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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 | |
75 | called so the specific filesystem implementation can do it's 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 | |
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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 | ||
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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 *); |
5ea626aa PE |
328 | int (*permission) (struct inode *, int, struct nameidata *); |
329 | int (*setattr) (struct dentry *, struct iattr *); | |
330 | int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); | |
331 | int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); | |
332 | ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); | |
333 | ssize_t (*listxattr) (struct dentry *, char *, size_t); | |
334 | int (*removexattr) (struct dentry *, const char *); | |
422b14c2 | 335 | void (*truncate_range)(struct inode *, loff_t, loff_t); |
1da177e4 LT |
336 | }; |
337 | ||
338 | Again, all methods are called without any locks being held, unless | |
339 | otherwise noted. | |
340 | ||
1da177e4 LT |
341 | create: called by the open(2) and creat(2) system calls. Only |
342 | required if you want to support regular files. The dentry you | |
343 | get should not have an inode (i.e. it should be a negative | |
344 | dentry). Here you will probably call d_instantiate() with the | |
345 | dentry and the newly created inode | |
346 | ||
347 | lookup: called when the VFS needs to look up an inode in a parent | |
348 | directory. The name to look for is found in the dentry. This | |
349 | method must call d_add() to insert the found inode into the | |
350 | dentry. The "i_count" field in the inode structure should be | |
351 | incremented. If the named inode does not exist a NULL inode | |
352 | should be inserted into the dentry (this is called a negative | |
353 | dentry). Returning an error code from this routine must only | |
354 | be done on a real error, otherwise creating inodes with system | |
355 | calls like create(2), mknod(2), mkdir(2) and so on will fail. | |
356 | If you wish to overload the dentry methods then you should | |
357 | initialise the "d_dop" field in the dentry; this is a pointer | |
358 | to a struct "dentry_operations". | |
359 | This method is called with the directory inode semaphore held | |
360 | ||
361 | link: called by the link(2) system call. Only required if you want | |
362 | to support hard links. You will probably need to call | |
363 | d_instantiate() just as you would in the create() method | |
364 | ||
365 | unlink: called by the unlink(2) system call. Only required if you | |
366 | want to support deleting inodes | |
367 | ||
368 | symlink: called by the symlink(2) system call. Only required if you | |
369 | want to support symlinks. You will probably need to call | |
370 | d_instantiate() just as you would in the create() method | |
371 | ||
372 | mkdir: called by the mkdir(2) system call. Only required if you want | |
373 | to support creating subdirectories. You will probably need to | |
374 | call d_instantiate() just as you would in the create() method | |
375 | ||
376 | rmdir: called by the rmdir(2) system call. Only required if you want | |
377 | to support deleting subdirectories | |
378 | ||
379 | mknod: called by the mknod(2) system call to create a device (char, | |
380 | block) inode or a named pipe (FIFO) or socket. Only required | |
381 | if you want to support creating these types of inodes. You | |
382 | will probably need to call d_instantiate() just as you would | |
383 | in the create() method | |
384 | ||
cc7d1f8f PE |
385 | rename: called by the rename(2) system call to rename the object to |
386 | have the parent and name given by the second inode and dentry. | |
387 | ||
1da177e4 LT |
388 | readlink: called by the readlink(2) system call. Only required if |
389 | you want to support reading symbolic links | |
390 | ||
391 | follow_link: called by the VFS to follow a symbolic link to the | |
5ea626aa | 392 | inode it points to. Only required if you want to support |
cc7d1f8f | 393 | symbolic links. This method returns a void pointer cookie |
5ea626aa PE |
394 | that is passed to put_link(). |
395 | ||
396 | put_link: called by the VFS to release resources allocated by | |
cc7d1f8f | 397 | follow_link(). The cookie returned by follow_link() is passed |
670e9f34 | 398 | to this method as the last parameter. It is used by |
cc7d1f8f PE |
399 | filesystems such as NFS where page cache is not stable |
