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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 | |
5ea626aa PE |
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 | |
cc7d1f8f PE |
14 | Introduction |
15 | ============ | |
1da177e4 | 16 | |
cc7d1f8f PE |
17 | The Virtual File System (also known as the Virtual Filesystem Switch) |
18 | is the software layer in the kernel that provides the filesystem | |
19 | interface to userspace programs. It also provides an abstraction | |
20 | within the kernel which allows different filesystem implementations to | |
21 | coexist. | |
1da177e4 | 22 | |
cc7d1f8f PE |
23 | VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so |
24 | on are called from a process context. Filesystem locking is described | |
25 | in the document Documentation/filesystems/Locking. | |
1da177e4 | 26 | |
1da177e4 | 27 | |
cc7d1f8f PE |
28 | Directory Entry Cache (dcache) |
29 | ------------------------------ | |
1da177e4 | 30 | |
cc7d1f8f PE |
31 | The VFS implements the open(2), stat(2), chmod(2), and similar system |
32 | calls. The pathname argument that is passed to them is used by the VFS | |
33 | to search through the directory entry cache (also known as the dentry | |
34 | cache or dcache). This provides a very fast look-up mechanism to | |
35 | translate a pathname (filename) into a specific dentry. Dentries live | |
36 | in RAM and are never saved to disc: they exist only for performance. | |
37 | ||
38 | The dentry cache is meant to be a view into your entire filespace. As | |
39 | most computers cannot fit all dentries in the RAM at the same time, | |
40 | some bits of the cache are missing. In order to resolve your pathname | |
41 | into a dentry, the VFS may have to resort to creating dentries along | |
42 | the way, and then loading the inode. This is done by looking up the | |
43 | inode. | |
44 | ||
45 | ||
46 | The Inode Object | |
47 | ---------------- | |
48 | ||
49 | An individual dentry usually has a pointer to an inode. Inodes are | |
50 | filesystem objects such as regular files, directories, FIFOs and other | |
51 | beasts. They live either on the disc (for block device filesystems) | |
52 | or in the memory (for pseudo filesystems). Inodes that live on the | |
53 | disc are copied into the memory when required and changes to the inode | |
54 | are written back to disc. A single inode can be pointed to by multiple | |
55 | dentries (hard links, for example, do this). | |
56 | ||
57 | To look up an inode requires that the VFS calls the lookup() method of | |
58 | the parent directory inode. This method is installed by the specific | |
59 | filesystem implementation that the inode lives in. Once the VFS has | |
60 | the required dentry (and hence the inode), we can do all those boring | |
61 | things like open(2) the file, or stat(2) it to peek at the inode | |
62 | data. The stat(2) operation is fairly simple: once the VFS has the | |
63 | dentry, it peeks at the inode data and passes some of it back to | |
64 | userspace. | |
65 | ||
66 | ||
67 | The File Object | |
68 | --------------- | |
1da177e4 LT |
69 | |
70 | Opening a file requires another operation: allocation of a file | |
71 | structure (this is the kernel-side implementation of file | |
5ea626aa | 72 | descriptors). The freshly allocated file structure is initialized with |
1da177e4 LT |
73 | a pointer to the dentry and a set of file operation member functions. |
74 | These are taken from the inode data. The open() file method is then | |
a33f3224 | 75 | called so the specific filesystem implementation can do its work. You |
cc7d1f8f PE |
76 | can see that this is another switch performed by the VFS. The file |
77 | structure is placed into the file descriptor table for the process. | |
1da177e4 LT |
78 | |
79 | Reading, writing and closing files (and other assorted VFS operations) | |
80 | is done by using the userspace file descriptor to grab the appropriate | |
cc7d1f8f PE |
81 | file structure, and then calling the required file structure method to |
82 | do whatever is required. For as long as the file is open, it keeps the | |
83 | dentry in use, which in turn means that the VFS inode is still in use. | |
1da177e4 | 84 | |
5ea626aa PE |
85 | |
86 | Registering and Mounting a Filesystem | |
cc7d1f8f | 87 | ===================================== |
1da177e4 | 88 | |
cc7d1f8f PE |
89 | To register and unregister a filesystem, use the following API |
90 | functions: | |
1da177e4 | 91 | |
cc7d1f8f | 92 | #include <linux/fs.h> |
1da177e4 | 93 | |
cc7d1f8f PE |
94 | extern int register_filesystem(struct file_system_type *); |
95 | extern int unregister_filesystem(struct file_system_type *); | |
1da177e4 | 96 | |
cc7d1f8f | 97 | The passed struct file_system_type describes your filesystem. When a |
1a102ff9 AV |
98 | request is made to mount a filesystem onto a directory in your namespace, |
99 | the VFS will call the appropriate mount() method for the specific | |
100 | filesystem. New vfsmount refering to the tree returned by ->mount() | |
101 | will be attached to the mountpoint, so that when pathname resolution | |
102 | reaches the mountpoint it will jump into the root of that vfsmount. | |
1da177e4 | 103 | |
cc7d1f8f PE |
104 | You can see all filesystems that are registered to the kernel in the |
105 | file /proc/filesystems. | |
1da177e4 LT |
106 | |
107 | ||
5ea626aa | 108 | struct file_system_type |
cc7d1f8f | 109 | ----------------------- |
1da177e4 | 110 | |
1a102ff9 | 111 | This describes the filesystem. As of kernel 2.6.39, the following |
1da177e4 LT |
112 | members are defined: |
113 | ||
114 | struct file_system_type { | |
115 | const char *name; | |
116 | int fs_flags; | |
1a102ff9 AV |
117 | struct dentry (*mount) (struct file_system_type *, int, |
118 | const char *, void *); | |
5ea626aa PE |
119 | void (*kill_sb) (struct super_block *); |
120 | struct module *owner; | |
121 | struct file_system_type * next; | |
122 | struct list_head fs_supers; | |
0746aec3 BP |
123 | struct lock_class_key s_lock_key; |
124 | struct lock_class_key s_umount_key; | |
1da177e4 LT |
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 |
1da177e4 LT |
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 |
5ea626aa PE |
137 | |
138 | owner: for internal VFS use: you should initialize this to THIS_MODULE in | |
139 | most cases. | |
1da177e4 | 140 | |
5ea626aa PE |
141 | next: for internal VFS use: you should initialize this to NULL |
142 | ||
0746aec3 BP |
143 | s_lock_key, s_umount_key: lockdep-specific |
144 | ||
1a102ff9 | 145 | The 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 |
5ea626aa PE |
149 | |
150 | int flags: mount flags | |
151 | ||
152 | const char *dev_name: the device name we are mounting. | |
1da177e4 LT |
153 | |
154 | void *data: arbitrary mount options, usually comes as an ASCII | |
f84e3f52 | 155 | string (see "Mount Options" section) |
1da177e4 | 156 | |
1a102ff9 AV |
157 | The mount() method must return the root dentry of the tree requested by |
158 | caller. An active reference to its superblock must be grabbed and the | |
159 | superblock must be locked. On failure it should return ERR_PTR(error). | |
1da177e4 | 160 | |
1a102ff9 AV |
