsequent requests on the same file descriptor should return
5940, unless further writeback errors have occurred since the previous file
595syncronization.
596
597Ideally, the kernel would report errors only on file descriptions on
598which writes were done that subsequently failed to be written back. The
599generic pagecache infrastructure does not track the file descriptions
600that have dirtied each individual page however, so determining which
601file descriptors should get back an error is not possible.
602
603Instead, the generic writeback error tracking infrastructure in the
604kernel settles for reporting errors to fsync on all file descriptions
605that were open at the time that the error occurred. In a situation with
606multiple writers, all of them will get back an error on a subsequent fsync,
607even if all of the writes done through that particular file descriptor
608succeeded (or even if there were no writes on that file descriptor at all).
609
610Filesystems that wish to use this infrastructure should call
611mapping_set_error to record the error in the address_space when it
612occurs. Then, after writing back data from the pagecache in their
613file->fsync operation, they should call file_check_and_advance_wb_err to
614ensure that the struct file's error cursor has advanced to the correct
615point in the stream of errors emitted by the backing device(s).
5ea626aa
PE
616
617struct address_space_operations
cc7d1f8f 618-------------------------------
5ea626aa
PE
619
620This describes how the VFS can manipulate mapping of a file to page cache in
d47992f8 621your filesystem. The following members are defined:
5ea626aa
PE
622
623struct address_space_operations {
624 int (*writepage)(struct page *page, struct writeback_control *wbc);
625 int (*readpage)(struct file *, struct page *);
5ea626aa
PE
626 int (*writepages)(struct address_space *, struct writeback_control *);
627 int (*set_page_dirty)(struct page *page);
628 int (*readpages)(struct file *filp, struct address_space *mapping,
629 struct list_head *pages, unsigned nr_pages);
afddba49
NP
630 int (*write_begin)(struct file *, struct address_space *mapping,
631 loff_t pos, unsigned len, unsigned flags,
632 struct page **pagep, void **fsdata);
633 int (*write_end)(struct file *, struct address_space *mapping,
634 loff_t pos, unsigned len, unsigned copied,
635 struct page *page, void *fsdata);
5ea626aa 636 sector_t (*bmap)(struct address_space *, sector_t);
d47992f8 637 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
5ea626aa 638 int (*releasepage) (struct page *, int);
6072d13c 639 void (*freepage)(struct page *);
c8b8e32d 640 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
bda807d4
MK
641 /* isolate a page for migration */
642 bool (*isolate_page) (struct page *, isolate_mode_t);
341546f5
N
643 /* migrate the contents of a page to the specified target */
644 int (*migratepage) (struct page *, struct page *);
bda807d4
MK
645 /* put migration-failed page back to right list */
646 void (*putback_page) (struct page *);
422b14c2 647 int (*launder_page) (struct page *);
bda807d4 648
c186afb4 649 int (*is_partially_uptodate) (struct page *, unsigned long,
26c0c5bf 650 unsigned long);
543cc115 651 void (*is_dirty_writeback) (struct page *, bool *, bool *);
25718736 652 int (*error_remove_page) (struct mapping *mapping, struct page *page);
62c230bc
MG
653 int (*swap_activate)(struct file *);
654 int (*swap_deactivate)(struct file *);
5ea626aa
PE
655};
656
341546f5 657 writepage: called by the VM to write a dirty page to backing store.
a9e102b6 658 This may happen for data integrity reasons (i.e. 'sync'), or
341546f5
N
659 to free up memory (flush). The difference can be seen in
660 wbc->sync_mode.
661 The PG_Dirty flag has been cleared and PageLocked is true.
662 writepage should start writeout, should set PG_Writeback,
663 and should make sure the page is unlocked, either synchronously
664 or asynchronously when the write operation completes.
665
666 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
a9e102b6
N
667 try too hard if there are problems, and may choose to write out
668 other pages from the mapping if that is easier (e.g. due to
669 internal dependencies). If it chooses not to start writeout, it
670 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
341546f5
N
671 calling ->writepage on that page.
672
673 See the file "Locking" for more details.
5ea626aa
PE
674
675 readpage: called by the VM to read a page from backing store.
341546f5
N
676 The page will be Locked when readpage is called, and should be
677 unlocked and marked uptodate once the read completes.