400 | (i.e. page that was installed when the symbolic link walk | |
401 | started might not be in the page cache at the end of the | |
402 | walk). | |
403 | ||
404 | truncate: called by the VFS to change the size of a file. The | |
405 | i_size field of the inode is set to the desired size by the | |
406 | VFS before this method is called. This method is called by | |
407 | the truncate(2) system call and related functionality. | |
5ea626aa PE |
408 | |
409 | permission: called by the VFS to check for access rights on a POSIX-like | |
410 | filesystem. | |
411 | ||
cc7d1f8f PE |
412 | setattr: called by the VFS to set attributes for a file. This method |
413 | is called by chmod(2) and related system calls. | |
5ea626aa | 414 | |
cc7d1f8f PE |
415 | getattr: called by the VFS to get attributes of a file. This method |
416 | is called by stat(2) and related system calls. | |
5ea626aa PE |
417 | |
418 | setxattr: called by the VFS to set an extended attribute for a file. | |
cc7d1f8f PE |
419 | Extended attribute is a name:value pair associated with an |
420 | inode. This method is called by setxattr(2) system call. | |
421 | ||
422 | getxattr: called by the VFS to retrieve the value of an extended | |
423 | attribute name. This method is called by getxattr(2) function | |
424 | call. | |
425 | ||
426 | listxattr: called by the VFS to list all extended attributes for a | |
427 | given file. This method is called by listxattr(2) system call. | |
5ea626aa | 428 | |
cc7d1f8f PE |
429 | removexattr: called by the VFS to remove an extended attribute from |
430 | a file. This method is called by removexattr(2) system call. | |
5ea626aa | 431 | |
422b14c2 BP |
432 | truncate_range: a method provided by the underlying filesystem to truncate a |
433 | range of blocks , i.e. punch a hole somewhere in a file. | |
434 | ||
5ea626aa | 435 | |
cc7d1f8f PE |
436 | The Address Space Object |
437 | ======================== | |
438 | ||
341546f5 N |
439 | The address space object is used to group and manage pages in the page |
440 | cache. It can be used to keep track of the pages in a file (or | |
441 | anything else) and also track the mapping of sections of the file into | |
442 | process address spaces. | |
443 | ||
444 | There are a number of distinct yet related services that an | |
445 | address-space can provide. These include communicating memory | |
446 | pressure, page lookup by address, and keeping track of pages tagged as | |
447 | Dirty or Writeback. | |
448 | ||
a9e102b6 | 449 | The first can be used independently to the others. The VM can try to |
341546f5 N |
450 | either write dirty pages in order to clean them, or release clean |
451 | pages in order to reuse them. To do this it can call the ->writepage | |
452 | method on dirty pages, and ->releasepage on clean pages with | |
453 | PagePrivate set. Clean pages without PagePrivate and with no external | |
454 | references will be released without notice being given to the | |
455 | address_space. | |
456 | ||
a9e102b6 | 457 | To achieve this functionality, pages need to be placed on an LRU with |
341546f5 N |
458 | lru_cache_add and mark_page_active needs to be called whenever the |
459 | page is used. | |
460 | ||
461 | Pages are normally kept in a radix tree index by ->index. This tree | |
462 | maintains information about the PG_Dirty and PG_Writeback status of | |
463 | each page, so that pages with either of these flags can be found | |
464 | quickly. | |
465 | ||
466 | The Dirty tag is primarily used by mpage_writepages - the default | |
467 | ->writepages method. It uses the tag to find dirty pages to call | |
468 | ->writepage on. If mpage_writepages is not used (i.e. the address | |
a9e102b6 | 469 | provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is |
341546f5 N |
470 | almost unused. write_inode_now and sync_inode do use it (through |
471 | __sync_single_inode) to check if ->writepages has been successful in | |
472 | writing out the whole address_space. | |
473 | ||
474 | The Writeback tag is used by filemap*wait* and sync_page* functions, | |
94004ed7 | 475 | via filemap_fdatawait_range, to wait for all writeback to |
341546f5 | 476 | complete. While waiting ->sync_page (if defined) will be called on |
a9e102b6 | 477 | each page that is found to require writeback. |
341546f5 N |
478 | |
479 | An address_space handler may attach extra information to a page, | |
480 | typically using the 'private' field in the 'struct page'. If such | |
481 | information is attached, the PG_Private flag should be set. This will | |
a9e102b6 | 482 | cause various VM routines to make extra calls into the address_space |