161 | The arguments match those of mount(2) and their interpretation |
162 | depends on filesystem type. E.g. for block filesystems, dev_name is | |
163 | interpreted as block device name, that device is opened and if it | |
164 | contains a suitable filesystem image the method creates and initializes | |
165 | struct super_block accordingly, returning its root dentry to caller. | |
166 | ||
167 | ->mount() may choose to return a subtree of existing filesystem - it | |
168 | doesn't have to create a new one. The main result from the caller's | |
169 | point of view is a reference to dentry at the root of (sub)tree to | |
170 | be attached; creation of new superblock is a common side effect. | |
1da177e4 LT |
171 | |
172 | The most interesting member of the superblock structure that the | |
1a102ff9 | 173 | mount() method fills in is the "s_op" field. This is a pointer to |
1da177e4 LT |
174 | a "struct super_operations" which describes the next level of the |
175 | filesystem implementation. | |
176 | ||
1a102ff9 AV |
177 | Usually, a filesystem uses one of the generic mount() implementations |
178 | and 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 |
5ea626aa PE |
185 | all mounts |
186 | ||
1a102ff9 | 187 | A fill_super() callback implementation has the following arguments: |
5ea626aa | 188 | |
1a102ff9 | 189 | struct super_block *sb: the superblock structure. The callback |
5ea626aa PE |
190 | must initialize this properly. |
191 | ||
192 | void *data: arbitrary mount options, usually comes as an ASCII | |
f84e3f52 | 193 | string (see "Mount Options" section) |
5ea626aa PE |
194 | |
195 | int silent: whether or not to be silent on error | |
196 | ||
197 | ||
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198 | The Superblock Object |
199 | ===================== | |
200 | ||
201 | A superblock object represents a mounted filesystem. | |
202 | ||
203 | ||
5ea626aa | 204 | struct super_operations |
cc7d1f8f | 205 | ----------------------- |
1da177e4 LT |
206 | |
207 | This describes how the VFS can manipulate the superblock of your | |
422b14c2 | 208 | filesystem. As of kernel 2.6.22, the following members are defined: |
1da177e4 LT |
209 | |
210 | struct super_operations { | |
5ea626aa PE |
211 | struct inode *(*alloc_inode)(struct super_block *sb); |
212 | void (*destroy_inode)(struct inode *); | |
213 | ||
5ea626aa PE |
214 | void (*dirty_inode) (struct inode *); |
215 | int (*write_inode) (struct inode *, int); | |
5ea626aa PE |
216 | void (*drop_inode) (struct inode *); |
217 | void (*delete_inode) (struct inode *); | |
218 | void (*put_super) (struct super_block *); | |
219 | void (*write_super) (struct super_block *); | |
220 | int (*sync_fs)(struct super_block *sb, int wait); | |
c4be0c1d TS |
221 | int (*freeze_fs) (struct super_block *); |
222 | int (*unfreeze_fs) (struct super_block *); | |
726c3342 | 223 | int (*statfs) (struct dentry *, struct kstatfs *); |
5ea626aa PE |
224 | int (*remount_fs) (struct super_block *, int *, char *); |
225 | void (*clear_inode) (struct inode *); | |
226 | void (*umount_begin) (struct super_block *); | |
227 | ||
5ea626aa PE |
228 | int (*show_options)(struct seq_file *, struct vfsmount *); |
229 | ||
230 | ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); | |
231 | ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); | |
1da177e4 LT |
232 | }; |
233 | ||
234 | All methods are called without any locks being held, unless otherwise | |
235 | noted. This means that most methods can block safely. All methods are | |
236 | only called from a process context (i.e. not from an interrupt handler | |
237 | or bottom half). | |
238 | ||
5ea626aa | 239 | alloc_inode: this method is called by inode_alloc() to allocate memory |
341546f5 N |
240 | for struct inode and initialize it. If this function is not |
241 | defined, a simple 'struct inode' is allocated. Normally | |
242 | alloc_inode will be used to allocate a larger structure which | |
243 | contains a 'struct inode' embedded within it. | |
5ea626aa PE |
244 | |
245 | destroy_inode: this method is called by destroy_inode() to release | |
341546f5 N |
246 | resources allocated for struct inode. It is only required if |
247 | ->alloc_inode was defined and simply undoes anything done by | |
248 | ->alloc_inode. | |
5ea626aa | 249 | |
5ea626aa | 250 | dirty_inode: this method is called by the VFS to mark an inode dirty. |
1da177e4 LT |
251 | |
252 | write_inode: this method is called when the VFS needs to write an | |
253 | inode to disc. The second parameter indicates whether the write | |
254 | should be synchronous or not, not all filesystems check this flag. | |
255 | ||
1da177e4 | 256 | drop_inode: called when the last access to the inode is dropped, |
f283c86a | 257 | with the inode->i_lock spinlock held. |
1da177e4 | 258 | |
5ea626aa | 259 | This method should be either NULL (normal UNIX filesystem |
1da177e4 LT |
260 | semantics) or "generic_delete_inode" (for filesystems that do not |
261 | want to cache inodes - causing "delete_inode" to always be | |
262 | called regardless of the value of i_nlink) | |
263 | ||
5ea626aa | 264 | The "generic_delete_inode()" behavior is equivalent to the |
1da177e4 LT |
265 | old practice of using "force_delete" in the put_inode() case, |
266 | but does not have the races that the "force_delete()" approach | |
267 | had. | |
268 | ||
269 | delete_inode: called when the VFS wants to delete an inode | |
270 | ||
1da177e4 LT |
271 | put_super: called when the VFS wishes to free the superblock |
272 | (i.e. unmount). This is called with the superblock lock held | |
273 | ||
274 | write_super: called when the VFS superblock needs to be written to | |
275 | disc. This method is optional | |
276 | ||
5ea626aa PE |
277 | sync_fs: called when VFS is writing out all dirty data associated with |
278 | a superblock. The second parameter indicates whether the method | |
279 | should wait until the write out has been completed. Optional. | |
280 | ||
c4be0c1d | 281 | freeze_fs: called when VFS is locking a filesystem and |
cc7d1f8f PE |
282 | forcing it into a consistent state. This method is currently |
283 | used by the Logical Volume Manager (LVM). | |
5ea626aa | 284 | |
c4be0c1d | 285 | unfreeze_fs: called when VFS is unlocking a filesystem and making it writable |
5ea626aa PE |
286 | again. |
287 | ||
66672fef | 288 | statfs: called when the VFS needs to get filesystem statistics. |
1da177e4 LT |
289 | |
290 | remount_fs: called when the filesystem is remounted. This is called | |
291 | with the kernel lock held | |
292 | ||
293 | clear_inode: called then the VFS clears the inode. Optional | |
294 | ||
5ea626aa PE |
295 | umount_begin: called when the VFS is unmounting a filesystem. |
296 | ||
f84e3f52 MS |
297 | show_options: called by the VFS to show mount options for |
298 | /proc/<pid>/mounts. (see "Mount Options" section) | |
5ea626aa PE |
299 | |
300 | quota_read: called by the VFS to read from filesystem quota file. | |
301 | ||
302 | quota_write: called by the VFS to write to filesystem quota file. | |
303 | ||
12debc42 DH |
304 | Whoever sets up the inode is responsible for filling in the "i_op" field. This |
305 | is a pointer to a "struct inode_operations" which describes the methods that | |
