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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 PE |
97 | The passed struct file_system_type describes your filesystem. When a |
98 | request is made to mount a device onto a directory in your filespace, | |
99 | the VFS will call the appropriate get_sb() method for the specific | |
100 | filesystem. The dentry for the mount point will then be updated to | |
101 | point to the root inode for the new filesystem. | |
1da177e4 | 102 | |
cc7d1f8f PE |
103 | You can see all filesystems that are registered to the kernel in the |
104 | file /proc/filesystems. | |
1da177e4 LT |
105 | |
106 | ||
5ea626aa | 107 | struct file_system_type |
cc7d1f8f | 108 | ----------------------- |
1da177e4 | 109 | |
0746aec3 | 110 | This describes the filesystem. As of kernel 2.6.22, the following |
1da177e4 LT |
111 | members are defined: |
112 | ||
113 | struct file_system_type { | |
114 | const char *name; | |
115 | int fs_flags; | |
5d8b2ebf JC |
116 | int (*get_sb) (struct file_system_type *, int, |
117 | const char *, void *, struct vfsmount *); | |
5ea626aa PE |
118 | void (*kill_sb) (struct super_block *); |
119 | struct module *owner; | |
120 | struct file_system_type * next; | |
121 | struct list_head fs_supers; | |
0746aec3 BP |
122 | struct lock_class_key s_lock_key; |
123 | struct lock_class_key s_umount_key; | |
1da177e4 LT |
124 | }; |
125 | ||
126 | name: the name of the filesystem type, such as "ext2", "iso9660", | |
127 | "msdos" and so on | |
128 | ||
129 | fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) | |
130 | ||
5ea626aa | 131 | get_sb: the method to call when a new instance of this |
1da177e4 LT |
132 | filesystem should be mounted |
133 | ||
5ea626aa PE |
134 | kill_sb: the method to call when an instance of this filesystem |
135 | should be unmounted | |
136 | ||
137 | owner: for internal VFS use: you should initialize this to THIS_MODULE in | |
138 | most cases. | |
1da177e4 | 139 | |
5ea626aa PE |
140 | next: for internal VFS use: you should initialize this to NULL |
141 | ||
0746aec3 BP |
142 | s_lock_key, s_umount_key: lockdep-specific |
143 | ||
5ea626aa | 144 | The get_sb() method has the following arguments: |
1da177e4 | 145 | |
d9195881 | 146 | struct file_system_type *fs_type: describes the filesystem, partly initialized |
0746aec3 | 147 | by the specific filesystem code |
5ea626aa PE |
148 | |
149 | int flags: mount flags | |
150 | ||
151 | const char *dev_name: the device name we are mounting. | |
1da177e4 LT |
152 | |
153 | void *data: arbitrary mount options, usually comes as an ASCII | |
f84e3f52 | 154 | string (see "Mount Options" section) |
1da177e4 | 155 | |
0746aec3 | 156 | struct vfsmount *mnt: a vfs-internal representation of a mount point |
1da177e4 | 157 | |
5ea626aa | 158 | The get_sb() method must determine if the block device specified |
0746aec3 BP |
159 | in the dev_name and fs_type contains a filesystem of the type the method |
160 | supports. If it succeeds in opening the named block device, it initializes a | |
161 | struct super_block descriptor for the filesystem contained by the block device. | |
162 | On failure it returns an error. | |
1da177e4 LT |
163 | |
164 | The most interesting member of the superblock structure that the | |
5ea626aa | 165 | get_sb() method fills in is the "s_op" field. This is a pointer to |
1da177e4 LT |
166 | a "struct super_operations" which describes the next level of the |
167 | filesystem implementation. | |
168 | ||
e3e1bfe4 JC |
169 | Usually, a filesystem uses one of the generic get_sb() implementations |
170 | and provides a fill_super() method instead. The generic methods are: | |
5ea626aa PE |
171 | |
172 | get_sb_bdev: mount a filesystem residing on a block device | |
1da177e4 | 173 | |
5ea626aa PE |
174 | get_sb_nodev: mount a filesystem that is not backed by a device |
175 | ||
176 | get_sb_single: mount a filesystem which shares the instance between | |
177 | all mounts | |
178 | ||
179 | A fill_super() method implementation has the following arguments: | |
180 | ||
181 | struct super_block *sb: the superblock structure. The method fill_super() | |
182 | must initialize this properly. | |
183 | ||
184 | void *data: arbitrary mount options, usually comes as an ASCII | |
f84e3f52 | 185 | string (see "Mount Options" section) |
5ea626aa PE |
186 | |
187 | int silent: whether or not to be silent on error | |
188 | ||
189 | ||
cc7d1f8f PE |
190 | The Superblock Object |
191 | ===================== | |
192 | ||
193 | A superblock object represents a mounted filesystem. | |
194 | ||
195 | ||
5ea626aa | 196 | struct super_operations |
cc7d1f8f | 197 | ----------------------- |
1da177e4 LT |
198 | |
199 | This describes how the VFS can manipulate the superblock of your | |
422b14c2 | 200 | filesystem. As of kernel 2.6.22, the following members are defined: |
1da177e4 LT |
201 | |
202 | struct super_operations { | |
5ea626aa PE |
203 | struct inode *(*alloc_inode)(struct super_block *sb); |
204 | void (*destroy_inode)(struct inode *); | |
205 | ||
5ea626aa PE |
206 | void (*dirty_inode) (struct inode *); |
207 | int (*write_inode) (struct inode *, int); | |
5ea626aa PE |
208 | void (*drop_inode) (struct inode *); |
209 | void (*delete_inode) (struct inode *); | |
210 | void (*put_super) (struct super_block *); | |
211 | void (*write_super) (struct super_block *); | |
212 | int (*sync_fs)(struct super_block *sb, int wait); | |
c4be0c1d TS |
213 | int (*freeze_fs) (struct super_block *); |
214 | int (*unfreeze_fs) (struct super_block *); | |
726c3342 | 215 | int (*statfs) (struct dentry *, struct kstatfs *); |
5ea626aa PE |
216 | int (*remount_fs) (struct super_block *, int *, char *); |
217 | void (*clear_inode) (struct inode *); | |
218 | void (*umount_begin) (struct super_block *); | |
219 | ||
5ea626aa PE |
220 | int (*show_options)(struct seq_file *, struct vfsmount *); |
221 | ||
222 | ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); | |
223 | ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); | |
1da177e4 LT |
224 | }; |
225 | ||
226 | All methods are called without any locks being held, unless otherwise | |
227 | noted. This means that most methods can block safely. All methods are | |
228 | only called from a process context (i.e. not from an interrupt handler | |
229 | or bottom half). | |
230 | ||
5ea626aa | 231 | alloc_inode: this method is called by inode_alloc() to allocate memory |
341546f5 N |
232 | for struct inode and initialize it. If this function is not |
233 | defined, a simple 'struct inode' is allocated. Normally | |
234 | alloc_inode will be used to allocate a larger structure which | |