341546f5 N |
483 | handler to deal with that data. |
484 | ||
485 | An address space acts as an intermediate between storage and | |
486 | application. Data is read into the address space a whole page at a | |
487 | time, and provided to the application either by copying of the page, | |
488 | or by memory-mapping the page. | |
489 | Data is written into the address space by the application, and then | |
490 | written-back to storage typically in whole pages, however the | |
a9e102b6 | 491 | address_space has finer control of write sizes. |
341546f5 N |
492 | |
493 | The read process essentially only requires 'readpage'. The write | |
4e02ed4b | 494 | process is more complicated and uses write_begin/write_end or |
341546f5 N |
495 | set_page_dirty to write data into the address_space, and writepage, |
496 | sync_page, and writepages to writeback data to storage. | |
497 | ||
498 | Adding and removing pages to/from an address_space is protected by the | |
499 | inode's i_mutex. | |
500 | ||
501 | When data is written to a page, the PG_Dirty flag should be set. It | |
502 | typically remains set until writepage asks for it to be written. This | |
503 | should clear PG_Dirty and set PG_Writeback. It can be actually | |
504 | written at any point after PG_Dirty is clear. Once it is known to be | |
505 | safe, PG_Writeback is cleared. | |
506 | ||
507 | Writeback makes use of a writeback_control structure... | |
5ea626aa PE |
508 | |
509 | struct address_space_operations | |
cc7d1f8f | 510 | ------------------------------- |
5ea626aa PE |
511 | |
512 | This describes how the VFS can manipulate mapping of a file to page cache in | |
422b14c2 | 513 | your filesystem. As of kernel 2.6.22, the following members are defined: |
5ea626aa PE |
514 | |
515 | struct address_space_operations { | |
516 | int (*writepage)(struct page *page, struct writeback_control *wbc); | |
517 | int (*readpage)(struct file *, struct page *); | |
518 | int (*sync_page)(struct page *); | |
519 | int (*writepages)(struct address_space *, struct writeback_control *); | |
520 | int (*set_page_dirty)(struct page *page); | |
521 | int (*readpages)(struct file *filp, struct address_space *mapping, | |
522 | struct list_head *pages, unsigned nr_pages); | |
afddba49 NP |
523 | int (*write_begin)(struct file *, struct address_space *mapping, |
524 | loff_t pos, unsigned len, unsigned flags, | |
525 | struct page **pagep, void **fsdata); | |
526 | int (*write_end)(struct file *, struct address_space *mapping, | |
527 | loff_t pos, unsigned len, unsigned copied, | |
528 | struct page *page, void *fsdata); | |
5ea626aa PE |
529 | sector_t (*bmap)(struct address_space *, sector_t); |
530 | int (*invalidatepage) (struct page *, unsigned long); | |
531 | int (*releasepage) (struct page *, int); | |
532 | ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, | |
533 | loff_t offset, unsigned long nr_segs); | |
534 | struct page* (*get_xip_page)(struct address_space *, sector_t, | |
535 | int); | |
341546f5 N |
536 | /* migrate the contents of a page to the specified target */ |
537 | int (*migratepage) (struct page *, struct page *); | |
422b14c2 | 538 | int (*launder_page) (struct page *); |
25718736 | 539 | int (*error_remove_page) (struct mapping *mapping, struct page *page); |
5ea626aa PE |
540 | }; |
541 | ||
341546f5 | 542 | writepage: called by the VM to write a dirty page to backing store. |
a9e102b6 | 543 | This may happen for data integrity reasons (i.e. 'sync'), or |
341546f5 N |
544 | to free up memory (flush). The difference can be seen in |
545 | wbc->sync_mode. | |
546 | The PG_Dirty flag has been cleared and PageLocked is true. | |
547 | writepage should start writeout, should set PG_Writeback, | |
548 | and should make sure the page is unlocked, either synchronously | |
549 | or asynchronously when the write operation completes. | |
550 | ||
551 | If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to | |
a9e102b6 N |
552 | try too hard if there are problems, and may choose to write out |
553 | other pages from the mapping if that is easier (e.g. due to | |
554 | internal dependencies). If it chooses not to start writeout, it | |
555 | should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep | |
341546f5 N |
556 | calling ->writepage on that page. |
557 | ||
558 | See the file "Locking" for more details. | |
5ea626aa PE |
559 | |
560 | readpage: called by the VM to read a page from backing store. | |
341546f5 N |
561 | The page will be Locked when readpage is called, and should be |
562 | unlocked and marked uptodate once the read completes. | |