306 | can be performed on individual inodes. | |
1da177e4 LT |
307 | |
308 | ||
cc7d1f8f PE |
309 | The Inode Object |
310 | ================ | |
311 | ||
312 | An inode object represents an object within the filesystem. | |
313 | ||
314 | ||
5ea626aa | 315 | struct inode_operations |
cc7d1f8f | 316 | ----------------------- |
1da177e4 LT |
317 | |
318 | This describes how the VFS can manipulate an inode in your | |
422b14c2 | 319 | filesystem. As of kernel 2.6.22, the following members are defined: |
1da177e4 LT |
320 | |
321 | struct inode_operations { | |
5ea626aa PE |
322 | int (*create) (struct inode *,struct dentry *,int, struct nameidata *); |
323 | struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *); | |
1da177e4 LT |
324 | int (*link) (struct dentry *,struct inode *,struct dentry *); |
325 | int (*unlink) (struct inode *,struct dentry *); | |
326 | int (*symlink) (struct inode *,struct dentry *,const char *); | |
327 | int (*mkdir) (struct inode *,struct dentry *,int); | |
328 | int (*rmdir) (struct inode *,struct dentry *); | |
329 | int (*mknod) (struct inode *,struct dentry *,int,dev_t); | |
330 | int (*rename) (struct inode *, struct dentry *, | |
331 | struct inode *, struct dentry *); | |
5ea626aa PE |
332 | int (*readlink) (struct dentry *, char __user *,int); |
333 | void * (*follow_link) (struct dentry *, struct nameidata *); | |
334 | void (*put_link) (struct dentry *, struct nameidata *, void *); | |
1da177e4 | 335 | void (*truncate) (struct inode *); |
b74c79e9 NP |
336 | int (*permission) (struct inode *, int, unsigned int); |
337 | int (*check_acl)(struct inode *, int, unsigned int); | |
5ea626aa PE |
338 | int (*setattr) (struct dentry *, struct iattr *); |
339 | int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); | |
340 | int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); | |
341 | ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); | |
342 | ssize_t (*listxattr) (struct dentry *, char *, size_t); | |
343 | int (*removexattr) (struct dentry *, const char *); | |
422b14c2 | 344 | void (*truncate_range)(struct inode *, loff_t, loff_t); |
1da177e4 LT |
345 | }; |
346 | ||
347 | Again, all methods are called without any locks being held, unless | |
348 | otherwise noted. | |
349 | ||
1da177e4 LT |
350 | create: called by the open(2) and creat(2) system calls. Only |
351 | required if you want to support regular files. The dentry you | |
352 | get should not have an inode (i.e. it should be a negative | |
353 | dentry). Here you will probably call d_instantiate() with the | |
354 | dentry and the newly created inode | |
355 | ||
356 | lookup: called when the VFS needs to look up an inode in a parent | |
357 | directory. The name to look for is found in the dentry. This | |
358 | method must call d_add() to insert the found inode into the | |
359 | dentry. The "i_count" field in the inode structure should be | |
360 | incremented. If the named inode does not exist a NULL inode | |
361 | should be inserted into the dentry (this is called a negative | |
362 | dentry). Returning an error code from this routine must only | |
363 | be done on a real error, otherwise creating inodes with system | |
364 | calls like create(2), mknod(2), mkdir(2) and so on will fail. | |
365 | If you wish to overload the dentry methods then you should | |
366 | initialise the "d_dop" field in the dentry; this is a pointer | |
367 | to a struct "dentry_operations". | |
368 | This method is called with the directory inode semaphore held | |
369 | ||
370 | link: called by the link(2) system call. Only required if you want | |
371 | to support hard links. You will probably need to call | |
372 | d_instantiate() just as you would in the create() method | |
373 | ||
374 | unlink: called by the unlink(2) system call. Only required if you | |
375 | want to support deleting inodes | |
376 | ||
377 | symlink: called by the symlink(2) system call. Only required if you | |
378 | want to support symlinks. You will probably need to call | |
379 | d_instantiate() just as you would in the create() method | |
380 | ||
381 | mkdir: called by the mkdir(2) system call. Only required if you want | |
382 | to support creating subdirectories. You will probably need to | |
383 | call d_instantiate() just as you would in the create() method | |
384 | ||
385 | rmdir: called by the rmdir(2) system call. Only required if you want | |
386 | to support deleting subdirectories | |
387 | ||
388 | mknod: called by the mknod(2) system call to create a device (char, | |
389 | block) inode or a named pipe (FIFO) or socket. Only required | |
390 | if you want to support creating these types of inodes. You | |
391 | will probably need to call d_instantiate() just as you would | |
392 | in the create() method | |
393 | ||
cc7d1f8f PE |
394 | rename: called by the rename(2) system call to rename the object to |
395 | have the parent and name given by the second inode and dentry. | |
396 | ||
1da177e4 LT |
397 | readlink: called by the readlink(2) system call. Only required if |
398 | you want to support reading symbolic links | |
399 | ||
400 | follow_link: called by the VFS to follow a symbolic link to the | |
5ea626aa | 401 | inode it points to. Only required if you want to support |
cc7d1f8f | 402 | symbolic links. This method returns a void pointer cookie |
5ea626aa PE |
403 | that is passed to put_link(). |
404 | ||
405 | put_link: called by the VFS to release resources allocated by | |
cc7d1f8f | 406 | follow_link(). The cookie returned by follow_link() is passed |
670e9f34 | 407 | to this method as the last parameter. It is used by |
cc7d1f8f PE |
408 | filesystems such as NFS where page cache is not stable |
409 | (i.e. page that was installed when the symbolic link walk | |
410 | started might not be in the page cache at the end of the | |
411 | walk). | |
412 | ||
7bb46a67 | 413 | truncate: Deprecated. This will not be called if ->setsize is defined. |
414 | Called by the VFS to change the size of a file. The | |
cc7d1f8f PE |
415 | i_size field of the inode is set to the desired size by the |
416 | VFS before this method is called. This method is called by | |
417 | the truncate(2) system call and related functionality. | |
5ea626aa | 418 | |
7bb46a67 | 419 | Note: ->truncate and vmtruncate are deprecated. Do not add new |
420 | instances/calls of these. Filesystems should be converted to do their | |
421 | truncate sequence via ->setattr(). | |
422 | ||
5ea626aa PE |
423 | permission: called by the VFS to check for access rights on a POSIX-like |
424 | filesystem. | |
425 | ||
a82416da NP |
426 | May be called in rcu-walk mode (flags & IPERM_FLAG_RCU). If in rcu-walk |
427 | mode, the filesystem must check the permission without blocking or | |
b74c79e9 NP |
428 | storing to the inode. |
429 | ||
430 | If a situation is encountered that rcu-walk cannot handle, return | |
431 | -ECHILD and it will be called again in ref-walk mode. | |
432 | ||
cc7d1f8f PE |
433 | setattr: called by the VFS to set attributes for a file. This method |
434 | is called by chmod(2) and related system calls. | |
5ea626aa | 435 | |
cc7d1f8f PE |
436 | getattr: called by the VFS to get attributes of a file. This method |