235 | contains a 'struct inode' embedded within it. | |
5ea626aa PE |
236 | |
237 | destroy_inode: this method is called by destroy_inode() to release | |
341546f5 N |
238 | resources allocated for struct inode. It is only required if |
239 | ->alloc_inode was defined and simply undoes anything done by | |
240 | ->alloc_inode. | |
5ea626aa | 241 | |
5ea626aa | 242 | dirty_inode: this method is called by the VFS to mark an inode dirty. |
1da177e4 LT |
243 | |
244 | write_inode: this method is called when the VFS needs to write an | |
245 | inode to disc. The second parameter indicates whether the write | |
246 | should be synchronous or not, not all filesystems check this flag. | |
247 | ||
1da177e4 LT |
248 | drop_inode: called when the last access to the inode is dropped, |
249 | with the inode_lock spinlock held. | |
250 | ||
5ea626aa | 251 | This method should be either NULL (normal UNIX filesystem |
1da177e4 LT |
252 | semantics) or "generic_delete_inode" (for filesystems that do not |
253 | want to cache inodes - causing "delete_inode" to always be | |
254 | called regardless of the value of i_nlink) | |
255 | ||
5ea626aa | 256 | The "generic_delete_inode()" behavior is equivalent to the |
1da177e4 LT |
257 | old practice of using "force_delete" in the put_inode() case, |
258 | but does not have the races that the "force_delete()" approach | |
259 | had. | |
260 | ||
261 | delete_inode: called when the VFS wants to delete an inode | |
262 | ||
1da177e4 LT |
263 | put_super: called when the VFS wishes to free the superblock |
264 | (i.e. unmount). This is called with the superblock lock held | |
265 | ||
266 | write_super: called when the VFS superblock needs to be written to | |
267 | disc. This method is optional | |
268 | ||
5ea626aa PE |
269 | sync_fs: called when VFS is writing out all dirty data associated with |
270 | a superblock. The second parameter indicates whether the method | |
271 | should wait until the write out has been completed. Optional. | |
272 | ||
c4be0c1d | 273 | freeze_fs: called when VFS is locking a filesystem and |
cc7d1f8f PE |
274 | forcing it into a consistent state. This method is currently |
275 | used by the Logical Volume Manager (LVM). | |
5ea626aa | 276 | |
c4be0c1d | 277 | unfreeze_fs: called when VFS is unlocking a filesystem and making it writable |
5ea626aa PE |
278 | again. |
279 | ||
66672fef | 280 | statfs: called when the VFS needs to get filesystem statistics. |
1da177e4 LT |
281 | |
282 | remount_fs: called when the filesystem is remounted. This is called | |
283 | with the kernel lock held | |
284 | ||
285 | clear_inode: called then the VFS clears the inode. Optional | |
286 | ||
5ea626aa PE |
287 | umount_begin: called when the VFS is unmounting a filesystem. |
288 | ||
f84e3f52 MS |
289 | show_options: called by the VFS to show mount options for |
290 | /proc/<pid>/mounts. (see "Mount Options" section) | |
5ea626aa PE |
291 | |
292 | quota_read: called by the VFS to read from filesystem quota file. | |
293 | ||
294 | quota_write: called by the VFS to write to filesystem quota file. | |
295 | ||
12debc42 DH |
296 | Whoever sets up the inode is responsible for filling in the "i_op" field. This |
297 | is a pointer to a "struct inode_operations" which describes the methods that | |
298 | can be performed on individual inodes. | |
1da177e4 LT |
299 | |
300 | ||
cc7d1f8f PE |
301 | The Inode Object |
302 | ================ | |
303 | ||
304 | An inode object represents an object within the filesystem. | |
305 | ||
306 | ||
5ea626aa | 307 | struct inode_operations |
cc7d1f8f | 308 | ----------------------- |
1da177e4 LT |
309 | |
310 | This describes how the VFS can manipulate an inode in your | |
422b14c2 | 311 | filesystem. As of kernel 2.6.22, the following members are defined: |
1da177e4 LT |
312 | |
313 | struct inode_operations { | |
5ea626aa PE |
314 | int (*create) (struct inode *,struct dentry *,int, struct nameidata *); |
315 | struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *); | |
1da177e4 LT |
316 | int (*link) (struct dentry *,struct inode *,struct dentry *); |
317 | int (*unlink) (struct inode *,struct dentry *); | |
318 | int (*symlink) (struct inode *,struct dentry *,const char *); | |
319 | int (*mkdir) (struct inode *,struct dentry *,int); | |
320 | int (*rmdir) (struct inode *,struct dentry *); | |
321 | int (*mknod) (struct inode *,struct dentry *,int,dev_t); | |
322 | int (*rename) (struct inode *, struct dentry *, | |
323 | struct inode *, struct dentry *); | |
5ea626aa PE |
324 | int (*readlink) (struct dentry *, char __user *,int); |
325 | void * (*follow_link) (struct dentry *, struct nameidata *); | |
326 | void (*put_link) (struct dentry *, struct nameidata *, void *); | |
1da177e4 | 327 | void (*truncate) (struct inode *); |
b74c79e9 NP |
328 | int (*permission) (struct inode *, int, unsigned int); |
329 | int (*check_acl)(struct inode *, int, unsigned int); | |
5ea626aa PE |
330 | int (*setattr) (struct dentry *, struct iattr *); |
331 | int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); | |
332 | int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); | |
333 | ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); | |
334 | ssize_t (*listxattr) (struct dentry *, char *, size_t); | |
335 | int (*removexattr) (struct dentry *, const char *); | |
422b14c2 | 336 | void (*truncate_range)(struct inode *, loff_t, loff_t); |
1da177e4 LT |
337 | }; |
338 | ||
339 | Again, all methods are called without any locks being held, unless | |
340 | otherwise noted. | |
341 | ||
1da177e4 LT |
342 | create: called by the open(2) and creat(2) system calls. Only |
343 | required if you want to support regular files. The dentry you | |
344 | get should not have an inode (i.e. it should be a negative | |
345 | dentry). Here you will probably call d_instantiate() with the | |
346 | dentry and the newly created inode | |
347 | ||
348 | lookup: called when the VFS needs to look up an inode in a parent | |
349 | directory. The name to look for is found in the dentry. This | |
350 | method must call d_add() to insert the found inode into the | |
351 | dentry. The "i_count" field in the inode structure should be | |
352 | incremented. If the named inode does not exist a NULL inode | |
353 | should be inserted into the dentry (this is called a negative | |
354 | dentry). Returning an error code from this routine must only | |
355 | be done on a real error, otherwise creating inodes with system | |
356 | calls like create(2), mknod(2), mkdir(2) and so on will fail. | |
357 | If you wish to overload the dentry methods then you should | |
358 | initialise the "d_dop" field in the dentry; this is a pointer | |