563 | If ->readpage discovers that it needs to unlock the page for | |
564 | some reason, it can do so, and then return AOP_TRUNCATED_PAGE. | |
a9e102b6 | 565 | In this case, the page will be relocated, relocked and if |
341546f5 | 566 | that all succeeds, ->readpage will be called again. |
5ea626aa PE |
567 | |
568 | sync_page: called by the VM to notify the backing store to perform all | |
569 | queued I/O operations for a page. I/O operations for other pages | |
570 | associated with this address_space object may also be performed. | |
571 | ||
341546f5 N |
572 | This function is optional and is called only for pages with |
573 | PG_Writeback set while waiting for the writeback to complete. | |
574 | ||
5ea626aa | 575 | writepages: called by the VM to write out pages associated with the |
a9e102b6 N |
576 | address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then |
577 | the writeback_control will specify a range of pages that must be | |
578 | written out. If it is WBC_SYNC_NONE, then a nr_to_write is given | |
341546f5 N |
579 | and that many pages should be written if possible. |
580 | If no ->writepages is given, then mpage_writepages is used | |
a9e102b6 | 581 | instead. This will choose pages from the address space that are |
341546f5 | 582 | tagged as DIRTY and will pass them to ->writepage. |
5ea626aa PE |
583 | |
584 | set_page_dirty: called by the VM to set a page dirty. | |
341546f5 N |
585 | This is particularly needed if an address space attaches |
586 | private data to a page, and that data needs to be updated when | |
587 | a page is dirtied. This is called, for example, when a memory | |
588 | mapped page gets modified. | |
589 | If defined, it should set the PageDirty flag, and the | |
590 | PAGECACHE_TAG_DIRTY tag in the radix tree. | |
5ea626aa PE |
591 | |
592 | readpages: called by the VM to read pages associated with the address_space | |
341546f5 N |
593 | object. This is essentially just a vector version of |
594 | readpage. Instead of just one page, several pages are | |
595 | requested. | |
a9e102b6 | 596 | readpages is only used for read-ahead, so read errors are |
341546f5 | 597 | ignored. If anything goes wrong, feel free to give up. |
1da177e4 | 598 | |
4e02ed4b | 599 | write_begin: |
afddba49 NP |
600 | Called by the generic buffered write code to ask the filesystem to |
601 | prepare to write len bytes at the given offset in the file. The | |
602 | address_space should check that the write will be able to complete, | |
603 | by allocating space if necessary and doing any other internal | |
604 | housekeeping. If the write will update parts of any basic-blocks on | |
605 | storage, then those blocks should be pre-read (if they haven't been | |
606 | read already) so that the updated blocks can be written out properly. | |
607 | ||
608 | The filesystem must return the locked pagecache page for the specified | |
609 | offset, in *pagep, for the caller to write into. | |
610 | ||
4e02ed4b NP |
611 | It must be able to cope with short writes (where the length passed to |
612 | write_begin is greater than the number of bytes copied into the page). | |
613 | ||
afddba49 NP |
614 | flags is a field for AOP_FLAG_xxx flags, described in |
615 | include/linux/fs.h. | |
616 | ||
617 | A void * may be returned in fsdata, which then gets passed into | |
618 | write_end. | |
619 | ||
620 | Returns 0 on success; < 0 on failure (which is the error code), in | |
621 | which case write_end is not called. | |
622 | ||
623 | write_end: After a successful write_begin, and data copy, write_end must | |
624 | be called. len is the original len passed to write_begin, and copied | |
625 | is the amount that was able to be copied (copied == len is always true | |
626 | if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag). | |
627 | ||
628 | The filesystem must take care of unlocking the page and releasing it | |
629 | refcount, and updating i_size. | |
630 | ||
631 | Returns < 0 on failure, otherwise the number of bytes (<= 'copied') | |
632 | that were able to be copied into pagecache. | |
633 | ||
5ea626aa | 634 | bmap: called by the VFS to map a logical block offset within object to |
a9e102b6 | 635 | physical block number. This method is used by the FIBMAP |
341546f5 | 636 | ioctl and for working with swap-files. To be able to swap to |
a9e102b6 | 637 | a file, the file must have a stable mapping to a block |
341546f5 N |
638 | device. The swap system does not go through the filesystem |
639 | but instead uses bmap to find out where the blocks in the file | |