678 If ->readpage discovers that it needs to unlock the page for
679 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
a9e102b6 680 In this case, the page will be relocated, relocked and if
341546f5 681 that all succeeds, ->readpage will be called again.
5ea626aa 682
5ea626aa 683 writepages: called by the VM to write out pages associated with the
a9e102b6
N
684 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
685 the writeback_control will specify a range of pages that must be
686 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
341546f5
N
687 and that many pages should be written if possible.
688 If no ->writepages is given, then mpage_writepages is used
a9e102b6 689 instead. This will choose pages from the address space that are
341546f5 690 tagged as DIRTY and will pass them to ->writepage.
5ea626aa
PE
691
692 set_page_dirty: called by the VM to set a page dirty.
341546f5
N
693 This is particularly needed if an address space attaches
694 private data to a page, and that data needs to be updated when
695 a page is dirtied. This is called, for example, when a memory
696 mapped page gets modified.
697 If defined, it should set the PageDirty flag, and the
698 PAGECACHE_TAG_DIRTY tag in the radix tree.
5ea626aa
PE
699
700 readpages: called by the VM to read pages associated with the address_space
341546f5
N
701 object. This is essentially just a vector version of
702 readpage. Instead of just one page, several pages are
703 requested.
a9e102b6 704 readpages is only used for read-ahead, so read errors are
341546f5 705 ignored. If anything goes wrong, feel free to give up.
1da177e4 706
4e02ed4b 707 write_begin:
afddba49
NP
708 Called by the generic buffered write code to ask the filesystem to
709 prepare to write len bytes at the given offset in the file. The
710 address_space should check that the write will be able to complete,
711 by allocating space if necessary and doing any other internal
712 housekeeping. If the write will update parts of any basic-blocks on
713 storage, then those blocks should be pre-read (if they haven't been
714 read already) so that the updated blocks can be written out properly.
715
716 The filesystem must return the locked pagecache page for the specified
717 offset, in *pagep, for the caller to write into.
718
4e02ed4b
NP
719 It must be able to cope with short writes (where the length passed to
720 write_begin is greater than the number of bytes copied into the page).
721
afddba49
NP
722 flags is a field for AOP_FLAG_xxx flags, described in
723 include/linux/fs.h.
724
725 A void * may be returned in fsdata, which then gets passed into
726 write_end.
727
728 Returns 0 on success; < 0 on failure (which is the error code), in
729 which case write_end is not called.
730
731 write_end: After a successful write_begin, and data copy, write_end must
732 be called. len is the original len passed to write_begin, and copied
c718a975 733 is the amount that was able to be copied.
afddba49
NP
734
735 The filesystem must take care of unlocking the page and releasing it
736 refcount, and updating i_size.
737
738 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
739 that were able to be copied into pagecache.
740
5ea626aa 741 bmap: called by the VFS to map a logical block offset within object to
a9e102b6 742 physical block number. This method is used by the FIBMAP
341546f5 743 ioctl and for working with swap-files. To be able to swap to
a9e102b6 744 a file, the file must have a stable mapping to a block
341546f5
N
745 device. The swap system does not go through the filesystem
746 but instead uses bmap to find out where the blocks in the file
747 are and uses those addresses directly.
748
341546f5
N
749 invalidatepage: If a page has PagePrivate set, then invalidatepage
750 will be called when part or all of the page is to be removed
a9e102b6 751 from the address space. This generally corresponds to either a
d47992f8
LC
752 truncation, punch hole or a complete invalidation of the address
753 space (in the latter case 'offset' will always be 0 and 'length'
ea1754a0 754 will be PAGE_SIZE). Any private data associated with the page
d47992f8 755 should be updated to reflect this truncation. If offset is 0 and
ea1754a0 756 length is PAGE_SIZE, then the private data should be released,
d47992f8
LC
757 because the page must be able to be completely discarded. This may
758 be done by calling the ->releasepage function, but in this case the
759 release MUST succeed.
341546f5
N
760
761 releasepage: releasepage is called on PagePrivate pages to indicate
762 that the page should be freed if possible. ->releasepage
763 should remove any private data from the page and clear the
4fe65cab
AM
764 PagePrivate flag. If releasepage() fails for some reason, it must
765 indicate failure with a 0 return value.