359 | to a struct "dentry_operations". | |
360 | This method is called with the directory inode semaphore held | |
361 | ||
362 | link: called by the link(2) system call. Only required if you want | |
363 | to support hard links. You will probably need to call | |
364 | d_instantiate() just as you would in the create() method | |
365 | ||
366 | unlink: called by the unlink(2) system call. Only required if you | |
367 | want to support deleting inodes | |
368 | ||
369 | symlink: called by the symlink(2) system call. Only required if you | |
370 | want to support symlinks. You will probably need to call | |
371 | d_instantiate() just as you would in the create() method | |
372 | ||
373 | mkdir: called by the mkdir(2) system call. Only required if you want | |
374 | to support creating subdirectories. You will probably need to | |
375 | call d_instantiate() just as you would in the create() method | |
376 | ||
377 | rmdir: called by the rmdir(2) system call. Only required if you want | |
378 | to support deleting subdirectories | |
379 | ||
380 | mknod: called by the mknod(2) system call to create a device (char, | |
381 | block) inode or a named pipe (FIFO) or socket. Only required | |
382 | if you want to support creating these types of inodes. You | |
383 | will probably need to call d_instantiate() just as you would | |
384 | in the create() method | |
385 | ||
cc7d1f8f PE |
386 | rename: called by the rename(2) system call to rename the object to |
387 | have the parent and name given by the second inode and dentry. | |
388 | ||
1da177e4 LT |
389 | readlink: called by the readlink(2) system call. Only required if |
390 | you want to support reading symbolic links | |
391 | ||
392 | follow_link: called by the VFS to follow a symbolic link to the | |
5ea626aa | 393 | inode it points to. Only required if you want to support |
cc7d1f8f | 394 | symbolic links. This method returns a void pointer cookie |
5ea626aa PE |
395 | that is passed to put_link(). |
396 | ||
397 | put_link: called by the VFS to release resources allocated by | |
cc7d1f8f | 398 | follow_link(). The cookie returned by follow_link() is passed |
670e9f34 | 399 | to this method as the last parameter. It is used by |
cc7d1f8f PE |
400 | filesystems such as NFS where page cache is not stable |
401 | (i.e. page that was installed when the symbolic link walk | |
402 | started might not be in the page cache at the end of the | |
403 | walk). | |
404 | ||
7bb46a67 | 405 | truncate: Deprecated. This will not be called if ->setsize is defined. |
406 | Called by the VFS to change the size of a file. The | |
cc7d1f8f PE |
407 | i_size field of the inode is set to the desired size by the |
408 | VFS before this method is called. This method is called by | |
409 | the truncate(2) system call and related functionality. | |
5ea626aa | 410 | |
7bb46a67 | 411 | Note: ->truncate and vmtruncate are deprecated. Do not add new |
412 | instances/calls of these. Filesystems should be converted to do their | |
413 | truncate sequence via ->setattr(). | |
414 | ||
5ea626aa PE |
415 | permission: called by the VFS to check for access rights on a POSIX-like |
416 | filesystem. | |
417 | ||
a82416da NP |
418 | May be called in rcu-walk mode (flags & IPERM_FLAG_RCU). If in rcu-walk |
419 | mode, the filesystem must check the permission without blocking or | |
b74c79e9 NP |
420 | storing to the inode. |
421 | ||
422 | If a situation is encountered that rcu-walk cannot handle, return | |
423 | -ECHILD and it will be called again in ref-walk mode. | |
424 | ||
cc7d1f8f PE |
425 | setattr: called by the VFS to set attributes for a file. This method |
426 | is called by chmod(2) and related system calls. | |
5ea626aa | 427 | |
cc7d1f8f PE |
428 | getattr: called by the VFS to get attributes of a file. This method |
429 | is called by stat(2) and related system calls. | |
5ea626aa PE |
430 | |
431 | setxattr: called by the VFS to set an extended attribute for a file. | |
cc7d1f8f PE |
432 | Extended attribute is a name:value pair associated with an |
433 | inode. This method is called by setxattr(2) system call. | |
434 | ||
435 | getxattr: called by the VFS to retrieve the value of an extended | |
436 | attribute name. This method is called by getxattr(2) function | |
437 | call. | |
438 | ||
439 | listxattr: called by the VFS to list all extended attributes for a | |
440 | given file. This method is called by listxattr(2) system call. | |
5ea626aa | 441 | |
cc7d1f8f PE |
442 | removexattr: called by the VFS to remove an extended attribute from |
443 | a file. This method is called by removexattr(2) system call. | |
5ea626aa | 444 | |
422b14c2 BP |
445 | truncate_range: a method provided by the underlying filesystem to truncate a |
446 | range of blocks , i.e. punch a hole somewhere in a file. | |
447 | ||
5ea626aa | 448 | |
cc7d1f8f PE |
449 | The Address Space Object |
450 | ======================== | |
451 | ||
341546f5 N |
452 | The address space object is used to group and manage pages in the page |
453 | cache. It can be used to keep track of the pages in a file (or | |
454 | anything else) and also track the mapping of sections of the file into | |
455 | process address spaces. | |
456 | ||
457 | There are a number of distinct yet related services that an | |
458 | address-space can provide. These include communicating memory | |
459 | pressure, page lookup by address, and keeping track of pages tagged as | |
460 | Dirty or Writeback. | |
461 | ||
a9e102b6 | 462 | The first can be used independently to the others. The VM can try to |
341546f5 N |
463 | either write dirty pages in order to clean them, or release clean |
464 | pages in order to reuse them. To do this it can call the ->writepage | |
465 | method on dirty pages, and ->releasepage on clean pages with | |
466 | PagePrivate set. Clean pages without PagePrivate and with no external | |
467 | references will be released without notice being given to the | |
468 | address_space. | |
469 | ||
a9e102b6 | 470 | To achieve this functionality, pages need to be placed on an LRU with |
341546f5 N |
471 | lru_cache_add and mark_page_active needs to be called whenever the |
472 | page is used. | |
473 | ||
474 | Pages are normally kept in a radix tree index by ->index. This tree | |
475 | maintains information about the PG_Dirty and PG_Writeback status of | |
476 | each page, so that pages with either of these flags can be found | |
477 | quickly. | |
478 | ||
479 | The Dirty tag is primarily used by mpage_writepages - the default | |
480 | ->writepages method. It uses the tag to find dirty pages to call | |
481 | ->writepage on. If mpage_writepages is not used (i.e. the address | |