640 | are and uses those addresses directly. | |
641 | ||
642 | ||
643 | invalidatepage: If a page has PagePrivate set, then invalidatepage | |
644 | will be called when part or all of the page is to be removed | |
a9e102b6 | 645 | from the address space. This generally corresponds to either a |
341546f5 N |
646 | truncation or a complete invalidation of the address space |
647 | (in the latter case 'offset' will always be 0). | |
648 | Any private data associated with the page should be updated | |
649 | to reflect this truncation. If offset is 0, then | |
650 | the private data should be released, because the page | |
651 | must be able to be completely discarded. This may be done by | |
652 | calling the ->releasepage function, but in this case the | |
653 | release MUST succeed. | |
654 | ||
655 | releasepage: releasepage is called on PagePrivate pages to indicate | |
656 | that the page should be freed if possible. ->releasepage | |
657 | should remove any private data from the page and clear the | |
658 | PagePrivate flag. It may also remove the page from the | |
659 | address_space. If this fails for some reason, it may indicate | |
660 | failure with a 0 return value. | |
661 | This is used in two distinct though related cases. The first | |
662 | is when the VM finds a clean page with no active users and | |
663 | wants to make it a free page. If ->releasepage succeeds, the | |
664 | page will be removed from the address_space and become free. | |
665 | ||
bc5b1d55 | 666 | The second case is when a request has been made to invalidate |
341546f5 N |
667 | some or all pages in an address_space. This can happen |
668 | through the fadvice(POSIX_FADV_DONTNEED) system call or by the | |
669 | filesystem explicitly requesting it as nfs and 9fs do (when | |
670 | they believe the cache may be out of date with storage) by | |
671 | calling invalidate_inode_pages2(). | |
672 | If the filesystem makes such a call, and needs to be certain | |
a9e102b6 | 673 | that all pages are invalidated, then its releasepage will |
341546f5 N |
674 | need to ensure this. Possibly it can clear the PageUptodate |
675 | bit if it cannot free private data yet. | |
676 | ||
677 | direct_IO: called by the generic read/write routines to perform | |
678 | direct_IO - that is IO requests which bypass the page cache | |
a9e102b6 | 679 | and transfer data directly between the storage and the |
341546f5 | 680 | application's address space. |
5ea626aa PE |
681 | |
682 | get_xip_page: called by the VM to translate a block number to a page. | |
683 | The page is valid until the corresponding filesystem is unmounted. | |
684 | Filesystems that want to use execute-in-place (XIP) need to implement | |
685 | it. An example implementation can be found in fs/ext2/xip.c. | |
686 | ||
341546f5 N |
687 | migrate_page: This is used to compact the physical memory usage. |
688 | If the VM wants to relocate a page (maybe off a memory card | |
689 | that is signalling imminent failure) it will pass a new page | |
690 | and an old page to this function. migrate_page should | |
691 | transfer any private data across and update any references | |
692 | that it has to the page. | |
5ea626aa | 693 | |
422b14c2 BP |
694 | launder_page: Called before freeing a page - it writes back the dirty page. To |
695 | prevent redirtying the page, it is kept locked during the whole | |
696 | operation. | |
697 | ||
25718736 AK |
698 | error_remove_page: normally set to generic_error_remove_page if truncation |
699 | is ok for this address space. Used for memory failure handling. | |
700 | Setting this implies you deal with pages going away under you, | |
701 | unless you have them locked or reference counts increased. | |
702 | ||
703 | ||
cc7d1f8f PE |
704 | The File Object |
705 | =============== | |
706 | ||
707 | A file object represents a file opened by a process. | |
708 | ||
709 | ||
5ea626aa | 710 | struct file_operations |
cc7d1f8f | 711 | ---------------------- |
1da177e4 LT |
712 | |
713 | This describes how the VFS can manipulate an open file. As of kernel | |
422b14c2 | 714 | 2.6.22, the following members are defined: |
1da177e4 LT |
715 | |
716 | struct file_operations { | |
422b14c2 | 717 | struct module *owner; |
1da177e4 | 718 | loff_t (*llseek) (struct file *, loff_t, int); |
5ea626aa | 719 | ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); |
5ea626aa | 720 | ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); |
027445c3 BP |
721 | ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
722 | ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); | |
1da177e4 LT |