437 | is called by stat(2) and related system calls. | |
5ea626aa PE |
438 | |
439 | setxattr: called by the VFS to set an extended attribute for a file. | |
cc7d1f8f PE |
440 | Extended attribute is a name:value pair associated with an |
441 | inode. This method is called by setxattr(2) system call. | |
442 | ||
443 | getxattr: called by the VFS to retrieve the value of an extended | |
444 | attribute name. This method is called by getxattr(2) function | |
445 | call. | |
446 | ||
447 | listxattr: called by the VFS to list all extended attributes for a | |
448 | given file. This method is called by listxattr(2) system call. | |
5ea626aa | 449 | |
cc7d1f8f PE |
450 | removexattr: called by the VFS to remove an extended attribute from |
451 | a file. This method is called by removexattr(2) system call. | |
5ea626aa | 452 | |
422b14c2 BP |
453 | truncate_range: a method provided by the underlying filesystem to truncate a |
454 | range of blocks , i.e. punch a hole somewhere in a file. | |
455 | ||
5ea626aa | 456 | |
cc7d1f8f PE |
457 | The Address Space Object |
458 | ======================== | |
459 | ||
341546f5 N |
460 | The address space object is used to group and manage pages in the page |
461 | cache. It can be used to keep track of the pages in a file (or | |
462 | anything else) and also track the mapping of sections of the file into | |
463 | process address spaces. | |
464 | ||
465 | There are a number of distinct yet related services that an | |
466 | address-space can provide. These include communicating memory | |
467 | pressure, page lookup by address, and keeping track of pages tagged as | |
468 | Dirty or Writeback. | |
469 | ||
a9e102b6 | 470 | The first can be used independently to the others. The VM can try to |
341546f5 N |
471 | either write dirty pages in order to clean them, or release clean |
472 | pages in order to reuse them. To do this it can call the ->writepage | |
473 | method on dirty pages, and ->releasepage on clean pages with | |
474 | PagePrivate set. Clean pages without PagePrivate and with no external | |
475 | references will be released without notice being given to the | |
476 | address_space. | |
477 | ||
a9e102b6 | 478 | To achieve this functionality, pages need to be placed on an LRU with |
341546f5 N |
479 | lru_cache_add and mark_page_active needs to be called whenever the |
480 | page is used. | |
481 | ||
482 | Pages are normally kept in a radix tree index by ->index. This tree | |
483 | maintains information about the PG_Dirty and PG_Writeback status of | |
484 | each page, so that pages with either of these flags can be found | |
485 | quickly. | |
486 | ||
487 | The Dirty tag is primarily used by mpage_writepages - the default | |
488 | ->writepages method. It uses the tag to find dirty pages to call | |
489 | ->writepage on. If mpage_writepages is not used (i.e. the address | |
a9e102b6 | 490 | provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is |
341546f5 N |
491 | almost unused. write_inode_now and sync_inode do use it (through |
492 | __sync_single_inode) to check if ->writepages has been successful in | |
493 | writing out the whole address_space. | |
494 | ||
495 | The Writeback tag is used by filemap*wait* and sync_page* functions, | |
94004ed7 | 496 | via filemap_fdatawait_range, to wait for all writeback to |
341546f5 | 497 | complete. While waiting ->sync_page (if defined) will be called on |
a9e102b6 | 498 | each page that is found to require writeback. |
341546f5 N |
499 | |
500 | An address_space handler may attach extra information to a page, | |
501 | typically using the 'private' field in the 'struct page'. If such | |
502 | information is attached, the PG_Private flag should be set. This will | |
a9e102b6 | 503 | cause various VM routines to make extra calls into the address_space |
341546f5 N |
504 | handler to deal with that data. |
505 | ||
506 | An address space acts as an intermediate between storage and | |
507 | application. Data is read into the address space a whole page at a | |
508 | time, and provided to the application either by copying of the page, | |
509 | or by memory-mapping the page. | |
510 | Data is written into the address space by the application, and then | |
511 | written-back to storage typically in whole pages, however the | |
a9e102b6 | 512 | address_space has finer control of write sizes. |
341546f5 N |
513 | |
514 | The read process essentially only requires 'readpage'. The write | |
4e02ed4b | 515 | process is more complicated and uses write_begin/write_end or |
341546f5 N |
516 | set_page_dirty to write data into the address_space, and writepage, |
517 | sync_page, and writepages to writeback data to storage. | |
518 | ||
519 | Adding and removing pages to/from an address_space is protected by the | |
520 | inode's i_mutex. | |
521 | ||
522 | When data is written to a page, the PG_Dirty flag should be set. It | |
523 | typically remains set until writepage asks for it to be written. This | |
524 | should clear PG_Dirty and set PG_Writeback. It can be actually | |
525 | written at any point after PG_Dirty is clear. Once it is known to be | |
526 | safe, PG_Writeback is cleared. | |
527 | ||
528 | Writeback makes use of a writeback_control structure... | |
5ea626aa PE |
529 | |
530 | struct address_space_operations | |
cc7d1f8f | 531 | ------------------------------- |
5ea626aa PE |
532 | |
533 | This describes how the VFS can manipulate mapping of a file to page cache in | |
422b14c2 | 534 | your filesystem. As of kernel 2.6.22, the following members are defined: |
5ea626aa PE |
535 | |
536 | struct address_space_operations { | |
537 | int (*writepage)(struct page *page, struct writeback_control *wbc); | |
538 | int (*readpage)(struct file *, struct page *); | |
539 | int (*sync_page)(struct page *); | |
540 | int (*writepages)(struct address_space *, struct writeback_control *); | |
541 | int (*set_page_dirty)(struct page *page); | |
542 | int (*readpages)(struct file *filp, struct address_space *mapping, | |
543 | struct list_head *pages, unsigned nr_pages); | |
afddba49 NP |
544 | int (*write_begin)(struct file *, struct address_space *mapping, |
545 | loff_t pos, unsigned len, unsigned flags, | |
546 | struct page **pagep, void **fsdata); | |
547 | int (*write_end)(struct file *, struct address_space *mapping, | |
548 | loff_t pos, unsigned len, unsigned copied, | |
549 | struct page *page, void *fsdata); | |
5ea626aa PE |
550 | sector_t (*bmap)(struct address_space *, sector_t); |
551 | int (*invalidatepage) (struct page *, unsigned long); | |
552 | int (*releasepage) (struct page *, int); | |
6072d13c | 553 | void (*freepage)(struct page *); |
5ea626aa PE |
554 | ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, |
555 | loff_t offset, unsigned long nr_segs); | |
556 | struct page* (*get_xip_page)(struct address_space *, sector_t, | |
557 | int); | |
341546f5 N |
558 | /* migrate the contents of a page to the specified target */ |
559 | int (*migratepage) (struct page *, struct page *); | |
422b14c2 | 560 | int (*launder_page) (struct page *); |
25718736 | 561 | int (*error_remove_page) (struct mapping *mapping, struct page *page); |
5ea626aa PE |
562 | }; |
563 | ||