766 releasepage() is used in two distinct though related cases. The
767 first is when the VM finds a clean page with no active users and
341546f5
N
768 wants to make it a free page. If ->releasepage succeeds, the
769 page will be removed from the address_space and become free.
770
bc5b1d55 771 The second case is when a request has been made to invalidate
341546f5 772 some or all pages in an address_space. This can happen
0c6cac1a 773 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
341546f5
N
774 filesystem explicitly requesting it as nfs and 9fs do (when
775 they believe the cache may be out of date with storage) by
776 calling invalidate_inode_pages2().
777 If the filesystem makes such a call, and needs to be certain
a9e102b6 778 that all pages are invalidated, then its releasepage will
341546f5
N
779 need to ensure this. Possibly it can clear the PageUptodate
780 bit if it cannot free private data yet.
781
6072d13c
LT
782 freepage: freepage is called once the page is no longer visible in
783 the page cache in order to allow the cleanup of any private
784 data. Since it may be called by the memory reclaimer, it
785 should not assume that the original address_space mapping still
786 exists, and it should not block.
787
341546f5
N
788 direct_IO: called by the generic read/write routines to perform
789 direct_IO - that is IO requests which bypass the page cache
a9e102b6 790 and transfer data directly between the storage and the
341546f5 791 application's address space.
5ea626aa 792
bda807d4
MK
793 isolate_page: Called by the VM when isolating a movable non-lru page.
794 If page is successfully isolated, VM marks the page as PG_isolated
795 via __SetPageIsolated.
796
341546f5
N
797 migrate_page: This is used to compact the physical memory usage.
798 If the VM wants to relocate a page (maybe off a memory card
799 that is signalling imminent failure) it will pass a new page
800 and an old page to this function. migrate_page should
801 transfer any private data across and update any references
802 that it has to the page.
5ea626aa 803
bda807d4
MK
804 putback_page: Called by the VM when isolated page's migration fails.
805
422b14c2
BP
806 launder_page: Called before freeing a page - it writes back the dirty page. To
807 prevent redirtying the page, it is kept locked during the whole
808 operation.
809
26c0c5bf
MG
810 is_partially_uptodate: Called by the VM when reading a file through the
811 pagecache when the underlying blocksize != pagesize. If the required
812 block is up to date then the read can complete without needing the IO
813 to bring the whole page up to date.
814
543cc115
MG
815 is_dirty_writeback: Called by the VM when attempting to reclaim a page.
816 The VM uses dirty and writeback information to determine if it needs
817 to stall to allow flushers a chance to complete some IO. Ordinarily
818 it can use PageDirty and PageWriteback but some filesystems have
819 more complex state (unstable pages in NFS prevent reclaim) or
c290ea01 820 do not set those flags due to locking problems. This callback
543cc115
MG
821 allows a filesystem to indicate to the VM if a page should be
822 treated as dirty or writeback for the purposes of stalling.
823
25718736
AK
824 error_remove_page: normally set to generic_error_remove_page if truncation
825 is ok for this address space. Used for memory failure handling.
826 Setting this implies you deal with pages going away under you,
827 unless you have them locked or reference counts increased.
828
62c230bc
MG
829 swap_activate: Called when swapon is used on a file to allocate
830 space if necessary and pin the block lookup information in
831 memory. A return value of zero indicates success,
832 in which case this file can be used to back swapspace. The
833 swapspace operations will be proxied to this address space's
834 ->swap_{out,in} methods.
835
836 swap_deactivate: Called during swapoff on files where swap_activate
837 was successful.
838
25718736 839
cc7d1f8f
PE
840The File Object
841===============
842
acbf3c34
JL
843A file object represents a file opened by a process. This is also known
844as an "open file description" in POSIX parlance.