723 | int (*readdir) (struct file *, void *, filldir_t); |
724 | unsigned int (*poll) (struct file *, struct poll_table_struct *); | |
725 | int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long); | |
5ea626aa PE |
726 | long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
727 | long (*compat_ioctl) (struct file *, unsigned int, unsigned long); | |
1da177e4 LT |
728 | int (*mmap) (struct file *, struct vm_area_struct *); |
729 | int (*open) (struct inode *, struct file *); | |
5ea626aa | 730 | int (*flush) (struct file *); |
1da177e4 | 731 | int (*release) (struct inode *, struct file *); |
5ea626aa PE |
732 | int (*fsync) (struct file *, struct dentry *, int datasync); |
733 | int (*aio_fsync) (struct kiocb *, int datasync); | |
734 | int (*fasync) (int, struct file *, int); | |
1da177e4 | 735 | int (*lock) (struct file *, int, struct file_lock *); |
5ea626aa PE |
736 | ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *); |
737 | ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *); | |
738 | ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *); | |
739 | ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); | |
740 | unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); | |
741 | int (*check_flags)(int); | |
5ea626aa | 742 | int (*flock) (struct file *, int, struct file_lock *); |
422b14c2 BP |
743 | ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int); |
744 | ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int); | |
1da177e4 LT |
745 | }; |
746 | ||
747 | Again, all methods are called without any locks being held, unless | |
748 | otherwise noted. | |
749 | ||
750 | llseek: called when the VFS needs to move the file position index | |
751 | ||
752 | read: called by read(2) and related system calls | |
753 | ||
5ea626aa PE |
754 | aio_read: called by io_submit(2) and other asynchronous I/O operations |
755 | ||
1da177e4 LT |
756 | write: called by write(2) and related system calls |
757 | ||
5ea626aa PE |
758 | aio_write: called by io_submit(2) and other asynchronous I/O operations |
759 | ||
1da177e4 LT |
760 | readdir: called when the VFS needs to read the directory contents |
761 | ||
762 | poll: called by the VFS when a process wants to check if there is | |
763 | activity on this file and (optionally) go to sleep until there | |
764 | is activity. Called by the select(2) and poll(2) system calls | |
765 | ||
766 | ioctl: called by the ioctl(2) system call | |
767 | ||
5ea626aa PE |
768 | unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not |
769 | require the BKL should use this method instead of the ioctl() above. | |
770 | ||
771 | compat_ioctl: called by the ioctl(2) system call when 32 bit system calls | |
772 | are used on 64 bit kernels. | |
773 | ||
1da177e4 LT |
774 | mmap: called by the mmap(2) system call |
775 | ||
776 | open: called by the VFS when an inode should be opened. When the VFS | |
5ea626aa PE |
777 | opens a file, it creates a new "struct file". It then calls the |
778 | open method for the newly allocated file structure. You might | |
779 | think that the open method really belongs in | |
780 | "struct inode_operations", and you may be right. I think it's | |
781 | done the way it is because it makes filesystems simpler to | |
782 | implement. The open() method is a good place to initialize the | |
783 | "private_data" member in the file structure if you want to point | |
784 | to a device structure | |
785 | ||
786 | flush: called by the close(2) system call to flush a file | |
1da177e4 LT |
787 | |
788 | release: called when the last reference to an open file is closed | |
789 | ||
790 | fsync: called by the fsync(2) system call | |
791 | ||
792 | fasync: called by the fcntl(2) system call when asynchronous | |
793 | (non-blocking) mode is enabled for a file | |
794 | ||
5ea626aa PE |
795 | lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW |
796 | commands | |
797 | ||
798 | readv: called by the readv(2) system call | |
799 | ||
800 | writev: called by the writev(2) system call | |
801 | ||
802 | sendfile: called by the sendfile(2) system call | |
803 | ||
804 | get_unmapped_area: called by the mmap(2) system call | |
805 | ||
806 | check_flags: called by the fcntl(2) system call for F_SETFL command | |
807 | ||
5ea626aa PE |
808 | flock: called by the flock(2) system call |
809 | ||
d1195c51 PE |
810 | splice_write: called by the VFS to splice data from a pipe to a file. This |
811 | method is used by the splice(2) system call | |
812 | ||
813 | splice_read: called by the VFS to splice data from file to a pipe. This | |
814 | method is used by the splice(2) system call | |
