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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 | |
cc7d1f8f | 6 | Last updated on October 28, 2005 |
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 | |
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 | |
5ea626aa | 110 | This describes the filesystem. As of kernel 2.6.13, the following |
1da177e4 LT |
111 | members are defined: |
112 | ||
113 | struct file_system_type { | |
114 | const char *name; | |
115 | int fs_flags; | |
454e2398 DH |
116 | struct 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; | |
1da177e4 LT |
122 | }; |
123 | ||
124 | name: the name of the filesystem type, such as "ext2", "iso9660", | |
125 | "msdos" and so on | |
126 | ||
127 | fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) | |
128 | ||
5ea626aa | 129 | get_sb: the method to call when a new instance of this |
1da177e4 LT |
130 | filesystem should be mounted |
131 | ||
5ea626aa PE |
132 | kill_sb: the method to call when an instance of this filesystem |
133 | should be unmounted | |
134 | ||
135 | owner: for internal VFS use: you should initialize this to THIS_MODULE in | |
136 | most cases. | |
1da177e4 | 137 | |
5ea626aa PE |
138 | next: for internal VFS use: you should initialize this to NULL |
139 | ||
140 | The get_sb() method has the following arguments: | |
1da177e4 LT |
141 | |
142 | struct super_block *sb: the superblock structure. This is partially | |
5ea626aa PE |
143 | initialized by the VFS and the rest must be initialized by the |
144 | get_sb() method | |
145 | ||
146 | int flags: mount flags | |
147 | ||
148 | const char *dev_name: the device name we are mounting. | |
1da177e4 LT |
149 | |
150 | void *data: arbitrary mount options, usually comes as an ASCII | |
151 | string | |
152 | ||
153 | int silent: whether or not to be silent on error | |
154 | ||
5ea626aa | 155 | The get_sb() method must determine if the block device specified |
1da177e4 LT |
156 | in the superblock contains a filesystem of the type the method |
157 | supports. On success the method returns the superblock pointer, on | |
158 | failure it returns NULL. | |
159 | ||
160 | The most interesting member of the superblock structure that the | |
5ea626aa | 161 | get_sb() method fills in is the "s_op" field. This is a pointer to |
1da177e4 LT |
162 | a "struct super_operations" which describes the next level of the |
163 | filesystem implementation. | |
164 | ||
e3e1bfe4 JC |
165 | Usually, a filesystem uses one of the generic get_sb() implementations |
166 | and provides a fill_super() method instead. The generic methods are: | |
5ea626aa PE |
167 | |
168 | get_sb_bdev: mount a filesystem residing on a block device | |
1da177e4 | 169 | |
5ea626aa PE |
170 | get_sb_nodev: mount a filesystem that is not backed by a device |
171 | ||
172 | get_sb_single: mount a filesystem which shares the instance between | |
173 | all mounts | |
174 | ||
175 | A fill_super() method implementation has the following arguments: | |
176 | ||
177 | struct super_block *sb: the superblock structure. The method fill_super() | |
178 | must initialize this properly. | |
179 | ||
180 | void *data: arbitrary mount options, usually comes as an ASCII | |
181 | string | |
182 | ||
183 | int silent: whether or not to be silent on error | |
184 | ||
185 | ||
cc7d1f8f PE |
186 | The Superblock Object |
187 | ===================== | |
188 | ||
189 | A superblock object represents a mounted filesystem. | |
190 | ||
191 | ||
5ea626aa | 192 | struct super_operations |
cc7d1f8f | 193 | ----------------------- |
1da177e4 LT |
194 | |
195 | This describes how the VFS can manipulate the superblock of your | |
5ea626aa | 196 | filesystem. As of kernel 2.6.13, the following members are defined: |
1da177e4 LT |
197 | |
198 | struct super_operations { | |
5ea626aa PE |
199 | struct inode *(*alloc_inode)(struct super_block *sb); |
200 | void (*destroy_inode)(struct inode *); | |
201 | ||
202 | void (*read_inode) (struct inode *); | |
203 | ||
204 | void (*dirty_inode) (struct inode *); | |
205 | int (*write_inode) (struct inode *, int); | |
206 | void (*put_inode) (struct inode *); | |
207 | void (*drop_inode) (struct inode *); | |
208 | void (*delete_inode) (struct inode *); | |
209 | void (*put_super) (struct super_block *); | |
210 | void (*write_super) (struct super_block *); | |
211 | int (*sync_fs)(struct super_block *sb, int wait); | |
212 | void (*write_super_lockfs) (struct super_block *); | |
213 | void (*unlockfs) (struct super_block *); | |
214 | int (*statfs) (struct super_block *, struct kstatfs *); | |
215 | int (*remount_fs) (struct super_block *, int *, char *); | |
216 | void (*clear_inode) (struct inode *); | |
217 | void (*umount_begin) (struct super_block *); | |
218 | ||
219 | void (*sync_inodes) (struct super_block *sb, | |
220 | struct writeback_control *wbc); | |
221 | int (*show_options)(struct seq_file *, struct vfsmount *); | |
222 | ||
223 | ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); | |
224 | ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); | |
1da177e4 LT |
225 | }; |
226 | ||
227 | All methods are called without any locks being held, unless otherwise | |
228 | noted. This means that most methods can block safely. All methods are | |
229 | only called from a process context (i.e. not from an interrupt handler | |