cc7d1f8f
PE
845
846
5ea626aa 847struct file_operations
cc7d1f8f 848----------------------
1da177e4
LT
849
850This describes how the VFS can manipulate an open file. As of kernel
0d03943c 8514.1, the following members are defined:
1da177e4
LT
852
853struct file_operations {
422b14c2 854 struct module *owner;
1da177e4 855 loff_t (*llseek) (struct file *, loff_t, int);
5ea626aa 856 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
5ea626aa 857 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
293bc982
AV
858 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
859 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
2233f31a 860 int (*iterate) (struct file *, struct dir_context *);
1da177e4 861 unsigned int (*poll) (struct file *, struct poll_table_struct *);
5ea626aa
PE
862 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
863 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1da177e4 864 int (*mmap) (struct file *, struct vm_area_struct *);
0d03943c 865 int (*mremap)(struct file *, struct vm_area_struct *);
1da177e4 866 int (*open) (struct inode *, struct file *);
0d03943c 867 int (*flush) (struct file *, fl_owner_t id);
1da177e4 868 int (*release) (struct inode *, struct file *);
02c24a82 869 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
5ea626aa 870 int (*fasync) (int, struct file *, int);
1da177e4 871 int (*lock) (struct file *, int, struct file_lock *);
5ea626aa
PE
872 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
873 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
874 int (*check_flags)(int);
5ea626aa 875 int (*flock) (struct file *, int, struct file_lock *);
0d03943c
TB
876 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
877 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
878 int (*setlease)(struct file *, long, struct file_lock **, void **);
879 long (*fallocate)(struct file *file, int mode, loff_t offset,
880 loff_t len);
a3816ab0 881 void (*show_fdinfo)(struct seq_file *m, struct file *f);
0d03943c
TB
882#ifndef CONFIG_MMU
883 unsigned (*mmap_capabilities)(struct file *);
884#endif
1da177e4
LT
885};
886
887Again, all methods are called without any locks being held, unless
888otherwise noted.
889
890 llseek: called when the VFS needs to move the file position index
891
892 read: called by read(2) and related system calls
893
293bc982 894 read_iter: possibly asynchronous read with iov_iter as destination
5ea626aa 895
1da177e4
LT
896 write: called by write(2) and related system calls
897
293bc982 898 write_iter: possibly asynchronous write with iov_iter as source
5ea626aa 899
2233f31a 900 iterate: called when the VFS needs to read the directory contents
1da177e4
LT
901
902 poll: called by the VFS when a process wants to check if there is
903 activity on this file and (optionally) go to sleep until there
904 is activity. Called by the select(2) and poll(2) system calls
905
b19dd42f 906 unlocked_ioctl: called by the ioctl(2) system call.
5ea626aa
PE
907
908 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
909 are used on 64 bit kernels.
910
1da177e4
LT
911 mmap: called by the mmap(2) system call
912
913 open: called by the VFS when an inode should be opened. When the VFS
5ea626aa
PE
914 opens a file, it creates a new "struct file". It then calls the
915 open method for the newly allocated file structure. You might
916 think that the open method really belongs in
917 "struct inode_operations", and you may be right. I think it's
918 done the way it is because it makes filesystems simpler to
919 implement. The open() method is a good place to initialize the
920 "private_data" member in the file structure if you want to point
921 to a device structure
922
923 flush: called by the close(2) system call to flush a file
1da177e4
LT
924
925 release: called when the last reference to an open file is closed
926
acbf3c34
JL
927 fsync: called by the fsync(2) system call. Also see the section above
928 entitled "Handling errors during writeback".
1da177e4
LT
929
930 fasync: called by the fcntl(2) system call when asynchronous
931 (non-blocking) mode is enabled for a file
932
5ea626aa
PE
933 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
934 commands
935
5ea626aa
PE
936 get_unmapped_area: called by the mmap(2) system call
937
938 check_flags: called by the fcntl(2) system call for F_SETFL command
939
5ea626aa
PE
940 flock: called by the flock(2) system call
941
d1195c51
PE
942 splice_write: called by the VFS to splice data from a pipe to a file. This
943 method is used by the splice(2) system call
944
945 splice_read: called by the VFS to splice data from file to a pipe. This
946 method is used by the splice(2) system call
947
f82b4b67
JL
948 setlease: called by the VFS to set or release a file lock lease. setlease
949 implementations should call generic_setlease to record or remove
950 the lease in the inode after setting it.
17cf28af
HD
951
952 fallocate: called by the VFS to preallocate blocks or punch a hole.