a9e102b6 | 482 | provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is |
341546f5 N |
483 | almost unused. write_inode_now and sync_inode do use it (through |
484 | __sync_single_inode) to check if ->writepages has been successful in | |
485 | writing out the whole address_space. | |
486 | ||
487 | The Writeback tag is used by filemap*wait* and sync_page* functions, | |
94004ed7 | 488 | via filemap_fdatawait_range, to wait for all writeback to |
341546f5 | 489 | complete. While waiting ->sync_page (if defined) will be called on |
a9e102b6 | 490 | each page that is found to require writeback. |
341546f5 N |
491 | |
492 | An address_space handler may attach extra information to a page, | |
493 | typically using the 'private' field in the 'struct page'. If such | |
494 | information is attached, the PG_Private flag should be set. This will | |
a9e102b6 | 495 | cause various VM routines to make extra calls into the address_space |
341546f5 N |
496 | handler to deal with that data. |
497 | ||
498 | An address space acts as an intermediate between storage and | |
499 | application. Data is read into the address space a whole page at a | |
500 | time, and provided to the application either by copying of the page, | |
501 | or by memory-mapping the page. | |
502 | Data is written into the address space by the application, and then | |
503 | written-back to storage typically in whole pages, however the | |
a9e102b6 | 504 | address_space has finer control of write sizes. |
341546f5 N |
505 | |
506 | The read process essentially only requires 'readpage'. The write | |
4e02ed4b | 507 | process is more complicated and uses write_begin/write_end or |
341546f5 N |
508 | set_page_dirty to write data into the address_space, and writepage, |
509 | sync_page, and writepages to writeback data to storage. | |
510 | ||
511 | Adding and removing pages to/from an address_space is protected by the | |
512 | inode's i_mutex. | |
513 | ||
514 | When data is written to a page, the PG_Dirty flag should be set. It | |
515 | typically remains set until writepage asks for it to be written. This | |
516 | should clear PG_Dirty and set PG_Writeback. It can be actually | |
517 | written at any point after PG_Dirty is clear. Once it is known to be | |
518 | safe, PG_Writeback is cleared. | |
519 | ||
520 | Writeback makes use of a writeback_control structure... | |
5ea626aa PE |
521 | |
522 | struct address_space_operations | |
cc7d1f8f | 523 | ------------------------------- |
5ea626aa PE |
524 | |
525 | This describes how the VFS can manipulate mapping of a file to page cache in | |
422b14c2 | 526 | your filesystem. As of kernel 2.6.22, the following members are defined: |
5ea626aa PE |
527 | |
528 | struct address_space_operations { | |
529 | int (*writepage)(struct page *page, struct writeback_control *wbc); | |
530 | int (*readpage)(struct file *, struct page *); | |
531 | int (*sync_page)(struct page *); | |
532 | int (*writepages)(struct address_space *, struct writeback_control *); | |
533 | int (*set_page_dirty)(struct page *page); | |
534 | int (*readpages)(struct file *filp, struct address_space *mapping, | |
535 | struct list_head *pages, unsigned nr_pages); | |
afddba49 NP |
536 | int (*write_begin)(struct file *, struct address_space *mapping, |
537 | loff_t pos, unsigned len, unsigned flags, | |
538 | struct page **pagep, void **fsdata); | |
539 | int (*write_end)(struct file *, struct address_space *mapping, | |
540 | loff_t pos, unsigned len, unsigned copied, | |
541 | struct page *page, void *fsdata); | |
5ea626aa PE |
542 | sector_t (*bmap)(struct address_space *, sector_t); |
543 | int (*invalidatepage) (struct page *, unsigned long); | |
544 | int (*releasepage) (struct page *, int); | |
6072d13c | 545 | void (*freepage)(struct page *); |
5ea626aa PE |
546 | ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, |
547 | loff_t offset, unsigned long nr_segs); | |
548 | struct page* (*get_xip_page)(struct address_space *, sector_t, | |
549 | int); | |
341546f5 N |
550 | /* migrate the contents of a page to the specified target */ |
551 | int (*migratepage) (struct page *, struct page *); | |
422b14c2 | 552 | int (*launder_page) (struct page *); |
25718736 | 553 | int (*error_remove_page) (struct mapping *mapping, struct page *page); |
5ea626aa PE |
554 | }; |
555 | ||
341546f5 | 556 | writepage: called by the VM to write a dirty page to backing store. |
a9e102b6 | 557 | This may happen for data integrity reasons (i.e. 'sync'), or |
341546f5 N |
558 | to free up memory (flush). The difference can be seen in |
559 | wbc->sync_mode. | |
560 | The PG_Dirty flag has been cleared and PageLocked is true. | |
561 | writepage should start writeout, should set PG_Writeback, | |
562 | and should make sure the page is unlocked, either synchronously | |
563 | or asynchronously when the write operation completes. | |
564 | ||
565 | If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to | |
a9e102b6 N |
566 | try too hard if there are problems, and may choose to write out |
567 | other pages from the mapping if that is easier (e.g. due to | |
568 | internal dependencies). If it chooses not to start writeout, it | |
569 | should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep | |
341546f5 N |
570 | calling ->writepage on that page. |
571 | ||
572 | See the file "Locking" for more details. | |
5ea626aa PE |
573 | |
574 | readpage: called by the VM to read a page from backing store. | |
341546f5 N |
575 | The page will be Locked when readpage is called, and should be |
576 | unlocked and marked uptodate once the read completes. | |
577 | If ->readpage discovers that it needs to unlock the page for | |
578 | some reason, it can do so, and then return AOP_TRUNCATED_PAGE. | |
a9e102b6 | 579 | In this case, the page will be relocated, relocked and if |
341546f5 | 580 | that all succeeds, ->readpage will be called again. |
5ea626aa PE |
581 | |
582 | sync_page: called by the VM to notify the backing store to perform all | |
583 | queued I/O operations for a page. I/O operations for other pages | |
584 | associated with this address_space object may also be performed. | |
585 | ||
341546f5 N |
586 | This function is optional and is called only for pages with |
587 | PG_Writeback set while waiting for the writeback to complete. | |
588 | ||
5ea626aa | 589 | writepages: called by the VM to write out pages associated with the |
a9e102b6 N |
590 | address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then |
591 | the writeback_control will specify a range of pages that must be | |
592 | written out. If it is WBC_SYNC_NONE, then a nr_to_write is given | |
341546f5 N |
593 | and that many pages should be written if possible. |