230 | or bottom half). | |
231 | ||
5ea626aa | 232 | alloc_inode: this method is called by inode_alloc() to allocate memory |
341546f5 N |
233 | for struct inode and initialize it. If this function is not |
234 | defined, a simple 'struct inode' is allocated. Normally | |
235 | alloc_inode will be used to allocate a larger structure which | |
236 | contains a 'struct inode' embedded within it. | |
5ea626aa PE |
237 | |
238 | destroy_inode: this method is called by destroy_inode() to release | |
341546f5 N |
239 | resources allocated for struct inode. It is only required if |
240 | ->alloc_inode was defined and simply undoes anything done by | |
241 | ->alloc_inode. | |
5ea626aa | 242 | |
1da177e4 | 243 | read_inode: this method is called to read a specific inode from the |
5ea626aa PE |
244 | mounted filesystem. The i_ino member in the struct inode is |
245 | initialized by the VFS to indicate which inode to read. Other | |
246 | members are filled in by this method. | |
247 | ||
248 | You can set this to NULL and use iget5_locked() instead of iget() | |
249 | to read inodes. This is necessary for filesystems for which the | |
250 | inode number is not sufficient to identify an inode. | |
251 | ||
252 | dirty_inode: this method is called by the VFS to mark an inode dirty. | |
1da177e4 LT |
253 | |
254 | write_inode: this method is called when the VFS needs to write an | |
255 | inode to disc. The second parameter indicates whether the write | |
256 | should be synchronous or not, not all filesystems check this flag. | |
257 | ||
258 | put_inode: called when the VFS inode is removed from the inode | |
5ea626aa | 259 | cache. |
1da177e4 LT |
260 | |
261 | drop_inode: called when the last access to the inode is dropped, | |
262 | with the inode_lock spinlock held. | |
263 | ||
5ea626aa | 264 | This method should be either NULL (normal UNIX filesystem |
1da177e4 LT |
265 | semantics) or "generic_delete_inode" (for filesystems that do not |
266 | want to cache inodes - causing "delete_inode" to always be | |
267 | called regardless of the value of i_nlink) | |
268 | ||
5ea626aa | 269 | The "generic_delete_inode()" behavior is equivalent to the |
1da177e4 LT |
270 | old practice of using "force_delete" in the put_inode() case, |
271 | but does not have the races that the "force_delete()" approach | |
272 | had. | |
273 | ||
274 | delete_inode: called when the VFS wants to delete an inode | |
275 | ||
1da177e4 LT |
276 | put_super: called when the VFS wishes to free the superblock |
277 | (i.e. unmount). This is called with the superblock lock held | |
278 | ||
279 | write_super: called when the VFS superblock needs to be written to | |
280 | disc. This method is optional | |
281 | ||
5ea626aa PE |
282 | sync_fs: called when VFS is writing out all dirty data associated with |
283 | a superblock. The second parameter indicates whether the method | |
284 | should wait until the write out has been completed. Optional. | |
285 | ||
cc7d1f8f PE |
286 | write_super_lockfs: called when VFS is locking a filesystem and |
287 | forcing it into a consistent state. This method is currently | |
288 | used by the Logical Volume Manager (LVM). | |
5ea626aa PE |
289 | |
290 | unlockfs: called when VFS is unlocking a filesystem and making it writable | |
291 | again. | |
292 | ||
1da177e4 LT |
293 | statfs: called when the VFS needs to get filesystem statistics. This |
294 | is called with the kernel lock held | |
295 | ||
296 | remount_fs: called when the filesystem is remounted. This is called | |
297 | with the kernel lock held | |
298 | ||
299 | clear_inode: called then the VFS clears the inode. Optional | |
300 | ||
5ea626aa PE |
301 | umount_begin: called when the VFS is unmounting a filesystem. |
302 | ||
303 | sync_inodes: called when the VFS is writing out dirty data associated with | |
304 | a superblock. | |
305 | ||
306 | show_options: called by the VFS to show mount options for /proc/<pid>/mounts. | |
307 | ||
308 | quota_read: called by the VFS to read from filesystem quota file. | |
309 | ||
310 | quota_write: called by the VFS to write to filesystem quota file. | |
311 | ||
1da177e4 LT |
312 | The read_inode() method is responsible for filling in the "i_op" |
313 | field. This is a pointer to a "struct inode_operations" which | |
314 | describes the methods that can be performed on individual inodes. | |
315 | ||
316 | ||
cc7d1f8f PE |
317 | The Inode Object |
318 | ================ | |
319 | ||
320 | An inode object represents an object within the filesystem. | |
321 | ||
322 | ||
5ea626aa | 323 | struct inode_operations |
cc7d1f8f | 324 | ----------------------- |
1da177e4 LT |
325 | |
326 | This describes how the VFS can manipulate an inode in your | |
5ea626aa | 327 | filesystem. As of kernel 2.6.13, the following members are defined: |
1da177e4 LT |
328 | |
329 | struct inode_operations { | |
5ea626aa PE |
330 | int (*create) (struct inode *,struct dentry *,int, struct nameidata *); |
331 | struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *); | |
1da177e4 LT |
332 | int (*link) (struct dentry *,struct inode *,struct dentry *); |
333 | int (*unlink) (struct inode *,struct dentry *); | |
334 | int (*symlink) (struct inode *,struct dentry *,const char *); | |
335 | int (*mkdir) (struct inode *,struct dentry *,int); | |
336 | int (*rmdir) (struct inode *,struct dentry *); | |