953
1da177e4
LT
954Note that the file operations are implemented by the specific
955filesystem in which the inode resides. When opening a device node
956(character or block special) most filesystems will call special
957support routines in the VFS which will locate the required device
958driver information. These support routines replace the filesystem file
959operations with those for the device driver, and then proceed to call
960the new open() method for the file. This is how opening a device file
961in the filesystem eventually ends up calling the device driver open()
5ea626aa 962method.
1da177e4
LT
963
964
5ea626aa
PE
965Directory Entry Cache (dcache)
966==============================
967
1da177e4
LT
968
969struct dentry_operations
5ea626aa 970------------------------
1da177e4
LT
971
972This describes how a filesystem can overload the standard dentry
973operations. Dentries and the dcache are the domain of the VFS and the
974individual filesystem implementations. Device drivers have no business
975here. These methods may be set to NULL, as they are either optional or
c23fbb6b 976the VFS uses a default. As of kernel 2.6.22, the following members are
1da177e4
LT
977defined:
978
979struct dentry_operations {
0b728e19 980 int (*d_revalidate)(struct dentry *, unsigned int);
ecf3d1f1 981 int (*d_weak_revalidate)(struct dentry *, unsigned int);
da53be12 982 int (*d_hash)(const struct dentry *, struct qstr *);
6fa67e70 983 int (*d_compare)(const struct dentry *,
621e155a 984 unsigned int, const char *, const struct qstr *);
fe15ce44 985 int (*d_delete)(const struct dentry *);
285b102d 986 int (*d_init)(struct dentry *);
1da177e4
LT
987 void (*d_release)(struct dentry *);
988 void (*d_iput)(struct dentry *, struct inode *);
c23fbb6b 989 char *(*d_dname)(struct dentry *, char *, int);
9875cf80 990 struct vfsmount *(*d_automount)(struct path *);
fb5f51c7 991 int (*d_manage)(const struct path *, bool);
e698b8a4
MS
992 struct dentry *(*d_real)(struct dentry *, const struct inode *,
993 unsigned int);
1da177e4
LT
994};
995
996 d_revalidate: called when the VFS needs to revalidate a dentry. This
997 is called whenever a name look-up finds a dentry in the
ecf3d1f1
JL
998 dcache. Most local filesystems leave this as NULL, because all their
999 dentries in the dcache are valid. Network filesystems are different
1000 since things can change on the server without the client necessarily
1001 being aware of it.
1002
1003 This function should return a positive value if the dentry is still
1004 valid, and zero or a negative error code if it isn't.
1da177e4 1005
0b728e19 1006 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
34286d66
NP
1007 If in rcu-walk mode, the filesystem must revalidate the dentry without
1008 blocking or storing to the dentry, d_parent and d_inode should not be
0b728e19
AV
1009 used without care (because they can change and, in d_inode case, even
1010 become NULL under us).
34286d66
NP
1011
1012 If a situation is encountered that rcu-walk cannot handle, return
1013 -ECHILD and it will be called again in ref-walk mode.
1014
ecf3d1f1
JL
1015 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
1016 This is called when a path-walk ends at dentry that was not acquired by
1017 doing a lookup in the parent directory. This includes "/", "." and "..",
1018 as well as procfs-style symlinks and mountpoint traversal.
1019
1020 In this case, we are less concerned with whether the dentry is still
1021 fully correct, but rather that the inode is still valid. As with
1022 d_revalidate, most local filesystems will set this to NULL since their
1023 dcache entries are always valid.
1024
1025 This function has the same return code semantics as d_revalidate.
1026
1027 d_weak_revalidate is only called after leaving rcu-walk mode.
1028
621e155a
NP
1029 d_hash: called when the VFS adds a dentry to the hash table. The first
1030 dentry passed to d_hash is the parent directory that the name is
da53be12 1031 to be hashed into.
b1e6a015
NP
1032
1033 Same locking and synchronisation rules as d_compare regarding
1034 what is safe to dereference etc.
1da177e4 1035
621e155a
NP
1036 d_compare: called to compare a dentry name with a given name. The first
1037 dentry is the parent of the dentry to be compared, the second is
da53be12
LT
1038 the child dentry. len and name string are properties of the dentry
1039 to be compared. qstr is the name to compare it with.
621e155a
NP
1040
1041 Must be constant and idempotent, and should not take locks if
da53be12
LT
1042 possible, and should not or store into the dentry.