815 | ||
1da177e4 LT |
816 | Note that the file operations are implemented by the specific |
817 | filesystem in which the inode resides. When opening a device node | |
818 | (character or block special) most filesystems will call special | |
819 | support routines in the VFS which will locate the required device | |
820 | driver information. These support routines replace the filesystem file | |
821 | operations with those for the device driver, and then proceed to call | |
822 | the new open() method for the file. This is how opening a device file | |
823 | in the filesystem eventually ends up calling the device driver open() | |
5ea626aa | 824 | method. |
1da177e4 LT |
825 | |
826 | ||
5ea626aa PE |
827 | Directory Entry Cache (dcache) |
828 | ============================== | |
829 | ||
1da177e4 LT |
830 | |
831 | struct dentry_operations | |
5ea626aa | 832 | ------------------------ |
1da177e4 LT |
833 | |
834 | This describes how a filesystem can overload the standard dentry | |
835 | operations. Dentries and the dcache are the domain of the VFS and the | |
836 | individual filesystem implementations. Device drivers have no business | |
837 | here. These methods may be set to NULL, as they are either optional or | |
c23fbb6b | 838 | the VFS uses a default. As of kernel 2.6.22, the following members are |
1da177e4 LT |
839 | defined: |
840 | ||
841 | struct dentry_operations { | |
5ea626aa | 842 | int (*d_revalidate)(struct dentry *, struct nameidata *); |
1da177e4 LT |
843 | int (*d_hash) (struct dentry *, struct qstr *); |
844 | int (*d_compare) (struct dentry *, struct qstr *, struct qstr *); | |
5ea626aa | 845 | int (*d_delete)(struct dentry *); |
1da177e4 LT |
846 | void (*d_release)(struct dentry *); |
847 | void (*d_iput)(struct dentry *, struct inode *); | |
c23fbb6b | 848 | char *(*d_dname)(struct dentry *, char *, int); |
1da177e4 LT |
849 | }; |
850 | ||
851 | d_revalidate: called when the VFS needs to revalidate a dentry. This | |
852 | is called whenever a name look-up finds a dentry in the | |
853 | dcache. Most filesystems leave this as NULL, because all their | |
854 | dentries in the dcache are valid | |
855 | ||
856 | d_hash: called when the VFS adds a dentry to the hash table | |
857 | ||
858 | d_compare: called when a dentry should be compared with another | |
859 | ||
860 | d_delete: called when the last reference to a dentry is | |
861 | deleted. This means no-one is using the dentry, however it is | |
862 | still valid and in the dcache | |
863 | ||
864 | d_release: called when a dentry is really deallocated | |
865 | ||
866 | d_iput: called when a dentry loses its inode (just prior to its | |
867 | being deallocated). The default when this is NULL is that the | |
868 | VFS calls iput(). If you define this method, you must call | |
869 | iput() yourself | |
870 | ||
c23fbb6b | 871 | d_dname: called when the pathname of a dentry should be generated. |
d9195881 | 872 | Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay |
c23fbb6b | 873 | pathname generation. (Instead of doing it when dentry is created, |
d9195881 | 874 | it's done only when the path is needed.). Real filesystems probably |
c23fbb6b ED |
875 | dont want to use it, because their dentries are present in global |
876 | dcache hash, so their hash should be an invariant. As no lock is | |
877 | held, d_dname() should not try to modify the dentry itself, unless | |
878 | appropriate SMP safety is used. CAUTION : d_path() logic is quite | |
879 | tricky. The correct way to return for example "Hello" is to put it | |
880 | at the end of the buffer, and returns a pointer to the first char. | |
881 | dynamic_dname() helper function is provided to take care of this. | |
882 | ||
883 | Example : | |
884 | ||
885 | static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) | |
886 | { | |
887 | return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", | |
888 | dentry->d_inode->i_ino); | |
889 | } | |
890 | ||
1da177e4 LT |
891 | Each dentry has a pointer to its parent dentry, as well as a hash list |
892 | of child dentries. Child dentries are basically like files in a | |
893 | directory. | |
894 | ||
5ea626aa | 895 | |
cc7d1f8f | 896 | Directory Entry Cache API |
1da177e4 LT |
897 | -------------------------- |
898 | ||
899 | There are a number of functions defined which permit a filesystem to | |
900 | manipulate dentries: | |
901 | ||
902 | dget: open a new handle for an existing dentry (this just increments | |
903 | the usage count) | |
904 | ||
905 | dput: close a handle for a dentry (decrements the usage count). If | |