337 | int (*mknod) (struct inode *,struct dentry *,int,dev_t); | |
338 | int (*rename) (struct inode *, struct dentry *, | |
339 | struct inode *, struct dentry *); | |
5ea626aa PE |
340 | int (*readlink) (struct dentry *, char __user *,int); |
341 | void * (*follow_link) (struct dentry *, struct nameidata *); | |
342 | void (*put_link) (struct dentry *, struct nameidata *, void *); | |
1da177e4 | 343 | void (*truncate) (struct inode *); |
5ea626aa PE |
344 | int (*permission) (struct inode *, int, struct nameidata *); |
345 | int (*setattr) (struct dentry *, struct iattr *); | |
346 | int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); | |
347 | int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); | |
348 | ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); | |
349 | ssize_t (*listxattr) (struct dentry *, char *, size_t); | |
350 | int (*removexattr) (struct dentry *, const char *); | |
1da177e4 LT |
351 | }; |
352 | ||
353 | Again, all methods are called without any locks being held, unless | |
354 | otherwise noted. | |
355 | ||
1da177e4 LT |
356 | create: called by the open(2) and creat(2) system calls. Only |
357 | required if you want to support regular files. The dentry you | |
358 | get should not have an inode (i.e. it should be a negative | |
359 | dentry). Here you will probably call d_instantiate() with the | |
360 | dentry and the newly created inode | |
361 | ||
362 | lookup: called when the VFS needs to look up an inode in a parent | |
363 | directory. The name to look for is found in the dentry. This | |
364 | method must call d_add() to insert the found inode into the | |
365 | dentry. The "i_count" field in the inode structure should be | |
366 | incremented. If the named inode does not exist a NULL inode | |
367 | should be inserted into the dentry (this is called a negative | |
368 | dentry). Returning an error code from this routine must only | |
369 | be done on a real error, otherwise creating inodes with system | |
370 | calls like create(2), mknod(2), mkdir(2) and so on will fail. | |
371 | If you wish to overload the dentry methods then you should | |
372 | initialise the "d_dop" field in the dentry; this is a pointer | |
373 | to a struct "dentry_operations". | |
374 | This method is called with the directory inode semaphore held | |
375 | ||
376 | link: called by the link(2) system call. Only required if you want | |
377 | to support hard links. You will probably need to call | |
378 | d_instantiate() just as you would in the create() method | |
379 | ||
380 | unlink: called by the unlink(2) system call. Only required if you | |
381 | want to support deleting inodes | |
382 | ||
383 | symlink: called by the symlink(2) system call. Only required if you | |
384 | want to support symlinks. You will probably need to call | |
385 | d_instantiate() just as you would in the create() method | |
386 | ||
387 | mkdir: called by the mkdir(2) system call. Only required if you want | |
388 | to support creating subdirectories. You will probably need to | |
389 | call d_instantiate() just as you would in the create() method | |
390 | ||
391 | rmdir: called by the rmdir(2) system call. Only required if you want | |
392 | to support deleting subdirectories | |
393 | ||
394 | mknod: called by the mknod(2) system call to create a device (char, | |
395 | block) inode or a named pipe (FIFO) or socket. Only required | |
396 | if you want to support creating these types of inodes. You | |
397 | will probably need to call d_instantiate() just as you would | |
398 | in the create() method | |
399 | ||
cc7d1f8f PE |
400 | rename: called by the rename(2) system call to rename the object to |
401 | have the parent and name given by the second inode and dentry. | |
402 | ||
1da177e4 LT |
403 | readlink: called by the readlink(2) system call. Only required if |
404 | you want to support reading symbolic links | |
405 | ||
406 | follow_link: called by the VFS to follow a symbolic link to the | |
5ea626aa | 407 | inode it points to. Only required if you want to support |
cc7d1f8f | 408 | symbolic links. This method returns a void pointer cookie |
5ea626aa PE |
409 | that is passed to put_link(). |
410 | ||
411 | put_link: called by the VFS to release resources allocated by | |
cc7d1f8f PE |
412 | follow_link(). The cookie returned by follow_link() is passed |
413 | to to this method as the last parameter. It is used by | |
414 | filesystems such as NFS where page cache is not stable | |
415 | (i.e. page that was installed when the symbolic link walk | |
416 | started might not be in the page cache at the end of the | |
417 | walk). | |
418 | ||
419 | truncate: called by the VFS to change the size of a file. The | |
420 | i_size field of the inode is set to the desired size by the | |
421 | VFS before this method is called. This method is called by | |
422 | the truncate(2) system call and related functionality. | |
5ea626aa PE |
423 | |
424 | permission: called by the VFS to check for access rights on a POSIX-like | |
425 | filesystem. | |
426 | ||
cc7d1f8f PE |
427 | setattr: called by the VFS to set attributes for a file. This method |
428 | is called by chmod(2) and related system calls. | |
5ea626aa | 429 | |
cc7d1f8f PE |
430 | getattr: called by the VFS to get attributes of a file. This method |
431 | is called by stat(2) and related system calls. | |
