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1
2 Overview of the Linux Virtual File System
3
4 Original author: Richard Gooch <rgooch@atnf.csiro.au>
5
6 Last updated on June 24, 2007.
7
8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
10
11 This file is released under the GPLv2.
12
13
14Introduction
15============
16
17The Virtual File System (also known as the Virtual Filesystem Switch)
18is the software layer in the kernel that provides the filesystem
19interface to userspace programs. It also provides an abstraction
20within the kernel which allows different filesystem implementations to
21coexist.
22
23VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
24on are called from a process context. Filesystem locking is described
25in the document Documentation/filesystems/Locking.
26
27
28Directory Entry Cache (dcache)
29------------------------------
30
31The VFS implements the open(2), stat(2), chmod(2), and similar system
32calls. The pathname argument that is passed to them is used by the VFS
33to search through the directory entry cache (also known as the dentry
34cache or dcache). This provides a very fast look-up mechanism to
35translate a pathname (filename) into a specific dentry. Dentries live
36in RAM and are never saved to disc: they exist only for performance.
37
38The dentry cache is meant to be a view into your entire filespace. As
39most computers cannot fit all dentries in the RAM at the same time,
40some bits of the cache are missing. In order to resolve your pathname
41into a dentry, the VFS may have to resort to creating dentries along
42the way, and then loading the inode. This is done by looking up the
43inode.
44
45
46The Inode Object
47----------------
48
49An individual dentry usually has a pointer to an inode. Inodes are
50filesystem objects such as regular files, directories, FIFOs and other
51beasts. They live either on the disc (for block device filesystems)
52or in the memory (for pseudo filesystems). Inodes that live on the
53disc are copied into the memory when required and changes to the inode
54are written back to disc. A single inode can be pointed to by multiple
55dentries (hard links, for example, do this).
56
57To look up an inode requires that the VFS calls the lookup() method of
58the parent directory inode. This method is installed by the specific
59filesystem implementation that the inode lives in. Once the VFS has
60the required dentry (and hence the inode), we can do all those boring
61things like open(2) the file, or stat(2) it to peek at the inode
62data. The stat(2) operation is fairly simple: once the VFS has the
63dentry, it peeks at the inode data and passes some of it back to
64userspace.
65
66
67The File Object
68---------------
69
70Opening a file requires another operation: allocation of a file
71structure (this is the kernel-side implementation of file
72descriptors). The freshly allocated file structure is initialized with
73a pointer to the dentry and a set of file operation member functions.
74These are taken from the inode data. The open() file method is then
75called so the specific filesystem implementation can do its work. You
76can see that this is another switch performed by the VFS. The file
77structure is placed into the file descriptor table for the process.
78
79Reading, writing and closing files (and other assorted VFS operations)
80is done by using the userspace file descriptor to grab the appropriate
81file structure, and then calling the required file structure method to
82do whatever is required. For as long as the file is open, it keeps the
83dentry in use, which in turn means that the VFS inode is still in use.
84
85
86Registering and Mounting a Filesystem
87=====================================
88
89To register and unregister a filesystem, use the following API
90functions:
91
92 #include <linux/fs.h>
93
94 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
96
97The passed struct file_system_type describes your filesystem. When a
98request is made to mount a filesystem onto a directory in your namespace,
99the VFS will call the appropriate mount() method for the specific
100filesystem. New vfsmount referring to the tree returned by ->mount()
101will be attached to the mountpoint, so that when pathname resolution
102reaches the mountpoint it will jump into the root of that vfsmount.
103
104You can see all filesystems that are registered to the kernel in the
105file /proc/filesystems.
106
107
108struct file_system_type
109-----------------------
110
111This describes the filesystem. As of kernel 2.6.39, the following
112members are defined:
113
114struct file_system_type {
115 const char *name;
116 int fs_flags;
117 struct dentry *(*mount) (struct file_system_type *, int,
118 const char *, void *);
119 void (*kill_sb) (struct super_block *);
120 struct module *owner;
121 struct file_system_type * next;
122 struct list_head fs_supers;
123 struct lock_class_key s_lock_key;
124 struct lock_class_key s_umount_key;
125};
126
127 name: the name of the filesystem type, such as "ext2", "iso9660",
128 "msdos" and so on
129
130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131
132 mount: the method to call when a new instance of this
133 filesystem should be mounted
134
135 kill_sb: the method to call when an instance of this filesystem
136 should be shut down
137
138 owner: for internal VFS use: you should initialize this to THIS_MODULE in
139 most cases.