341546f5 | 564 | writepage: called by the VM to write a dirty page to backing store. |
a9e102b6 | 565 | This may happen for data integrity reasons (i.e. 'sync'), or |
341546f5 N |
566 | to free up memory (flush). The difference can be seen in |
567 | wbc->sync_mode. | |
568 | The PG_Dirty flag has been cleared and PageLocked is true. | |
569 | writepage should start writeout, should set PG_Writeback, | |
570 | and should make sure the page is unlocked, either synchronously | |
571 | or asynchronously when the write operation completes. | |
572 | ||
573 | If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to | |
a9e102b6 N |
574 | try too hard if there are problems, and may choose to write out |
575 | other pages from the mapping if that is easier (e.g. due to | |
576 | internal dependencies). If it chooses not to start writeout, it | |
577 | should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep | |
341546f5 N |
578 | calling ->writepage on that page. |
579 | ||
580 | See the file "Locking" for more details. | |
5ea626aa PE |
581 | |
582 | readpage: called by the VM to read a page from backing store. | |
341546f5 N |
583 | The page will be Locked when readpage is called, and should be |
584 | unlocked and marked uptodate once the read completes. | |
585 | If ->readpage discovers that it needs to unlock the page for | |
586 | some reason, it can do so, and then return AOP_TRUNCATED_PAGE. | |
a9e102b6 | 587 | In this case, the page will be relocated, relocked and if |
341546f5 | 588 | that all succeeds, ->readpage will be called again. |
5ea626aa PE |
589 | |
590 | sync_page: called by the VM to notify the backing store to perform all | |
591 | queued I/O operations for a page. I/O operations for other pages | |
592 | associated with this address_space object may also be performed. | |
593 | ||
341546f5 N |
594 | This function is optional and is called only for pages with |
595 | PG_Writeback set while waiting for the writeback to complete. | |
596 | ||
5ea626aa | 597 | writepages: called by the VM to write out pages associated with the |
a9e102b6 N |
598 | address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then |
599 | the writeback_control will specify a range of pages that must be | |
600 | written out. If it is WBC_SYNC_NONE, then a nr_to_write is given | |
341546f5 N |
601 | and that many pages should be written if possible. |
602 | If no ->writepages is given, then mpage_writepages is used | |
a9e102b6 | 603 | instead. This will choose pages from the address space that are |
341546f5 | 604 | tagged as DIRTY and will pass them to ->writepage. |
5ea626aa PE |
605 | |
606 | set_page_dirty: called by the VM to set a page dirty. | |
341546f5 N |
607 | This is particularly needed if an address space attaches |
608 | private data to a page, and that data needs to be updated when | |
609 | a page is dirtied. This is called, for example, when a memory | |
610 | mapped page gets modified. | |
611 | If defined, it should set the PageDirty flag, and the | |
612 | PAGECACHE_TAG_DIRTY tag in the radix tree. | |
5ea626aa PE |
613 | |
614 | readpages: called by the VM to read pages associated with the address_space | |
341546f5 N |
615 | object. This is essentially just a vector version of |
616 | readpage. Instead of just one page, several pages are | |
617 | requested. | |
a9e102b6 | 618 | readpages is only used for read-ahead, so read errors are |
341546f5 | 619 | ignored. If anything goes wrong, feel free to give up. |
1da177e4 | 620 | |
4e02ed4b | 621 | write_begin: |
afddba49 NP |
622 | Called by the generic buffered write code to ask the filesystem to |
623 | prepare to write len bytes at the given offset in the file. The | |
624 | address_space should check that the write will be able to complete, | |
625 | by allocating space if necessary and doing any other internal | |
626 | housekeeping. If the write will update parts of any basic-blocks on | |
627 | storage, then those blocks should be pre-read (if they haven't been | |
628 | read already) so that the updated blocks can be written out properly. | |
629 | ||
630 | The filesystem must return the locked pagecache page for the specified | |
631 | offset, in *pagep, for the caller to write into. | |
632 | ||
4e02ed4b NP |
633 | It must be able to cope with short writes (where the length passed to |
634 | write_begin is greater than the number of bytes copied into the page). | |
635 | ||
afddba49 NP |
636 | flags is a field for AOP_FLAG_xxx flags, described in |
637 | include/linux/fs.h. | |
638 | ||
639 | A void * may be returned in fsdata, which then gets passed into | |
640 | write_end. | |
641 | ||
642 | Returns 0 on success; < 0 on failure (which is the error code), in | |
643 | which case write_end is not called. | |
644 | ||
645 | write_end: After a successful write_begin, and data copy, write_end must | |
646 | be called. len is the original len passed to write_begin, and copied | |
647 | is the amount that was able to be copied (copied == len is always true | |
648 | if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag). | |
649 | ||
650 | The filesystem must take care of unlocking the page and releasing it | |
651 | refcount, and updating i_size. | |
652 | ||
653 | Returns < 0 on failure, otherwise the number of bytes (<= 'copied') | |
654 | that were able to be copied into pagecache. | |
655 | ||
5ea626aa | 656 | bmap: called by the VFS to map a logical block offset within object to |
a9e102b6 | 657 | physical block number. This method is used by the FIBMAP |
341546f5 | 658 | ioctl and for working with swap-files. To be able to swap to |
a9e102b6 | 659 | a file, the file must have a stable mapping to a block |
341546f5 N |
660 | device. The swap system does not go through the filesystem |
661 | but instead uses bmap to find out where the blocks in the file | |
662 | are and uses those addresses directly. | |
663 | ||
664 | ||
665 | invalidatepage: If a page has PagePrivate set, then invalidatepage | |
666 | will be called when part or all of the page is to be removed | |
a9e102b6 | 667 | from the address space. This generally corresponds to either a |
341546f5 N |
668 | truncation or a complete invalidation of the address space |
669 | (in the latter case 'offset' will always be 0). | |
670 | Any private data associated with the page should be updated | |
671 | to reflect this truncation. If offset is 0, then | |
672 | the private data should be released, because the page | |
673 | must be able to be completely discarded. This may be done by | |
674 | calling the ->releasepage function, but in this case the | |
675 | release MUST succeed. | |
676 | ||
677 | releasepage: releasepage is called on PagePrivate pages to indicate | |
678 | that the page should be freed if possible. ->releasepage | |
679 | should remove any private data from the page and clear the | |
4fe65cab AM |
680 | PagePrivate flag. If releasepage() fails for some reason, it must |
681 | indicate failure with a 0 return value. | |
682 | releasepage() is used in two distinct though related cases. The | |
683 | first is when the VM finds a clean page with no active users and | |
341546f5 N |
684 | wants to make it a free page. If ->releasepage succeeds, the |