5ea626aa PE |
432 | |
433 | setxattr: called by the VFS to set an extended attribute for a file. | |
cc7d1f8f PE |
434 | Extended attribute is a name:value pair associated with an |
435 | inode. This method is called by setxattr(2) system call. | |
436 | ||
437 | getxattr: called by the VFS to retrieve the value of an extended | |
438 | attribute name. This method is called by getxattr(2) function | |
439 | call. | |
440 | ||
441 | listxattr: called by the VFS to list all extended attributes for a | |
442 | given file. This method is called by listxattr(2) system call. | |
5ea626aa | 443 | |
cc7d1f8f PE |
444 | removexattr: called by the VFS to remove an extended attribute from |
445 | a file. This method is called by removexattr(2) system call. | |
5ea626aa | 446 | |
5ea626aa | 447 | |
cc7d1f8f PE |
448 | The Address Space Object |
449 | ======================== | |
450 | ||
341546f5 N |
451 | The address space object is used to group and manage pages in the page |
452 | cache. It can be used to keep track of the pages in a file (or | |
453 | anything else) and also track the mapping of sections of the file into | |
454 | process address spaces. | |
455 | ||
456 | There are a number of distinct yet related services that an | |
457 | address-space can provide. These include communicating memory | |
458 | pressure, page lookup by address, and keeping track of pages tagged as | |
459 | Dirty or Writeback. | |
460 | ||
a9e102b6 | 461 | The first can be used independently to the others. The VM can try to |
341546f5 N |
462 | either write dirty pages in order to clean them, or release clean |
463 | pages in order to reuse them. To do this it can call the ->writepage | |
464 | method on dirty pages, and ->releasepage on clean pages with | |
465 | PagePrivate set. Clean pages without PagePrivate and with no external | |
466 | references will be released without notice being given to the | |
467 | address_space. | |
468 | ||
a9e102b6 | 469 | To achieve this functionality, pages need to be placed on an LRU with |
341546f5 N |
470 | lru_cache_add and mark_page_active needs to be called whenever the |
471 | page is used. | |
472 | ||
473 | Pages are normally kept in a radix tree index by ->index. This tree | |
474 | maintains information about the PG_Dirty and PG_Writeback status of | |
475 | each page, so that pages with either of these flags can be found | |
476 | quickly. | |
477 | ||
478 | The Dirty tag is primarily used by mpage_writepages - the default | |
479 | ->writepages method. It uses the tag to find dirty pages to call | |
480 | ->writepage on. If mpage_writepages is not used (i.e. the address | |
a9e102b6 | 481 | provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is |
341546f5 N |
482 | almost unused. write_inode_now and sync_inode do use it (through |
483 | __sync_single_inode) to check if ->writepages has been successful in | |
484 | writing out the whole address_space. | |
485 | ||
486 | The Writeback tag is used by filemap*wait* and sync_page* functions, | |
a9e102b6 | 487 | via wait_on_page_writeback_range, to wait for all writeback to |
341546f5 | 488 | complete. While waiting ->sync_page (if defined) will be called on |
a9e102b6 | 489 | each page that is found to require writeback. |
341546f5 N |
490 | |
491 | An address_space handler may attach extra information to a page, | |
492 | typically using the 'private' field in the 'struct page'. If such | |
493 | information is attached, the PG_Private flag should be set. This will | |
a9e102b6 | 494 | cause various VM routines to make extra calls into the address_space |
341546f5 N |
495 | handler to deal with that data. |
496 | ||
497 | An address space acts as an intermediate between storage and | |
498 | application. Data is read into the address space a whole page at a | |
499 | time, and provided to the application either by copying of the page, | |
500 | or by memory-mapping the page. | |
501 | Data is written into the address space by the application, and then | |
502 | written-back to storage typically in whole pages, however the | |
a9e102b6 | 503 | address_space has finer control of write sizes. |
341546f5 N |
504 | |
505 | The read process essentially only requires 'readpage'. The write | |
506 | process is more complicated and uses prepare_write/commit_write or | |
507 | set_page_dirty to write data into the address_space, and writepage, | |
508 | sync_page, and writepages to writeback data to storage. | |
509 | ||
510 | Adding and removing pages to/from an address_space is protected by the | |
511 | inode's i_mutex. | |
512 | ||
513 | When data is written to a page, the PG_Dirty flag should be set. It | |
514 | typically remains set until writepage asks for it to be written. This | |
515 | should clear PG_Dirty and set PG_Writeback. It can be actually | |
516 | written at any point after PG_Dirty is clear. Once it is known to be | |
517 | safe, PG_Writeback is cleared. | |
518 | ||
519 | Writeback makes use of a writeback_control structure... | |
5ea626aa PE |
520 | |
521 | struct address_space_operations | |
cc7d1f8f | 522 | ------------------------------- |
5ea626aa PE |
523 | |
524 | This describes how the VFS can manipulate mapping of a file to page cache in | |
341546f5 | 525 | your filesystem. As of kernel 2.6.16, the following members are defined: |
5ea626aa PE |
526 | |