140
141 next: for internal VFS use: you should initialize this to NULL
142
143 s_lock_key, s_umount_key: lockdep-specific
144
145The mount() method has the following arguments:
146
147 struct file_system_type *fs_type: describes the filesystem, partly initialized
148 by the specific filesystem code
149
150 int flags: mount flags
151
152 const char *dev_name: the device name we are mounting.
153
154 void *data: arbitrary mount options, usually comes as an ASCII
155 string (see "Mount Options" section)
156
157The mount() method must return the root dentry of the tree requested by
158caller. An active reference to its superblock must be grabbed and the
159superblock must be locked. On failure it should return ERR_PTR(error).
160
161The arguments match those of mount(2) and their interpretation
162depends on filesystem type. E.g. for block filesystems, dev_name is
163interpreted as block device name, that device is opened and if it
164contains a suitable filesystem image the method creates and initializes
165struct super_block accordingly, returning its root dentry to caller.
166
167->mount() may choose to return a subtree of existing filesystem - it
168doesn't have to create a new one. The main result from the caller's
169point of view is a reference to dentry at the root of (sub)tree to
170be attached; creation of new superblock is a common side effect.
171
172The most interesting member of the superblock structure that the
173mount() method fills in is the "s_op" field. This is a pointer to
174a "struct super_operations" which describes the next level of the
175filesystem implementation.
176
177Usually, a filesystem uses one of the generic mount() implementations
178and provides a fill_super() callback instead. The generic variants are:
179
180 mount_bdev: mount a filesystem residing on a block device
181
182 mount_nodev: mount a filesystem that is not backed by a device
183
184 mount_single: mount a filesystem which shares the instance between
185 all mounts
186
187A fill_super() callback implementation has the following arguments:
188
189 struct super_block *sb: the superblock structure. The callback
190 must initialize this properly.
191
192 void *data: arbitrary mount options, usually comes as an ASCII
193 string (see "Mount Options" section)
194
195 int silent: whether or not to be silent on error
196
197
198The Superblock Object
199=====================
200
201A superblock object represents a mounted filesystem.
202
203
204struct super_operations
205-----------------------
206
207This describes how the VFS can manipulate the superblock of your
208filesystem. As of kernel 2.6.22, the following members are defined:
209
210struct super_operations {
211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
213
214 void (*dirty_inode) (struct inode *, int flags);
215 int (*write_inode) (struct inode *, int);
216 void (*drop_inode) (struct inode *);
217 void (*delete_inode) (struct inode *);
218 void (*put_super) (struct super_block *);
219 int (*sync_fs)(struct super_block *sb, int wait);
220 int (*freeze_fs) (struct super_block *);
221 int (*unfreeze_fs) (struct super_block *);
222 int (*statfs) (struct dentry *, struct kstatfs *);
223 int (*remount_fs) (struct super_block *, int *, char *);
224 void (*clear_inode) (struct inode *);
225 void (*umount_begin) (struct super_block *);
226
227 int (*show_options)(struct seq_file *, struct dentry *);
228
229 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
230 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
231 int (*nr_cached_objects)(struct super_block *);
232 void (*free_cached_objects)(struct super_block *, int);
233};
234
235All methods are called without any locks being held, unless otherwise
236noted. This means that most methods can block safely. All methods are
237only called from a process context (i.e. not from an interrupt handler
238or bottom half).
239
240 alloc_inode: this method is called by alloc_inode() to allocate memory
241 for struct inode and initialize it. If this function is not
242 defined, a simple 'struct inode' is allocated. Normally
243 alloc_inode will be used to allocate a larger structure which
244 contains a 'struct inode' embedded within it.
245
246 destroy_inode: this method is called by destroy_inode() to release
247 resources allocated for struct inode. It is only required if
248 ->alloc_inode was defined and simply undoes anything done by
249 ->alloc_inode.
250
251 dirty_inode: this method is called by the VFS to mark an inode dirty.
252
253 write_inode: this method is called when the VFS needs to write an
254 inode to disc. The second parameter indicates whether the write
255 should be synchronous or not, not all filesystems check this flag.
256
257 drop_inode: called when the last access to the inode is dropped,
258 with the inode->i_lock spinlock held.
259
260 This method should be either NULL (normal UNIX filesystem
261 semantics) or "generic_delete_inode" (for filesystems that do not
262 want to cache inodes - causing "delete_inode" to always be
263 called regardless of the value of i_nlink)
264
265 The "generic_delete_inode()" behavior is equivalent to the
266 old practice of using "force_delete" in the put_inode() case,
267 but does not have the races that the "force_delete()" approach
268 had.