594 | If no ->writepages is given, then mpage_writepages is used | |
a9e102b6 | 595 | instead. This will choose pages from the address space that are |
341546f5 | 596 | tagged as DIRTY and will pass them to ->writepage. |
5ea626aa PE |
597 | |
598 | set_page_dirty: called by the VM to set a page dirty. | |
341546f5 N |
599 | This is particularly needed if an address space attaches |
600 | private data to a page, and that data needs to be updated when | |
601 | a page is dirtied. This is called, for example, when a memory | |
602 | mapped page gets modified. | |
603 | If defined, it should set the PageDirty flag, and the | |
604 | PAGECACHE_TAG_DIRTY tag in the radix tree. | |
5ea626aa PE |
605 | |
606 | readpages: called by the VM to read pages associated with the address_space | |
341546f5 N |
607 | object. This is essentially just a vector version of |
608 | readpage. Instead of just one page, several pages are | |
609 | requested. | |
a9e102b6 | 610 | readpages is only used for read-ahead, so read errors are |
341546f5 | 611 | ignored. If anything goes wrong, feel free to give up. |
1da177e4 | 612 | |
4e02ed4b | 613 | write_begin: |
afddba49 NP |
614 | Called by the generic buffered write code to ask the filesystem to |
615 | prepare to write len bytes at the given offset in the file. The | |
616 | address_space should check that the write will be able to complete, | |
617 | by allocating space if necessary and doing any other internal | |
618 | housekeeping. If the write will update parts of any basic-blocks on | |
619 | storage, then those blocks should be pre-read (if they haven't been | |
620 | read already) so that the updated blocks can be written out properly. | |
621 | ||
622 | The filesystem must return the locked pagecache page for the specified | |
623 | offset, in *pagep, for the caller to write into. | |
624 | ||
4e02ed4b NP |
625 | It must be able to cope with short writes (where the length passed to |
626 | write_begin is greater than the number of bytes copied into the page). | |
627 | ||
afddba49 NP |
628 | flags is a field for AOP_FLAG_xxx flags, described in |
629 | include/linux/fs.h. | |
630 | ||
631 | A void * may be returned in fsdata, which then gets passed into | |
632 | write_end. | |
633 | ||
634 | Returns 0 on success; < 0 on failure (which is the error code), in | |
635 | which case write_end is not called. | |
636 | ||
637 | write_end: After a successful write_begin, and data copy, write_end must | |
638 | be called. len is the original len passed to write_begin, and copied | |
639 | is the amount that was able to be copied (copied == len is always true | |
640 | if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag). | |
641 | ||
642 | The filesystem must take care of unlocking the page and releasing it | |
643 | refcount, and updating i_size. | |
644 | ||
645 | Returns < 0 on failure, otherwise the number of bytes (<= 'copied') | |
646 | that were able to be copied into pagecache. | |
647 | ||
5ea626aa | 648 | bmap: called by the VFS to map a logical block offset within object to |
a9e102b6 | 649 | physical block number. This method is used by the FIBMAP |
341546f5 | 650 | ioctl and for working with swap-files. To be able to swap to |
a9e102b6 | 651 | a file, the file must have a stable mapping to a block |
341546f5 N |
652 | device. The swap system does not go through the filesystem |
653 | but instead uses bmap to find out where the blocks in the file | |
654 | are and uses those addresses directly. | |
655 | ||
656 | ||
657 | invalidatepage: If a page has PagePrivate set, then invalidatepage | |
658 | will be called when part or all of the page is to be removed | |
a9e102b6 | 659 | from the address space. This generally corresponds to either a |
341546f5 N |
660 | truncation or a complete invalidation of the address space |
661 | (in the latter case 'offset' will always be 0). | |
662 | Any private data associated with the page should be updated | |
663 | to reflect this truncation. If offset is 0, then | |
664 | the private data should be released, because the page | |
665 | must be able to be completely discarded. This may be done by | |
666 | calling the ->releasepage function, but in this case the | |
667 | release MUST succeed. | |
668 | ||
669 | releasepage: releasepage is called on PagePrivate pages to indicate | |
670 | that the page should be freed if possible. ->releasepage | |
671 | should remove any private data from the page and clear the | |
4fe65cab AM |
672 | PagePrivate flag. If releasepage() fails for some reason, it must |
673 | indicate failure with a 0 return value. | |
674 | releasepage() is used in two distinct though related cases. The | |
675 | first is when the VM finds a clean page with no active users and | |
341546f5 N |
676 | wants to make it a free page. If ->releasepage succeeds, the |
677 | page will be removed from the address_space and become free. | |
678 | ||
bc5b1d55 | 679 | The second case is when a request has been made to invalidate |
341546f5 N |
680 | some or all pages in an address_space. This can happen |
681 | through the fadvice(POSIX_FADV_DONTNEED) system call or by the | |
682 | filesystem explicitly requesting it as nfs and 9fs do (when | |
683 | they believe the cache may be out of date with storage) by | |
684 | calling invalidate_inode_pages2(). | |
685 | If the filesystem makes such a call, and needs to be certain | |
a9e102b6 | 686 | that all pages are invalidated, then its releasepage will |
341546f5 N |
687 | need to ensure this. Possibly it can clear the PageUptodate |
688 | bit if it cannot free private data yet. | |
689 | ||
6072d13c LT |
690 | freepage: freepage is called once the page is no longer visible in |
691 | the page cache in order to allow the cleanup of any private | |
692 | data. Since it may be called by the memory reclaimer, it | |
693 | should not assume that the original address_space mapping still | |
694 | exists, and it should not block. | |
695 | ||
341546f5 N |
696 | direct_IO: called by the generic read/write routines to perform |
697 | direct_IO - that is IO requests which bypass the page cache | |
a9e102b6 | 698 | and transfer data directly between the storage and the |
341546f5 | 699 | application's address space. |
5ea626aa PE |
700 | |
701 | get_xip_page: called by the VM to translate a block number to a page. | |
702 | The page is valid until the corresponding filesystem is unmounted. | |
703 | Filesystems that want to use execute-in-place (XIP) need to implement | |
704 | it. An example implementation can be found in fs/ext2/xip.c. | |
705 | ||
341546f5 N |
706 | migrate_page: This is used to compact the physical memory usage. |