527 | struct address_space_operations { | |
528 | int (*writepage)(struct page *page, struct writeback_control *wbc); | |
529 | int (*readpage)(struct file *, struct page *); | |
530 | int (*sync_page)(struct page *); | |
531 | int (*writepages)(struct address_space *, struct writeback_control *); | |
532 | int (*set_page_dirty)(struct page *page); | |
533 | int (*readpages)(struct file *filp, struct address_space *mapping, | |
534 | struct list_head *pages, unsigned nr_pages); | |
535 | int (*prepare_write)(struct file *, struct page *, unsigned, unsigned); | |
536 | int (*commit_write)(struct file *, struct page *, unsigned, unsigned); | |
537 | sector_t (*bmap)(struct address_space *, sector_t); | |
538 | int (*invalidatepage) (struct page *, unsigned long); | |
539 | int (*releasepage) (struct page *, int); | |
540 | ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, | |
541 | loff_t offset, unsigned long nr_segs); | |
542 | struct page* (*get_xip_page)(struct address_space *, sector_t, | |
543 | int); | |
341546f5 N |
544 | /* migrate the contents of a page to the specified target */ |
545 | int (*migratepage) (struct page *, struct page *); | |
5ea626aa PE |
546 | }; |
547 | ||
341546f5 | 548 | writepage: called by the VM to write a dirty page to backing store. |
a9e102b6 | 549 | This may happen for data integrity reasons (i.e. 'sync'), or |
341546f5 N |
550 | to free up memory (flush). The difference can be seen in |
551 | wbc->sync_mode. | |
552 | The PG_Dirty flag has been cleared and PageLocked is true. | |
553 | writepage should start writeout, should set PG_Writeback, | |
554 | and should make sure the page is unlocked, either synchronously | |
555 | or asynchronously when the write operation completes. | |
556 | ||
557 | If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to | |
a9e102b6 N |
558 | try too hard if there are problems, and may choose to write out |
559 | other pages from the mapping if that is easier (e.g. due to | |
560 | internal dependencies). If it chooses not to start writeout, it | |
561 | should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep | |
341546f5 N |
562 | calling ->writepage on that page. |
563 | ||
564 | See the file "Locking" for more details. | |
5ea626aa PE |
565 | |
566 | readpage: called by the VM to read a page from backing store. | |
341546f5 N |
567 | The page will be Locked when readpage is called, and should be |
568 | unlocked and marked uptodate once the read completes. | |
569 | If ->readpage discovers that it needs to unlock the page for | |
570 | some reason, it can do so, and then return AOP_TRUNCATED_PAGE. | |
a9e102b6 | 571 | In this case, the page will be relocated, relocked and if |
341546f5 | 572 | that all succeeds, ->readpage will be called again. |
5ea626aa PE |
573 | |
574 | sync_page: called by the VM to notify the backing store to perform all | |
575 | queued I/O operations for a page. I/O operations for other pages | |
576 | associated with this address_space object may also be performed. | |
577 | ||
341546f5 N |
578 | This function is optional and is called only for pages with |
579 | PG_Writeback set while waiting for the writeback to complete. | |
580 | ||
5ea626aa | 581 | writepages: called by the VM to write out pages associated with the |
a9e102b6 N |
582 | address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then |
583 | the writeback_control will specify a range of pages that must be | |
584 | written out. If it is WBC_SYNC_NONE, then a nr_to_write is given | |
341546f5 N |
585 | and that many pages should be written if possible. |
586 | If no ->writepages is given, then mpage_writepages is used | |
a9e102b6 | 587 | instead. This will choose pages from the address space that are |
341546f5 | 588 | tagged as DIRTY and will pass them to ->writepage. |
5ea626aa PE |
589 | |
590 | set_page_dirty: called by the VM to set a page dirty. | |
341546f5 N |
591 | This is particularly needed if an address space attaches |
592 | private data to a page, and that data needs to be updated when | |
593 | a page is dirtied. This is called, for example, when a memory | |
594 | mapped page gets modified. | |
595 | If defined, it should set the PageDirty flag, and the | |
596 | PAGECACHE_TAG_DIRTY tag in the radix tree. | |
5ea626aa PE |
597 | |
598 | readpages: called by the VM to read pages associated with the address_space | |
341546f5 N |
599 | object. This is essentially just a vector version of |
600 | readpage. Instead of just one page, several pages are | |
601 | requested. | |
a9e102b6 | 602 | readpages is only used for read-ahead, so read errors are |
341546f5 | 603 | ignored. If anything goes wrong, feel free to give up. |
1da177e4 | 604 | |
5ea626aa | 605 | prepare_write: called by the generic write path in VM to set up a write |
341546f5 | 606 | request for a page. This indicates to the address space that |
a9e102b6 | 607 | the given range of bytes is about to be written. The |
341546f5 N |
608 | address_space should check that the write will be able to |
609 | complete, by allocating space if necessary and doing any other | |
a9e102b6 | 610 | internal housekeeping. If the write will update parts of |
341546f5 N |
611 | any basic-blocks on storage, then those blocks should be |
612 | pre-read (if they haven't been read already) so that the | |
613 | updated blocks can be written out properly. | |