1043 Should not dereference pointers outside the dentry without
621e155a
NP
1044 lots of care (eg. d_parent, d_inode, d_name should not be used).
1045
1046 However, our vfsmount is pinned, and RCU held, so the dentries and
1047 inodes won't disappear, neither will our sb or filesystem module.
da53be12 1048 ->d_sb may be used.
621e155a
NP
1049
1050 It is a tricky calling convention because it needs to be called under
1051 "rcu-walk", ie. without any locks or references on things.
1da177e4 1052
fe15ce44
NP
1053 d_delete: called when the last reference to a dentry is dropped and the
1054 dcache is deciding whether or not to cache it. Return 1 to delete
1055 immediately, or 0 to cache the dentry. Default is NULL which means to
1056 always cache a reachable dentry. d_delete must be constant and
1057 idempotent.
1da177e4 1058
285b102d
MS
1059 d_init: called when a dentry is allocated
1060
1da177e4
LT
1061 d_release: called when a dentry is really deallocated
1062
1063 d_iput: called when a dentry loses its inode (just prior to its
1064 being deallocated). The default when this is NULL is that the
1065 VFS calls iput(). If you define this method, you must call
1066 iput() yourself
1067
c23fbb6b 1068 d_dname: called when the pathname of a dentry should be generated.
d9195881 1069 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
c23fbb6b 1070 pathname generation. (Instead of doing it when dentry is created,
d9195881 1071 it's done only when the path is needed.). Real filesystems probably
c23fbb6b
ED
1072 dont want to use it, because their dentries are present in global
1073 dcache hash, so their hash should be an invariant. As no lock is
1074 held, d_dname() should not try to modify the dentry itself, unless
1075 appropriate SMP safety is used. CAUTION : d_path() logic is quite
1076 tricky. The correct way to return for example "Hello" is to put it
1077 at the end of the buffer, and returns a pointer to the first char.
1078 dynamic_dname() helper function is provided to take care of this.
1079
0cac643c
MS
1080 Example :
1081
1082 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1083 {
1084 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1085 dentry->d_inode->i_ino);
1086 }
1087
9875cf80 1088 d_automount: called when an automount dentry is to be traversed (optional).
ea5b778a
DH
1089 This should create a new VFS mount record and return the record to the
1090 caller. The caller is supplied with a path parameter giving the
1091 automount directory to describe the automount target and the parent
1092 VFS mount record to provide inheritable mount parameters. NULL should
1093 be returned if someone else managed to make the automount first. If
1094 the vfsmount creation failed, then an error code should be returned.
1095 If -EISDIR is returned, then the directory will be treated as an
1096 ordinary directory and returned to pathwalk to continue walking.
1097
1098 If a vfsmount is returned, the caller will attempt to mount it on the
1099 mountpoint and will remove the vfsmount from its expiration list in
1100 the case of failure. The vfsmount should be returned with 2 refs on
1101 it to prevent automatic expiration - the caller will clean up the
1102 additional ref.
9875cf80
DH
1103
1104 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1105 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1106 inode being added.
1107
cc53ce53
DH
1108 d_manage: called to allow the filesystem to manage the transition from a
1109 dentry (optional). This allows autofs, for example, to hold up clients
1110 waiting to explore behind a 'mountpoint' whilst letting the daemon go
1111 past and construct the subtree there. 0 should be returned to let the
1112 calling process continue. -EISDIR can be returned to tell pathwalk to
1113 use this directory as an ordinary directory and to ignore anything
1114 mounted on it and not to check the automount flag. Any other error
1115 code will abort pathwalk completely.
1116
ab90911f
DH
1117 If the 'rcu_walk' parameter is true, then the caller is doing a
1118 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
40e47125 1119 and the caller can be asked to leave it and call again by returning
b8faf035
N
1120 -ECHILD. -EISDIR may also be returned to tell pathwalk to
1121 ignore d_automount or any mounts.
ab90911f 1122
cc53ce53
DH
1123 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1124 dentry being transited from.
1125
e698b8a4
MS
1126 d_real: overlay/union type filesystems implement this method to return one of
1127 the underlying dentries hidden by the overlay. It is used in three
1128 different modes:
c23fbb6b 1129
e698b8a4
MS
1130 Called from open it may need to copy-up the file depending on the
1131 supplied open flags. This mode is selected with a non-zero flags
1132 argument. In this mode the d_real method can return an error.