707 | If the VM wants to relocate a page (maybe off a memory card | |
708 | that is signalling imminent failure) it will pass a new page | |
709 | and an old page to this function. migrate_page should | |
710 | transfer any private data across and update any references | |
711 | that it has to the page. | |
5ea626aa | 712 | |
422b14c2 BP |
713 | launder_page: Called before freeing a page - it writes back the dirty page. To |
714 | prevent redirtying the page, it is kept locked during the whole | |
715 | operation. | |
716 | ||
25718736 AK |
717 | error_remove_page: normally set to generic_error_remove_page if truncation |
718 | is ok for this address space. Used for memory failure handling. | |
719 | Setting this implies you deal with pages going away under you, | |
720 | unless you have them locked or reference counts increased. | |
721 | ||
722 | ||
cc7d1f8f PE |
723 | The File Object |
724 | =============== | |
725 | ||
726 | A file object represents a file opened by a process. | |
727 | ||
728 | ||
5ea626aa | 729 | struct file_operations |
cc7d1f8f | 730 | ---------------------- |
1da177e4 LT |
731 | |
732 | This describes how the VFS can manipulate an open file. As of kernel | |
422b14c2 | 733 | 2.6.22, the following members are defined: |
1da177e4 LT |
734 | |
735 | struct file_operations { | |
422b14c2 | 736 | struct module *owner; |
1da177e4 | 737 | loff_t (*llseek) (struct file *, loff_t, int); |
5ea626aa | 738 | ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); |
5ea626aa | 739 | ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); |
027445c3 BP |
740 | ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
741 | ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); | |
1da177e4 LT |
742 | int (*readdir) (struct file *, void *, filldir_t); |
743 | unsigned int (*poll) (struct file *, struct poll_table_struct *); | |
5ea626aa PE |
744 | long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
745 | long (*compat_ioctl) (struct file *, unsigned int, unsigned long); | |
1da177e4 LT |
746 | int (*mmap) (struct file *, struct vm_area_struct *); |
747 | int (*open) (struct inode *, struct file *); | |
5ea626aa | 748 | int (*flush) (struct file *); |
1da177e4 | 749 | int (*release) (struct inode *, struct file *); |
7ea80859 | 750 | int (*fsync) (struct file *, int datasync); |
5ea626aa PE |
751 | int (*aio_fsync) (struct kiocb *, int datasync); |
752 | int (*fasync) (int, struct file *, int); | |
1da177e4 | 753 | int (*lock) (struct file *, int, struct file_lock *); |
5ea626aa PE |
754 | ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *); |
755 | ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *); | |
756 | ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *); | |
757 | ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); | |
758 | unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); | |
759 | int (*check_flags)(int); | |
5ea626aa | 760 | int (*flock) (struct file *, int, struct file_lock *); |
422b14c2 BP |
761 | ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int); |
762 | ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int); | |
1da177e4 LT |
763 | }; |
764 | ||
765 | Again, all methods are called without any locks being held, unless | |
766 | otherwise noted. | |
767 | ||
768 | llseek: called when the VFS needs to move the file position index | |
769 | ||
770 | read: called by read(2) and related system calls | |
771 | ||
5ea626aa PE |
772 | aio_read: called by io_submit(2) and other asynchronous I/O operations |
773 | ||
1da177e4 LT |
774 | write: called by write(2) and related system calls |
775 | ||
5ea626aa PE |
776 | aio_write: called by io_submit(2) and other asynchronous I/O operations |
777 | ||
1da177e4 LT |
778 | readdir: called when the VFS needs to read the directory contents |
779 | ||
780 | poll: called by the VFS when a process wants to check if there is | |
781 | activity on this file and (optionally) go to sleep until there | |
782 | is activity. Called by the select(2) and poll(2) system calls | |
783 | ||
b19dd42f | 784 | unlocked_ioctl: called by the ioctl(2) system call. |
5ea626aa PE |
785 | |
786 | compat_ioctl: called by the ioctl(2) system call when 32 bit system calls | |
787 | are used on 64 bit kernels. | |
788 | ||
1da177e4 LT |
789 | mmap: called by the mmap(2) system call |
790 | ||
791 | open: called by the VFS when an inode should be opened. When the VFS | |
5ea626aa PE |
792 | opens a file, it creates a new "struct file". It then calls the |
793 | open method for the newly allocated file structure. You might | |
794 | think that the open method really belongs in | |
795 | "struct inode_operations", and you may be right. I think it's | |
796 | done the way it is because it makes filesystems simpler to | |
797 | implement. The open() method is a good place to initialize the | |
798 | "private_data" member in the file structure if you want to point | |
799 | to a device structure | |
800 | ||
801 | flush: called by the close(2) system call to flush a file | |
1da177e4 LT |
802 | |
803 | release: called when the last reference to an open file is closed | |
804 | ||
805 | fsync: called by the fsync(2) system call | |
806 | ||
807 | fasync: called by the fcntl(2) system call when asynchronous | |
808 | (non-blocking) mode is enabled for a file | |
809 | ||
5ea626aa PE |
810 | lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW |
811 | commands | |
812 | ||
813 | readv: called by the readv(2) system call | |
814 | ||
815 | writev: called by the writev(2) system call | |
816 | ||
817 | sendfile: called by the sendfile(2) system call | |
818 | ||
819 | get_unmapped_area: called by the mmap(2) system call | |
820 | ||
821 | check_flags: called by the fcntl(2) system call for F_SETFL command | |
822 | ||
5ea626aa PE |
823 | flock: called by the flock(2) system call |
824 | ||
d1195c51 PE |
825 | splice_write: called by the VFS to splice data from a pipe to a file. This |
826 | method is used by the splice(2) system call | |
827 | ||
828 | splice_read: called by the VFS to splice data from file to a pipe. This | |
829 | method is used by the splice(2) system call | |
830 | ||
1da177e4 LT |
831 | Note that the file operations are implemented by the specific |
832 | filesystem in which the inode resides. When opening a device node | |
833 | (character or block special) most filesystems will call special | |
834 | support routines in the VFS which will locate the required device | |
835 | driver information. These support routines replace the filesystem file | |
836 | operations with those for the device driver, and then proceed to call | |
837 | the new open() method for the file. This is how opening a device file | |