614 | The page will be locked. If prepare_write wants to unlock the | |
615 | page it, like readpage, may do so and return | |
616 | AOP_TRUNCATED_PAGE. | |
617 | In this case the prepare_write will be retried one the lock is | |
618 | regained. | |
619 | ||
620 | commit_write: If prepare_write succeeds, new data will be copied | |
621 | into the page and then commit_write will be called. It will | |
622 | typically update the size of the file (if appropriate) and | |
623 | mark the inode as dirty, and do any other related housekeeping | |
624 | operations. It should avoid returning an error if possible - | |
625 | errors should have been handled by prepare_write. | |
5ea626aa PE |
626 | |
627 | bmap: called by the VFS to map a logical block offset within object to | |
a9e102b6 | 628 | physical block number. This method is used by the FIBMAP |
341546f5 | 629 | ioctl and for working with swap-files. To be able to swap to |
a9e102b6 | 630 | a file, the file must have a stable mapping to a block |
341546f5 N |
631 | device. The swap system does not go through the filesystem |
632 | but instead uses bmap to find out where the blocks in the file | |
633 | are and uses those addresses directly. | |
634 | ||
635 | ||
636 | invalidatepage: If a page has PagePrivate set, then invalidatepage | |
637 | will be called when part or all of the page is to be removed | |
a9e102b6 | 638 | from the address space. This generally corresponds to either a |
341546f5 N |
639 | truncation or a complete invalidation of the address space |
640 | (in the latter case 'offset' will always be 0). | |
641 | Any private data associated with the page should be updated | |
642 | to reflect this truncation. If offset is 0, then | |
643 | the private data should be released, because the page | |
644 | must be able to be completely discarded. This may be done by | |
645 | calling the ->releasepage function, but in this case the | |
646 | release MUST succeed. | |
647 | ||
648 | releasepage: releasepage is called on PagePrivate pages to indicate | |
649 | that the page should be freed if possible. ->releasepage | |
650 | should remove any private data from the page and clear the | |
651 | PagePrivate flag. It may also remove the page from the | |
652 | address_space. If this fails for some reason, it may indicate | |
653 | failure with a 0 return value. | |
654 | This is used in two distinct though related cases. The first | |
655 | is when the VM finds a clean page with no active users and | |
656 | wants to make it a free page. If ->releasepage succeeds, the | |
657 | page will be removed from the address_space and become free. | |
658 | ||
659 | The second case if when a request has been made to invalidate | |
660 | some or all pages in an address_space. This can happen | |
661 | through the fadvice(POSIX_FADV_DONTNEED) system call or by the | |
662 | filesystem explicitly requesting it as nfs and 9fs do (when | |
663 | they believe the cache may be out of date with storage) by | |
664 | calling invalidate_inode_pages2(). | |
665 | If the filesystem makes such a call, and needs to be certain | |
a9e102b6 | 666 | that all pages are invalidated, then its releasepage will |
341546f5 N |
667 | need to ensure this. Possibly it can clear the PageUptodate |
668 | bit if it cannot free private data yet. | |
669 | ||
670 | direct_IO: called by the generic read/write routines to perform | |
671 | direct_IO - that is IO requests which bypass the page cache | |
a9e102b6 | 672 | and transfer data directly between the storage and the |
341546f5 | 673 | application's address space. |
5ea626aa PE |
674 | |
675 | get_xip_page: called by the VM to translate a block number to a page. | |
676 | The page is valid until the corresponding filesystem is unmounted. | |
677 | Filesystems that want to use execute-in-place (XIP) need to implement | |
678 | it. An example implementation can be found in fs/ext2/xip.c. | |
679 | ||
341546f5 N |
680 | migrate_page: This is used to compact the physical memory usage. |
681 | If the VM wants to relocate a page (maybe off a memory card | |
682 | that is signalling imminent failure) it will pass a new page | |
683 | and an old page to this function. migrate_page should | |
684 | transfer any private data across and update any references | |
685 | that it has to the page. | |
5ea626aa | 686 | |
cc7d1f8f PE |
687 | The File Object |
688 | =============== | |
689 | ||
690 | A file object represents a file opened by a process. | |
691 | ||
692 | ||
5ea626aa | 693 | struct file_operations |
cc7d1f8f | 694 | ---------------------- |
1da177e4 LT |
695 | |
696 | This describes how the VFS can manipulate an open file. As of kernel | |
d1195c51 | 697 | 2.6.17, the following members are defined: |
1da177e4 LT |
698 | |
699 | struct file_operations { | |
700 | loff_t (*llseek) (struct file *, loff_t, int); | |
5ea626aa PE |
701 | ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); |
702 | ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t); | |
703 | ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); | |
704 | ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t); | |
1da177e4 LT |
705 | int (*readdir) (struct file *, void *, filldir_t); |
706 | unsigned int (*poll) (struct file *, struct poll_table_struct *); | |
707 | int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long); | |