906 | the usage count drops to 0, the "d_delete" method is called | |
907 | and the dentry is placed on the unused list if the dentry is | |
908 | still in its parents hash list. Putting the dentry on the | |
909 | unused list just means that if the system needs some RAM, it | |
910 | goes through the unused list of dentries and deallocates them. | |
911 | If the dentry has already been unhashed and the usage count | |
912 | drops to 0, in this case the dentry is deallocated after the | |
913 | "d_delete" method is called | |
914 | ||
915 | d_drop: this unhashes a dentry from its parents hash list. A | |
5ea626aa | 916 | subsequent call to dput() will deallocate the dentry if its |
1da177e4 LT |
917 | usage count drops to 0 |
918 | ||
919 | d_delete: delete a dentry. If there are no other open references to | |
920 | the dentry then the dentry is turned into a negative dentry | |
921 | (the d_iput() method is called). If there are other | |
922 | references, then d_drop() is called instead | |
923 | ||
924 | d_add: add a dentry to its parents hash list and then calls | |
925 | d_instantiate() | |
926 | ||
927 | d_instantiate: add a dentry to the alias hash list for the inode and | |
928 | updates the "d_inode" member. The "i_count" member in the | |
929 | inode structure should be set/incremented. If the inode | |
930 | pointer is NULL, the dentry is called a "negative | |
931 | dentry". This function is commonly called when an inode is | |
932 | created for an existing negative dentry | |
933 | ||
934 | d_lookup: look up a dentry given its parent and path name component | |
935 | It looks up the child of that given name from the dcache | |
936 | hash table. If it is found, the reference count is incremented | |
be42c4c4 | 937 | and the dentry is returned. The caller must use dput() |
1da177e4 LT |
938 | to free the dentry when it finishes using it. |
939 | ||
cbf8f0f3 PE |
940 | For further information on dentry locking, please refer to the document |
941 | Documentation/filesystems/dentry-locking.txt. | |
cc7d1f8f | 942 | |
f84e3f52 MS |
943 | Mount Options |
944 | ============= | |
945 | ||
946 | Parsing options | |
947 | --------------- | |
948 | ||
949 | On mount and remount the filesystem is passed a string containing a | |
950 | comma separated list of mount options. The options can have either of | |
951 | these forms: | |
952 | ||
953 | option | |
954 | option=value | |
955 | ||
956 | The <linux/parser.h> header defines an API that helps parse these | |
957 | options. There are plenty of examples on how to use it in existing | |
958 | filesystems. | |
959 | ||
960 | Showing options | |
961 | --------------- | |
962 | ||
963 | If a filesystem accepts mount options, it must define show_options() | |
964 | to show all the currently active options. The rules are: | |
965 | ||
966 | - options MUST be shown which are not default or their values differ | |
967 | from the default | |
968 | ||
969 | - options MAY be shown which are enabled by default or have their | |
970 | default value | |
971 | ||
972 | Options used only internally between a mount helper and the kernel | |
973 | (such as file descriptors), or which only have an effect during the | |
974 | mounting (such as ones controlling the creation of a journal) are exempt | |
975 | from the above rules. | |
976 | ||
977 | The underlying reason for the above rules is to make sure, that a | |
978 | mount can be accurately replicated (e.g. umounting and mounting again) | |
979 | based on the information found in /proc/mounts. | |
980 | ||
981 | A simple method of saving options at mount/remount time and showing | |
982 | them is provided with the save_mount_options() and | |
983 | generic_show_options() helper functions. Please note, that using | |
984 | these may have drawbacks. For more info see header comments for these | |
985 | functions in fs/namespace.c. | |
cc7d1f8f PE |
986 | |
987 | Resources | |
988 | ========= | |
989 | ||
990 | (Note some of these resources are not up-to-date with the latest kernel | |
991 | version.) | |
992 | ||
993 | Creating Linux virtual filesystems. 2002 | |
994 | <http://lwn.net/Articles/13325/> | |
995 | ||
996 | The Linux Virtual File-system Layer by Neil Brown. 1999 | |
997 | <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> | |
998 | ||
999 | A tour of the Linux VFS by Michael K. Johnson. 1996 | |
1000 | <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> | |
1001 | ||
1002 | A small trail through the Linux kernel by Andries Brouwer. 2001 | |
1003 | <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> |