685 | page will be removed from the address_space and become free. | |
686 | ||
bc5b1d55 | 687 | The second case is when a request has been made to invalidate |
341546f5 N |
688 | some or all pages in an address_space. This can happen |
689 | through the fadvice(POSIX_FADV_DONTNEED) system call or by the | |
690 | filesystem explicitly requesting it as nfs and 9fs do (when | |
691 | they believe the cache may be out of date with storage) by | |
692 | calling invalidate_inode_pages2(). | |
693 | If the filesystem makes such a call, and needs to be certain | |
a9e102b6 | 694 | that all pages are invalidated, then its releasepage will |
341546f5 N |
695 | need to ensure this. Possibly it can clear the PageUptodate |
696 | bit if it cannot free private data yet. | |
697 | ||
6072d13c LT |
698 | freepage: freepage is called once the page is no longer visible in |
699 | the page cache in order to allow the cleanup of any private | |
700 | data. Since it may be called by the memory reclaimer, it | |
701 | should not assume that the original address_space mapping still | |
702 | exists, and it should not block. | |
703 | ||
341546f5 N |
704 | direct_IO: called by the generic read/write routines to perform |
705 | direct_IO - that is IO requests which bypass the page cache | |
a9e102b6 | 706 | and transfer data directly between the storage and the |
341546f5 | 707 | application's address space. |
5ea626aa PE |
708 | |
709 | get_xip_page: called by the VM to translate a block number to a page. | |
710 | The page is valid until the corresponding filesystem is unmounted. | |
711 | Filesystems that want to use execute-in-place (XIP) need to implement | |
712 | it. An example implementation can be found in fs/ext2/xip.c. | |
713 | ||
341546f5 N |
714 | migrate_page: This is used to compact the physical memory usage. |
715 | If the VM wants to relocate a page (maybe off a memory card | |
716 | that is signalling imminent failure) it will pass a new page | |
717 | and an old page to this function. migrate_page should | |
718 | transfer any private data across and update any references | |
719 | that it has to the page. | |
5ea626aa | 720 | |
422b14c2 BP |
721 | launder_page: Called before freeing a page - it writes back the dirty page. To |
722 | prevent redirtying the page, it is kept locked during the whole | |
723 | operation. | |
724 | ||
25718736 AK |
725 | error_remove_page: normally set to generic_error_remove_page if truncation |
726 | is ok for this address space. Used for memory failure handling. | |
727 | Setting this implies you deal with pages going away under you, | |
728 | unless you have them locked or reference counts increased. | |
729 | ||
730 | ||
cc7d1f8f PE |
731 | The File Object |
732 | =============== | |
733 | ||
734 | A file object represents a file opened by a process. | |
735 | ||
736 | ||
5ea626aa | 737 | struct file_operations |
cc7d1f8f | 738 | ---------------------- |
1da177e4 LT |
739 | |
740 | This describes how the VFS can manipulate an open file. As of kernel | |
422b14c2 | 741 | 2.6.22, the following members are defined: |
1da177e4 LT |
742 | |
743 | struct file_operations { | |
422b14c2 | 744 | struct module *owner; |
1da177e4 | 745 | loff_t (*llseek) (struct file *, loff_t, int); |
5ea626aa | 746 | ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); |
5ea626aa | 747 | ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); |
027445c3 BP |
748 | ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
749 | ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); | |
1da177e4 LT |
750 | int (*readdir) (struct file *, void *, filldir_t); |
751 | unsigned int (*poll) (struct file *, struct poll_table_struct *); | |
5ea626aa PE |
752 | long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
753 | long (*compat_ioctl) (struct file *, unsigned int, unsigned long); | |
1da177e4 LT |
754 | int (*mmap) (struct file *, struct vm_area_struct *); |
755 | int (*open) (struct inode *, struct file *); | |
5ea626aa | 756 | int (*flush) (struct file *); |
1da177e4 | 757 | int (*release) (struct inode *, struct file *); |
7ea80859 | 758 | int (*fsync) (struct file *, int datasync); |
5ea626aa PE |
759 | int (*aio_fsync) (struct kiocb *, int datasync); |
760 | int (*fasync) (int, struct file *, int); | |
1da177e4 | 761 | int (*lock) (struct file *, int, struct file_lock *); |
5ea626aa PE |
762 | ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *); |
763 | ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *); | |
764 | ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *); | |
765 | ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); | |
766 | unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); | |
767 | int (*check_flags)(int); | |
5ea626aa | 768 | int (*flock) (struct file *, int, struct file_lock *); |
422b14c2 BP |
769 | ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int); |
770 | ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int); | |
1da177e4 LT |
771 | }; |
772 | ||
773 | Again, all methods are called without any locks being held, unless | |
774 | otherwise noted. | |
775 | ||
776 | llseek: called when the VFS needs to move the file position index | |
777 | ||
778 | read: called by read(2) and related system calls | |
779 | ||
5ea626aa PE |
780 | aio_read: called by io_submit(2) and other asynchronous I/O operations |
781 | ||
1da177e4 LT |
782 | write: called by write(2) and related system calls |
783 | ||
5ea626aa PE |
784 | aio_write: called by io_submit(2) and other asynchronous I/O operations |
785 | ||
1da177e4 LT |
786 | readdir: called when the VFS needs to read the directory contents |
787 | ||
788 | poll: called by the VFS when a process wants to check if there is | |
789 | activity on this file and (optionally) go to sleep until there | |
790 | is activity. Called by the select(2) and poll(2) system calls | |
791 | ||
b19dd42f | 792 | unlocked_ioctl: called by the ioctl(2) system call. |
5ea626aa PE |
793 | |
794 | compat_ioctl: called by the ioctl(2) system call when 32 bit system calls | |
795 | are used on 64 bit kernels. | |
796 | ||
1da177e4 LT |
797 | mmap: called by the mmap(2) system call |
798 | ||
799 | open: called by the VFS when an inode should be opened. When the VFS | |
5ea626aa PE |
800 | opens a file, it creates a new "struct file". It then calls the |
801 | open method for the newly allocated file structure. You might | |
802 | think that the open method really belongs in | |
803 | "struct inode_operations", and you may be right. I think it's | |
804 | done the way it is because it makes filesystems simpler to | |
805 | implement. The open() method is a good place to initialize the | |
806 | "private_data" member in the file structure if you want to point | |
807 | to a device structure | |
808 | ||
809 | flush: called by the close(2) system call to flush a file | |
1da177e4 LT |
810 | |
811 | release: called when the last reference to an open file is closed | |
812 | ||
813 | fsync: called by the fsync(2) system call | |
814 | ||
815 | fasync: called by the fcntl(2) system call when asynchronous | |
816 | (non-blocking) mode is enabled for a file | |
817 | ||
5ea626aa PE |
818 | lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW |
819 | commands | |
820 | ||
821 | readv: called by the readv(2) system call | |
822 | ||
823 | writev: called by the writev(2) system call | |
824 | ||
825 | sendfile: called by the sendfile(2) system call | |
826 | ||
827 | get_unmapped_area: called by the mmap(2) system call | |
828 | ||
829 | check_flags: called by the fcntl(2) system call for F_SETFL command | |
830 | ||
5ea626aa PE |
831 | flock: called by the flock(2) system call |
832 | ||
d1195c51 PE |
833 | splice_write: called by the VFS to splice data from a pipe to a file. This |
834 | method is used by the splice(2) system call | |
835 | ||
836 | splice_read: called by the VFS to splice data from file to a pipe. This | |
837 | method is used by the splice(2) system call | |
838 | ||
1da177e4 LT |
839 | Note that the file operations are implemented by the specific |
840 | filesystem in which the inode resides. When opening a device node | |
841 | (character or block special) most filesystems will call special | |
842 | support routines in the VFS which will locate the required device | |
843 | driver information. These support routines replace the filesystem file | |
844 | operations with those for the device driver, and then proceed to call | |
845 | the new open() method for the file. This is how opening a device file | |
846 | in the filesystem eventually ends up calling the device driver open() | |
5ea626aa | 847 | method. |
1da177e4 LT |
848 | |
849 | ||
5ea626aa PE |
850 | Directory Entry Cache (dcache) |
851 | ============================== | |
852 | ||
1da177e4 LT |
853 | |
854 | struct dentry_operations | |
5ea626aa | 855 | ------------------------ |
1da177e4 LT |
856 | |
857 | This describes how a filesystem can overload the standard dentry | |
858 | operations. Dentries and the dcache are the domain of the VFS and the | |
859 | individual filesystem implementations. Device drivers have no business | |
860 | here. These methods may be set to NULL, as they are either optional or | |
c23fbb6b | 861 | the VFS uses a default. As of kernel 2.6.22, the following members are |
1da177e4 LT |
862 | defined: |
863 | ||
864 | struct dentry_operations { | |
5ea626aa | 865 | int (*d_revalidate)(struct dentry *, struct nameidata *); |
b1e6a015 NP |
866 | int (*d_hash)(const struct dentry *, const struct inode *, |
867 | struct qstr *); | |
621e155a NP |
868 | int (*d_compare)(const struct dentry *, const struct inode *, |
869 | const struct dentry *, const struct inode *, | |
870 | unsigned int, const char *, const struct qstr *); | |
fe15ce44 | 871 | int (*d_delete)(const struct dentry *); |
1da177e4 LT |
872 | void (*d_release)(struct dentry *); |
873 | void (*d_iput)(struct dentry *, struct inode *); | |
c23fbb6b | 874 | char *(*d_dname)(struct dentry *, char *, int); |
9875cf80 | 875 | struct vfsmount *(*d_automount)(struct path *); |
1aed3e42 | 876 | int (*d_manage)(struct dentry *, bool); |
1da177e4 LT |
877 | }; |
878 | ||
879 | d_revalidate: called when the VFS needs to revalidate a dentry. This | |
880 | is called whenever a name look-up finds a dentry in the | |
881 | dcache. Most filesystems leave this as NULL, because all their | |
882 | dentries in the dcache are valid | |
883 | ||
34286d66 NP |
884 | d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU). |
885 | If in rcu-walk mode, the filesystem must revalidate the dentry without | |
886 | blocking or storing to the dentry, d_parent and d_inode should not be | |
887 | used without care (because they can go NULL), instead nd->inode should | |
888 | be used. | |
889 | ||
890 | If a situation is encountered that rcu-walk cannot handle, return | |
891 | -ECHILD and it will be called again in ref-walk mode. | |
892 | ||
621e155a NP |
893 | d_hash: called when the VFS adds a dentry to the hash table. The first |
894 | dentry passed to d_hash is the parent directory that the name is | |
b1e6a015 NP |
895 | to be hashed into. The inode is the dentry's inode. |
896 | ||
897 | Same locking and synchronisation rules as d_compare regarding | |
898 | what is safe to dereference etc. | |
1da177e4 | 899 | |
621e155a NP |
900 | d_compare: called to compare a dentry name with a given name. The first |
901 | dentry is the parent of the dentry to be compared, the second is | |
902 | the parent's inode, then the dentry and inode (may be NULL) of the | |
903 | child dentry. len and name string are properties of the dentry to be | |
904 | compared. qstr is the name to compare it with. | |
905 | ||
906 | Must be constant and idempotent, and should not take locks if | |
907 | possible, and should not or store into the dentry or inodes. | |
908 | Should not dereference pointers outside the dentry or inodes without | |
909 | lots of care (eg. d_parent, d_inode, d_name should not be used). | |
910 | ||
911 | However, our vfsmount is pinned, and RCU held, so the dentries and | |
912 | inodes won't disappear, neither will our sb or filesystem module. | |
913 | ->i_sb and ->d_sb may be used. | |
914 | ||
915 | It is a tricky calling convention because it needs to be called under | |
916 | "rcu-walk", ie. without any locks or references on things. | |
1da177e4 | 917 | |
fe15ce44 NP |
918 | d_delete: called when the last reference to a dentry is dropped and the |
919 | dcache is deciding whether or not to cache it. Return 1 to delete | |
920 | immediately, or 0 to cache the dentry. Default is NULL which means to | |
921 | always cache a reachable dentry. d_delete must be constant and | |
922 | idempotent. | |
1da177e4 LT |
923 | |
924 | d_release: called when a dentry is really deallocated | |
925 | ||
926 | d_iput: called when a dentry loses its inode (just prior to its | |
927 | being deallocated). The default when this is NULL is that the | |
928 | VFS calls iput(). If you define this method, you must call | |
929 | iput() yourself | |
930 | ||
c23fbb6b | 931 | d_dname: called when the pathname of a dentry should be generated. |
d9195881 | 932 | Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay |
c23fbb6b | 933 | pathname generation. (Instead of doing it when dentry is created, |
d9195881 | 934 | it's done only when the path is needed.). Real filesystems probably |
c23fbb6b ED |
935 | dont want to use it, because their dentries are present in global |
936 | dcache hash, so their hash should be an invariant. As no lock is | |
937 | held, d_dname() should not try to modify the dentry itself, unless | |
938 | appropriate SMP safety is used. CAUTION : d_path() logic is quite | |
939 | tricky. The correct way to return for example "Hello" is to put it | |
940 | at the end of the buffer, and returns a pointer to the first char. | |
941 | dynamic_dname() helper function is provided to take care of this. | |
942 | ||
9875cf80 | 943 | d_automount: called when an automount dentry is to be traversed (optional). |
ea5b778a DH |
944 | This should create a new VFS mount record and return the record to the |
945 | caller. The caller is supplied with a path parameter giving the | |
946 | automount directory to describe the automount target and the parent | |
947 | VFS mount record to provide inheritable mount parameters. NULL should | |
948 | be returned if someone else managed to make the automount first. If | |