5ea626aa PE |
708 | long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
709 | long (*compat_ioctl) (struct file *, unsigned int, unsigned long); | |
1da177e4 LT |
710 | int (*mmap) (struct file *, struct vm_area_struct *); |
711 | int (*open) (struct inode *, struct file *); | |
5ea626aa | 712 | int (*flush) (struct file *); |
1da177e4 | 713 | int (*release) (struct inode *, struct file *); |
5ea626aa PE |
714 | int (*fsync) (struct file *, struct dentry *, int datasync); |
715 | int (*aio_fsync) (struct kiocb *, int datasync); | |
716 | int (*fasync) (int, struct file *, int); | |
1da177e4 | 717 | int (*lock) (struct file *, int, struct file_lock *); |
5ea626aa PE |
718 | ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *); |
719 | ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *); | |
720 | ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *); | |
721 | ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); | |
722 | unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); | |
723 | int (*check_flags)(int); | |
724 | int (*dir_notify)(struct file *filp, unsigned long arg); | |
725 | int (*flock) (struct file *, int, struct file_lock *); | |
d1195c51 PE |
726 | ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned |
727 | int); | |
728 | ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned | |
729 | int); | |
1da177e4 LT |
730 | }; |
731 | ||
732 | Again, all methods are called without any locks being held, unless | |
733 | otherwise noted. | |
734 | ||
735 | llseek: called when the VFS needs to move the file position index | |
736 | ||
737 | read: called by read(2) and related system calls | |
738 | ||
5ea626aa PE |
739 | aio_read: called by io_submit(2) and other asynchronous I/O operations |
740 | ||
1da177e4 LT |
741 | write: called by write(2) and related system calls |
742 | ||
5ea626aa PE |
743 | aio_write: called by io_submit(2) and other asynchronous I/O operations |
744 | ||
1da177e4 LT |
745 | readdir: called when the VFS needs to read the directory contents |
746 | ||
747 | poll: called by the VFS when a process wants to check if there is | |
748 | activity on this file and (optionally) go to sleep until there | |
749 | is activity. Called by the select(2) and poll(2) system calls | |
750 | ||
751 | ioctl: called by the ioctl(2) system call | |
752 | ||
5ea626aa PE |
753 | unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not |
754 | require the BKL should use this method instead of the ioctl() above. | |
755 | ||
756 | compat_ioctl: called by the ioctl(2) system call when 32 bit system calls | |
757 | are used on 64 bit kernels. | |
758 | ||
1da177e4 LT |
759 | mmap: called by the mmap(2) system call |
760 | ||
761 | open: called by the VFS when an inode should be opened. When the VFS | |
5ea626aa PE |
762 | opens a file, it creates a new "struct file". It then calls the |
763 | open method for the newly allocated file structure. You might | |
764 | think that the open method really belongs in | |
765 | "struct inode_operations", and you may be right. I think it's | |
766 | done the way it is because it makes filesystems simpler to | |
767 | implement. The open() method is a good place to initialize the | |
768 | "private_data" member in the file structure if you want to point | |
769 | to a device structure | |
770 | ||
771 | flush: called by the close(2) system call to flush a file | |
1da177e4 LT |
772 | |
773 | release: called when the last reference to an open file is closed | |
774 | ||
775 | fsync: called by the fsync(2) system call | |
776 | ||
777 | fasync: called by the fcntl(2) system call when asynchronous | |
778 | (non-blocking) mode is enabled for a file | |
779 | ||
5ea626aa PE |
780 | lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW |
781 | commands | |
782 | ||
783 | readv: called by the readv(2) system call | |
784 | ||
785 | writev: called by the writev(2) system call | |
786 | ||
787 | sendfile: called by the sendfile(2) system call | |
788 | ||
789 | get_unmapped_area: called by the mmap(2) system call | |
790 | ||
791 | check_flags: called by the fcntl(2) system call for F_SETFL command | |
792 | ||
793 | dir_notify: called by the fcntl(2) system call for F_NOTIFY command | |
794 | ||
795 | flock: called by the flock(2) system call | |
796 | ||
d1195c51 PE |
797 | splice_write: called by the VFS to splice data from a pipe to a file. This |
798 | method is used by the splice(2) system call | |
799 | ||
800 | splice_read: called by the VFS to splice data from file to a pipe. This | |
801 | method is used by the splice(2) system call | |
802 | ||
1da177e4 LT |
803 | Note that the file operations are implemented by the specific |
804 | filesystem in which the inode resides. When opening a device node | |
805 | (character or block special) most filesystems will call special | |
806 | support routines in the VFS which will locate the required device | |
807 | driver information. These support routines replace the filesystem file | |
808 | operations with those for the device driver, and then proceed to call | |
809 | the new open() method for the file. This is how opening a device file | |
810 | in the filesystem eventually ends up calling the device driver open() | |
5ea626aa | 811 | method. |
1da177e4 LT |
812 | |
813 | ||
5ea626aa PE |
814 | Directory Entry Cache (dcache) |