1133
1134 Called from file_dentry() it returns the real dentry matching the inode
1135 argument. The real dentry may be from a lower layer already copied up,
1136 but still referenced from the file. This mode is selected with a
1137 non-NULL inode argument. This will always succeed.
1138
1139 With NULL inode and zero flags the topmost real underlying dentry is
1140 returned. This will always succeed.
1141
1142 This method is never called with both non-NULL inode and non-zero flags.
c23fbb6b 1143
1da177e4
LT
1144Each dentry has a pointer to its parent dentry, as well as a hash list
1145of child dentries. Child dentries are basically like files in a
1146directory.
1147
5ea626aa 1148
cc7d1f8f 1149Directory Entry Cache API
1da177e4
LT
1150--------------------------
1151
1152There are a number of functions defined which permit a filesystem to
1153manipulate dentries:
1154
1155 dget: open a new handle for an existing dentry (this just increments
1156 the usage count)
1157
1158 dput: close a handle for a dentry (decrements the usage count). If
fe15ce44
NP
1159 the usage count drops to 0, and the dentry is still in its
1160 parent's hash, the "d_delete" method is called to check whether
1161 it should be cached. If it should not be cached, or if the dentry
1162 is not hashed, it is deleted. Otherwise cached dentries are put
1163 into an LRU list to be reclaimed on memory shortage.
1da177e4
LT
1164
1165 d_drop: this unhashes a dentry from its parents hash list. A
5ea626aa 1166 subsequent call to dput() will deallocate the dentry if its
1da177e4
LT
1167 usage count drops to 0
1168
1169 d_delete: delete a dentry. If there are no other open references to
1170 the dentry then the dentry is turned into a negative dentry
1171 (the d_iput() method is called). If there are other
1172 references, then d_drop() is called instead
1173
1174 d_add: add a dentry to its parents hash list and then calls
1175 d_instantiate()
1176
1177 d_instantiate: add a dentry to the alias hash list for the inode and
1178 updates the "d_inode" member. The "i_count" member in the
1179 inode structure should be set/incremented. If the inode
1180 pointer is NULL, the dentry is called a "negative
1181 dentry". This function is commonly called when an inode is
1182 created for an existing negative dentry
1183
1184 d_lookup: look up a dentry given its parent and path name component
1185 It looks up the child of that given name from the dcache
1186 hash table. If it is found, the reference count is incremented
be42c4c4 1187 and the dentry is returned. The caller must use dput()
1da177e4
LT
1188 to free the dentry when it finishes using it.
1189
f84e3f52
MS
1190Mount Options
1191=============
1192
1193Parsing options
1194---------------
1195
1196On mount and remount the filesystem is passed a string containing a
1197comma separated list of mount options. The options can have either of
1198these forms:
1199
1200 option
1201 option=value
1202
1203The <linux/parser.h> header defines an API that helps parse these
1204options. There are plenty of examples on how to use it in existing
1205filesystems.
1206
1207Showing options
1208---------------
1209
1210If a filesystem accepts mount options, it must define show_options()
1211to show all the currently active options. The rules are:
1212
1213 - options MUST be shown which are not default or their values differ
1214 from the default
1215
1216 - options MAY be shown which are enabled by default or have their
1217 default value
1218
1219Options used only internally between a mount helper and the kernel
1220(such as file descriptors), or which only have an effect during the
1221mounting (such as ones controlling the creation of a journal) are exempt
1222from the above rules.
1223
1224The underlying reason for the above rules is to make sure, that a
1225mount can be accurately replicated (e.g. umounting and mounting again)
1226based on the information found in /proc/mounts.
1227
cc7d1f8f
PE
1228Resources
1229=========
1230
1231(Note some of these resources are not up-to-date with the latest kernel
1232 version.)
1233
1234Creating Linux virtual filesystems. 2002
1235 <http://lwn.net/Articles/13325/>
1236
1237The Linux Virtual File-system Layer by Neil Brown. 1999
1238 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1239
1240A tour of the Linux VFS by Michael K. Johnson. 1996
1241 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1242
1243A small trail through the Linux kernel by Andries Brouwer. 2001
1244 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>