949 | the vfsmount creation failed, then an error code should be returned. | |
950 | If -EISDIR is returned, then the directory will be treated as an | |
951 | ordinary directory and returned to pathwalk to continue walking. | |
952 | ||
953 | If a vfsmount is returned, the caller will attempt to mount it on the | |
954 | mountpoint and will remove the vfsmount from its expiration list in | |
955 | the case of failure. The vfsmount should be returned with 2 refs on | |
956 | it to prevent automatic expiration - the caller will clean up the | |
957 | additional ref. | |
9875cf80 DH |
958 | |
959 | This function is only used if DCACHE_NEED_AUTOMOUNT is set on the | |
960 | dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the | |
961 | inode being added. | |
962 | ||
cc53ce53 DH |
963 | d_manage: called to allow the filesystem to manage the transition from a |
964 | dentry (optional). This allows autofs, for example, to hold up clients | |
965 | waiting to explore behind a 'mountpoint' whilst letting the daemon go | |
966 | past and construct the subtree there. 0 should be returned to let the | |
967 | calling process continue. -EISDIR can be returned to tell pathwalk to | |
968 | use this directory as an ordinary directory and to ignore anything | |
969 | mounted on it and not to check the automount flag. Any other error | |
970 | code will abort pathwalk completely. | |
971 | ||
ab90911f DH |
972 | If the 'rcu_walk' parameter is true, then the caller is doing a |
973 | pathwalk in RCU-walk mode. Sleeping is not permitted in this mode, | |
974 | and the caller can be asked to leave it and call again by returing | |
975 | -ECHILD. | |
976 | ||
cc53ce53 DH |
977 | This function is only used if DCACHE_MANAGE_TRANSIT is set on the |
978 | dentry being transited from. | |
979 | ||
c23fbb6b ED |
980 | Example : |
981 | ||
982 | static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) | |
983 | { | |
984 | return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", | |
985 | dentry->d_inode->i_ino); | |
986 | } | |
987 | ||
1da177e4 LT |
988 | Each dentry has a pointer to its parent dentry, as well as a hash list |
989 | of child dentries. Child dentries are basically like files in a | |
990 | directory. | |
991 | ||
5ea626aa | 992 | |
cc7d1f8f | 993 | Directory Entry Cache API |
1da177e4 LT |
994 | -------------------------- |
995 | ||
996 | There are a number of functions defined which permit a filesystem to | |
997 | manipulate dentries: | |
998 | ||
999 | dget: open a new handle for an existing dentry (this just increments | |
1000 | the usage count) | |
1001 | ||
1002 | dput: close a handle for a dentry (decrements the usage count). If | |
fe15ce44 NP |
1003 | the usage count drops to 0, and the dentry is still in its |
1004 | parent's hash, the "d_delete" method is called to check whether | |
1005 | it should be cached. If it should not be cached, or if the dentry | |
1006 | is not hashed, it is deleted. Otherwise cached dentries are put | |
1007 | into an LRU list to be reclaimed on memory shortage. | |
1da177e4 LT |
1008 | |
1009 | d_drop: this unhashes a dentry from its parents hash list. A | |
5ea626aa | 1010 | subsequent call to dput() will deallocate the dentry if its |
1da177e4 LT |
1011 | usage count drops to 0 |
1012 | ||
1013 | d_delete: delete a dentry. If there are no other open references to | |
1014 | the dentry then the dentry is turned into a negative dentry | |
1015 | (the d_iput() method is called). If there are other | |
1016 | references, then d_drop() is called instead | |
1017 | ||
1018 | d_add: add a dentry to its parents hash list and then calls | |
1019 | d_instantiate() | |
1020 | ||
1021 | d_instantiate: add a dentry to the alias hash list for the inode and | |
1022 | updates the "d_inode" member. The "i_count" member in the | |
1023 | inode structure should be set/incremented. If the inode | |
1024 | pointer is NULL, the dentry is called a "negative | |
1025 | dentry". This function is commonly called when an inode is | |
1026 | created for an existing negative dentry | |
1027 | ||
1028 | d_lookup: look up a dentry given its parent and path name component | |
1029 | It looks up the child of that given name from the dcache | |
1030 | hash table. If it is found, the reference count is incremented | |
be42c4c4 | 1031 | and the dentry is returned. The caller must use dput() |
1da177e4 LT |
1032 | to free the dentry when it finishes using it. |
1033 | ||
cbf8f0f3 PE |
1034 | For further information on dentry locking, please refer to the document |
1035 | Documentation/filesystems/dentry-locking.txt. | |
cc7d1f8f | 1036 | |
f84e3f52 MS |
1037 | Mount Options |
1038 | ============= | |
1039 | ||
1040 | Parsing options | |
1041 | --------------- | |
1042 | ||
1043 | On mount and remount the filesystem is passed a string containing a | |
1044 | comma separated list of mount options. The options can have either of | |
1045 | these forms: | |
1046 | ||
1047 | option | |
1048 | option=value | |
1049 | ||
1050 | The <linux/parser.h> header defines an API that helps parse these | |
1051 | options. There are plenty of examples on how to use it in existing | |
1052 | filesystems. | |
1053 | ||
1054 | Showing options | |
1055 | --------------- | |
1056 | ||
1057 | If a filesystem accepts mount options, it must define show_options() | |
1058 | to show all the currently active options. The rules are: | |
1059 | ||
1060 | - options MUST be shown which are not default or their values differ | |
1061 | from the default | |
1062 | ||
1063 | - options MAY be shown which are enabled by default or have their | |
1064 | default value | |
1065 | ||
1066 | Options used only internally between a mount helper and the kernel | |
1067 | (such as file descriptors), or which only have an effect during the | |
1068 | mounting (such as ones controlling the creation of a journal) are exempt | |
1069 | from the above rules. | |
1070 | ||
1071 | The underlying reason for the above rules is to make sure, that a | |
1072 | mount can be accurately replicated (e.g. umounting and mounting again) | |
1073 | based on the information found in /proc/mounts. | |
1074 | ||
1075 | A simple method of saving options at mount/remount time and showing | |
1076 | them is provided with the save_mount_options() and | |
1077 | generic_show_options() helper functions. Please note, that using | |
1078 | these may have drawbacks. For more info see header comments for these | |
1079 | functions in fs/namespace.c. | |
cc7d1f8f PE |
1080 | |
1081 | Resources | |
1082 | ========= | |
1083 | ||
1084 | (Note some of these resources are not up-to-date with the latest kernel | |
1085 | version.) | |
1086 | ||
1087 | Creating Linux virtual filesystems. 2002 | |
1088 | <http://lwn.net/Articles/13325/> | |
1089 | ||
1090 | The Linux Virtual File-system Layer by Neil Brown. 1999 | |
1091 | <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> | |
1092 | ||
1093 | A tour of the Linux VFS by Michael K. Johnson. 1996 | |
1094 | <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> | |
1095 | ||
1096 | A small trail through the Linux kernel by Andries Brouwer. 2001 | |
1097 | <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> |