838 | in the filesystem eventually ends up calling the device driver open() | |
5ea626aa | 839 | method. |
1da177e4 LT |
840 | |
841 | ||
5ea626aa PE |
842 | Directory Entry Cache (dcache) |
843 | ============================== | |
844 | ||
1da177e4 LT |
845 | |
846 | struct dentry_operations | |
5ea626aa | 847 | ------------------------ |
1da177e4 LT |
848 | |
849 | This describes how a filesystem can overload the standard dentry | |
850 | operations. Dentries and the dcache are the domain of the VFS and the | |
851 | individual filesystem implementations. Device drivers have no business | |
852 | here. These methods may be set to NULL, as they are either optional or | |
c23fbb6b | 853 | the VFS uses a default. As of kernel 2.6.22, the following members are |
1da177e4 LT |
854 | defined: |
855 | ||
856 | struct dentry_operations { | |
5ea626aa | 857 | int (*d_revalidate)(struct dentry *, struct nameidata *); |
b1e6a015 NP |
858 | int (*d_hash)(const struct dentry *, const struct inode *, |
859 | struct qstr *); | |
621e155a NP |
860 | int (*d_compare)(const struct dentry *, const struct inode *, |
861 | const struct dentry *, const struct inode *, | |
862 | unsigned int, const char *, const struct qstr *); | |
fe15ce44 | 863 | int (*d_delete)(const struct dentry *); |
1da177e4 LT |
864 | void (*d_release)(struct dentry *); |
865 | void (*d_iput)(struct dentry *, struct inode *); | |
c23fbb6b | 866 | char *(*d_dname)(struct dentry *, char *, int); |
9875cf80 | 867 | struct vfsmount *(*d_automount)(struct path *); |
ab90911f | 868 | int (*d_manage)(struct dentry *, bool, bool); |
1da177e4 LT |
869 | }; |
870 | ||
871 | d_revalidate: called when the VFS needs to revalidate a dentry. This | |
872 | is called whenever a name look-up finds a dentry in the | |
873 | dcache. Most filesystems leave this as NULL, because all their | |
874 | dentries in the dcache are valid | |
875 | ||
34286d66 NP |
876 | d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU). |
877 | If in rcu-walk mode, the filesystem must revalidate the dentry without | |
878 | blocking or storing to the dentry, d_parent and d_inode should not be | |
879 | used without care (because they can go NULL), instead nd->inode should | |
880 | be used. | |
881 | ||
882 | If a situation is encountered that rcu-walk cannot handle, return | |
883 | -ECHILD and it will be called again in ref-walk mode. | |
884 | ||
621e155a NP |
885 | d_hash: called when the VFS adds a dentry to the hash table. The first |
886 | dentry passed to d_hash is the parent directory that the name is | |
b1e6a015 NP |
887 | to be hashed into. The inode is the dentry's inode. |
888 | ||
889 | Same locking and synchronisation rules as d_compare regarding | |
890 | what is safe to dereference etc. | |
1da177e4 | 891 | |
621e155a NP |
892 | d_compare: called to compare a dentry name with a given name. The first |
893 | dentry is the parent of the dentry to be compared, the second is | |
894 | the parent's inode, then the dentry and inode (may be NULL) of the | |
895 | child dentry. len and name string are properties of the dentry to be | |
896 | compared. qstr is the name to compare it with. | |
897 | ||
898 | Must be constant and idempotent, and should not take locks if | |
899 | possible, and should not or store into the dentry or inodes. | |
900 | Should not dereference pointers outside the dentry or inodes without | |
901 | lots of care (eg. d_parent, d_inode, d_name should not be used). | |
902 | ||
903 | However, our vfsmount is pinned, and RCU held, so the dentries and | |
904 | inodes won't disappear, neither will our sb or filesystem module. | |
905 | ->i_sb and ->d_sb may be used. | |
906 | ||
907 | It is a tricky calling convention because it needs to be called under | |
908 | "rcu-walk", ie. without any locks or references on things. | |
1da177e4 | 909 | |
fe15ce44 NP |
910 | d_delete: called when the last reference to a dentry is dropped and the |
911 | dcache is deciding whether or not to cache it. Return 1 to delete | |
912 | immediately, or 0 to cache the dentry. Default is NULL which means to | |
913 | always cache a reachable dentry. d_delete must be constant and | |
914 | idempotent. | |
1da177e4 LT |
915 | |
916 | d_release: called when a dentry is really deallocated | |
917 | ||
918 | d_iput: called when a dentry loses its inode (just prior to its | |
919 | being deallocated). The default when this is NULL is that the | |
920 | VFS calls iput(). If you define this method, you must call | |
921 | iput() yourself | |
922 | ||
c23fbb6b | 923 | d_dname: called when the pathname of a dentry should be generated. |
d9195881 | 924 | Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay |
c23fbb6b | 925 | pathname generation. (Instead of doing it when dentry is created, |
d9195881 | 926 | it's done only when the path is needed.). Real filesystems probably |
c23fbb6b ED |
927 | dont want to use it, because their dentries are present in global |
928 | dcache hash, so their hash should be an invariant. As no lock is | |
929 | held, d_dname() should not try to modify the dentry itself, unless | |
930 | appropriate SMP safety is used. CAUTION : d_path() logic is quite | |
931 | tricky. The correct way to return for example "Hello" is to put it | |
932 | at the end of the buffer, and returns a pointer to the first char. | |
933 | dynamic_dname() helper function is provided to take care of this. | |
934 | ||
9875cf80 | 935 | d_automount: called when an automount dentry is to be traversed (optional). |
ea5b778a DH |
936 | This should create a new VFS mount record and return the record to the |
937 | caller. The caller is supplied with a path parameter giving the | |
938 | automount directory to describe the automount target and the parent | |
939 | VFS mount record to provide inheritable mount parameters. NULL should | |
940 | be returned if someone else managed to make the automount first. If | |
941 | the vfsmount creation failed, then an error code should be returned. | |
942 | If -EISDIR is returned, then the directory will be treated as an | |
943 | ordinary directory and returned to pathwalk to continue walking. | |
944 | ||
945 | If a vfsmount is returned, the caller will attempt to mount it on the | |
946 | mountpoint and will remove the vfsmount from its expiration list in | |
947 | the case of failure. The vfsmount should be returned with 2 refs on | |
948 | it to prevent automatic expiration - the caller will clean up the | |
949 | additional ref. | |
9875cf80 DH |
950 | |
951 | This function is only used if DCACHE_NEED_AUTOMOUNT is set on the | |
952 | dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the | |