815 | ============================== | |
816 | ||
1da177e4 LT |
817 | |
818 | struct dentry_operations | |
5ea626aa | 819 | ------------------------ |
1da177e4 LT |
820 | |
821 | This describes how a filesystem can overload the standard dentry | |
822 | operations. Dentries and the dcache are the domain of the VFS and the | |
823 | individual filesystem implementations. Device drivers have no business | |
824 | here. These methods may be set to NULL, as they are either optional or | |
5ea626aa | 825 | the VFS uses a default. As of kernel 2.6.13, the following members are |
1da177e4 LT |
826 | defined: |
827 | ||
828 | struct dentry_operations { | |
5ea626aa | 829 | int (*d_revalidate)(struct dentry *, struct nameidata *); |
1da177e4 LT |
830 | int (*d_hash) (struct dentry *, struct qstr *); |
831 | int (*d_compare) (struct dentry *, struct qstr *, struct qstr *); | |
5ea626aa | 832 | int (*d_delete)(struct dentry *); |
1da177e4 LT |
833 | void (*d_release)(struct dentry *); |
834 | void (*d_iput)(struct dentry *, struct inode *); | |
835 | }; | |
836 | ||
837 | d_revalidate: called when the VFS needs to revalidate a dentry. This | |
838 | is called whenever a name look-up finds a dentry in the | |
839 | dcache. Most filesystems leave this as NULL, because all their | |
840 | dentries in the dcache are valid | |
841 | ||
842 | d_hash: called when the VFS adds a dentry to the hash table | |
843 | ||
844 | d_compare: called when a dentry should be compared with another | |
845 | ||
846 | d_delete: called when the last reference to a dentry is | |
847 | deleted. This means no-one is using the dentry, however it is | |
848 | still valid and in the dcache | |
849 | ||
850 | d_release: called when a dentry is really deallocated | |
851 | ||
852 | d_iput: called when a dentry loses its inode (just prior to its | |
853 | being deallocated). The default when this is NULL is that the | |
854 | VFS calls iput(). If you define this method, you must call | |
855 | iput() yourself | |
856 | ||
857 | Each dentry has a pointer to its parent dentry, as well as a hash list | |
858 | of child dentries. Child dentries are basically like files in a | |
859 | directory. | |
860 | ||
5ea626aa | 861 | |
cc7d1f8f | 862 | Directory Entry Cache API |
1da177e4 LT |
863 | -------------------------- |
864 | ||
865 | There are a number of functions defined which permit a filesystem to | |
866 | manipulate dentries: | |
867 | ||
868 | dget: open a new handle for an existing dentry (this just increments | |
869 | the usage count) | |
870 | ||
871 | dput: close a handle for a dentry (decrements the usage count). If | |
872 | the usage count drops to 0, the "d_delete" method is called | |
873 | and the dentry is placed on the unused list if the dentry is | |
874 | still in its parents hash list. Putting the dentry on the | |
875 | unused list just means that if the system needs some RAM, it | |
876 | goes through the unused list of dentries and deallocates them. | |
877 | If the dentry has already been unhashed and the usage count | |
878 | drops to 0, in this case the dentry is deallocated after the | |
879 | "d_delete" method is called | |
880 | ||
881 | d_drop: this unhashes a dentry from its parents hash list. A | |
5ea626aa | 882 | subsequent call to dput() will deallocate the dentry if its |
1da177e4 LT |
883 | usage count drops to 0 |
884 | ||
885 | d_delete: delete a dentry. If there are no other open references to | |
886 | the dentry then the dentry is turned into a negative dentry | |
887 | (the d_iput() method is called). If there are other | |
888 | references, then d_drop() is called instead | |
889 | ||
890 | d_add: add a dentry to its parents hash list and then calls | |
891 | d_instantiate() | |
892 | ||
893 | d_instantiate: add a dentry to the alias hash list for the inode and | |
894 | updates the "d_inode" member. The "i_count" member in the | |
895 | inode structure should be set/incremented. If the inode | |
896 | pointer is NULL, the dentry is called a "negative | |
897 | dentry". This function is commonly called when an inode is | |
898 | created for an existing negative dentry | |
899 | ||
900 | d_lookup: look up a dentry given its parent and path name component | |
901 | It looks up the child of that given name from the dcache | |
902 | hash table. If it is found, the reference count is incremented | |
903 | and the dentry is returned. The caller must use d_put() | |
904 | to free the dentry when it finishes using it. | |
905 | ||
cbf8f0f3 PE |
906 | For further information on dentry locking, please refer to the document |
907 | Documentation/filesystems/dentry-locking.txt. | |
cc7d1f8f PE |
908 | |
909 | ||
910 | Resources | |
911 | ========= | |
912 | ||
913 | (Note some of these resources are not up-to-date with the latest kernel | |
914 | version.) | |
915 | ||
916 | Creating Linux virtual filesystems. 2002 | |
917 | <http://lwn.net/Articles/13325/> | |
918 | ||
919 | The Linux Virtual File-system Layer by Neil Brown. 1999 | |
920 | <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> | |
921 | ||
922 | A tour of the Linux VFS by Michael K. Johnson. 1996 | |
923 | <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> | |
924 | ||
925 | A small trail through the Linux kernel by Andries Brouwer. 2001 | |
926 | <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> |