953 | inode being added. | |
954 | ||
cc53ce53 DH |
955 | d_manage: called to allow the filesystem to manage the transition from a |
956 | dentry (optional). This allows autofs, for example, to hold up clients | |
957 | waiting to explore behind a 'mountpoint' whilst letting the daemon go | |
958 | past and construct the subtree there. 0 should be returned to let the | |
959 | calling process continue. -EISDIR can be returned to tell pathwalk to | |
960 | use this directory as an ordinary directory and to ignore anything | |
961 | mounted on it and not to check the automount flag. Any other error | |
962 | code will abort pathwalk completely. | |
963 | ||
964 | If the 'mounting_here' parameter is true, then namespace_sem is being | |
965 | held by the caller and the function should not initiate any mounts or | |
966 | unmounts that it will then wait for. | |
967 | ||
ab90911f DH |
968 | If the 'rcu_walk' parameter is true, then the caller is doing a |
969 | pathwalk in RCU-walk mode. Sleeping is not permitted in this mode, | |
970 | and the caller can be asked to leave it and call again by returing | |
971 | -ECHILD. | |
972 | ||
cc53ce53 DH |
973 | This function is only used if DCACHE_MANAGE_TRANSIT is set on the |
974 | dentry being transited from. | |
975 | ||
c23fbb6b ED |
976 | Example : |
977 | ||
978 | static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) | |
979 | { | |
980 | return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", | |
981 | dentry->d_inode->i_ino); | |
982 | } | |
983 | ||
1da177e4 LT |
984 | Each dentry has a pointer to its parent dentry, as well as a hash list |
985 | of child dentries. Child dentries are basically like files in a | |
986 | directory. | |
987 | ||
5ea626aa | 988 | |
cc7d1f8f | 989 | Directory Entry Cache API |
1da177e4 LT |
990 | -------------------------- |
991 | ||
992 | There are a number of functions defined which permit a filesystem to | |
993 | manipulate dentries: | |
994 | ||
995 | dget: open a new handle for an existing dentry (this just increments | |
996 | the usage count) | |
997 | ||
998 | dput: close a handle for a dentry (decrements the usage count). If | |
fe15ce44 NP |
999 | the usage count drops to 0, and the dentry is still in its |
1000 | parent's hash, the "d_delete" method is called to check whether | |
1001 | it should be cached. If it should not be cached, or if the dentry | |
1002 | is not hashed, it is deleted. Otherwise cached dentries are put | |
1003 | into an LRU list to be reclaimed on memory shortage. | |
1da177e4 LT |
1004 | |
1005 | d_drop: this unhashes a dentry from its parents hash list. A | |
5ea626aa | 1006 | subsequent call to dput() will deallocate the dentry if its |
1da177e4 LT |
1007 | usage count drops to 0 |
1008 | ||
1009 | d_delete: delete a dentry. If there are no other open references to | |
1010 | the dentry then the dentry is turned into a negative dentry | |
1011 | (the d_iput() method is called). If there are other | |
1012 | references, then d_drop() is called instead | |
1013 | ||
1014 | d_add: add a dentry to its parents hash list and then calls | |
1015 | d_instantiate() | |
1016 | ||
1017 | d_instantiate: add a dentry to the alias hash list for the inode and | |
1018 | updates the "d_inode" member. The "i_count" member in the | |
1019 | inode structure should be set/incremented. If the inode | |
1020 | pointer is NULL, the dentry is called a "negative | |
1021 | dentry". This function is commonly called when an inode is | |
1022 | created for an existing negative dentry | |
1023 | ||
1024 | d_lookup: look up a dentry given its parent and path name component | |
1025 | It looks up the child of that given name from the dcache | |
1026 | hash table. If it is found, the reference count is incremented | |
be42c4c4 | 1027 | and the dentry is returned. The caller must use dput() |
1da177e4 LT |
1028 | to free the dentry when it finishes using it. |
1029 | ||
cbf8f0f3 PE |
1030 | For further information on dentry locking, please refer to the document |
1031 | Documentation/filesystems/dentry-locking.txt. | |
cc7d1f8f | 1032 | |
f84e3f52 MS |
1033 | Mount Options |
1034 | ============= | |
1035 | ||
1036 | Parsing options | |
1037 | --------------- | |
1038 | ||
1039 | On mount and remount the filesystem is passed a string containing a | |
1040 | comma separated list of mount options. The options can have either of | |
1041 | these forms: | |
1042 | ||
1043 | option | |
1044 | option=value | |
1045 | ||
1046 | The <linux/parser.h> header defines an API that helps parse these | |
1047 | options. There are plenty of examples on how to use it in existing | |
1048 | filesystems. | |
1049 | ||
1050 | Showing options | |
1051 | --------------- | |
1052 | ||
1053 | If a filesystem accepts mount options, it must define show_options() | |
1054 | to show all the currently active options. The rules are: | |
1055 | ||
1056 | - options MUST be shown which are not default or their values differ | |
1057 | from the default | |
1058 | ||
1059 | - options MAY be shown which are enabled by default or have their | |
1060 | default value | |
1061 | ||
1062 | Options used only internally between a mount helper and the kernel | |
1063 | (such as file descriptors), or which only have an effect during the | |
1064 | mounting (such as ones controlling the creation of a journal) are exempt | |
1065 | from the above rules. | |
1066 | ||
1067 | The underlying reason for the above rules is to make sure, that a | |
1068 | mount can be accurately replicated (e.g. umounting and mounting again) | |
1069 | based on the information found in /proc/mounts. | |
1070 | ||
1071 | A simple method of saving options at mount/remount time and showing | |
1072 | them is provided with the save_mount_options() and | |
1073 | generic_show_options() helper functions. Please note, that using | |
1074 | these may have drawbacks. For more info see header comments for these | |
1075 | functions in fs/namespace.c. | |
cc7d1f8f PE |
1076 | |
1077 | Resources | |
1078 | ========= | |
1079 | ||
1080 | (Note some of these resources are not up-to-date with the latest kernel | |
1081 | version.) | |
1082 | ||
1083 | Creating Linux virtual filesystems. 2002 | |
1084 | <http://lwn.net/Articles/13325/> | |
1085 | ||
1086 | The Linux Virtual File-system Layer by Neil Brown. 1999 | |
1087 | <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> | |
1088 | ||
1089 | A tour of the Linux VFS by Michael K. Johnson. 1996 | |
1090 | <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> | |
1091 | ||
1092 | A small trail through the Linux kernel by Andries Brouwer. 2001 | |
1093 | <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> |