269
270 delete_inode: called when the VFS wants to delete an inode
271
272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
274
275 sync_fs: called when VFS is writing out all dirty data associated with
276 a superblock. The second parameter indicates whether the method
277 should wait until the write out has been completed. Optional.
278
279 freeze_fs: called when VFS is locking a filesystem and
280 forcing it into a consistent state. This method is currently
281 used by the Logical Volume Manager (LVM).
282
283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
284 again.
285
286 statfs: called when the VFS needs to get filesystem statistics.
287
288 remount_fs: called when the filesystem is remounted. This is called
289 with the kernel lock held
290
291 clear_inode: called then the VFS clears the inode. Optional
292
293 umount_begin: called when the VFS is unmounting a filesystem.
294
295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
297
298 quota_read: called by the VFS to read from filesystem quota file.
299
300 quota_write: called by the VFS to write to filesystem quota file.
301
302 nr_cached_objects: called by the sb cache shrinking function for the
303 filesystem to return the number of freeable cached objects it contains.
304 Optional.
305
306 free_cache_objects: called by the sb cache shrinking function for the
307 filesystem to scan the number of objects indicated to try to free them.
308 Optional, but any filesystem implementing this method needs to also
309 implement ->nr_cached_objects for it to be called correctly.
310
311 We can't do anything with any errors that the filesystem might
312 encountered, hence the void return type. This will never be called if
313 the VM is trying to reclaim under GFP_NOFS conditions, hence this
314 method does not need to handle that situation itself.
315
316 Implementations must include conditional reschedule calls inside any
317 scanning loop that is done. This allows the VFS to determine
318 appropriate scan batch sizes without having to worry about whether
319 implementations will cause holdoff problems due to large scan batch
320 sizes.
321
322Whoever sets up the inode is responsible for filling in the "i_op" field. This
323is a pointer to a "struct inode_operations" which describes the methods that
324can be performed on individual inodes.
325
326
327The Inode Object
328================
329
330An inode object represents an object within the filesystem.
331
332
333struct inode_operations
334-----------------------
335
336This describes how the VFS can manipulate an inode in your
337filesystem. As of kernel 2.6.22, the following members are defined:
338
339struct inode_operations {
340 int (*create) (struct inode *,struct dentry *, umode_t, bool);
341 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
342 int (*link) (struct dentry *,struct inode *,struct dentry *);
343 int (*unlink) (struct inode *,struct dentry *);
344 int (*symlink) (struct inode *,struct dentry *,const char *);
345 int (*mkdir) (struct inode *,struct dentry *,umode_t);
346 int (*rmdir) (struct inode *,struct dentry *);
347 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
348 int (*rename) (struct inode *, struct dentry *,
349 struct inode *, struct dentry *);
350 int (*rename2) (struct inode *, struct dentry *,
351 struct inode *, struct dentry *, unsigned int);
352 int (*readlink) (struct dentry *, char __user *,int);
353 const char *(*get_link) (struct dentry *, struct inode *,
354 struct delayed_call *);
355 int (*permission) (struct inode *, int);
356 int (*get_acl)(struct inode *, int);
357 int (*setattr) (struct dentry *, struct iattr *);
358 int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
359 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
360 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
361 ssize_t (*listxattr) (struct dentry *, char *, size_t);
362 int (*removexattr) (struct dentry *, const char *);
363 void (*update_time)(struct inode *, struct timespec *, int);
364 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
365 unsigned open_flag, umode_t create_mode, int *opened);
366 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
367 int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
368};
369
370Again, all methods are called without any locks being held, unless
371otherwise noted.
372
373 create: called by the open(2) and creat(2) system calls. Only
374 required if you want to support regular files. The dentry you
375 get should not have an inode (i.e. it should be a negative
376 dentry). Here you will probably call d_instantiate() with the
377 dentry and the newly created inode
378
379 lookup: called when the VFS needs to look up an inode in a parent
380 directory. The name to look for is found in the dentry. This
381 method must call d_add() to insert the found inode into the
382 dentry. The "i_count" field in the inode structure should be
383 incremented. If the named inode does not exist a NULL inode
384 should be inserted into the dentry (this is called a negative
385 dentry). Returning an error code from this routine must only
386 be done on a real error, otherwise creating inodes with system
387 calls like create(2), mknod(2), mkdir(2) and so on will fail.
388 If you wish to overload the dentry methods then you should
389 initialise the "d_dop" field in the dentry; this is a pointer
390 to a struct "dentry_operations".
391 This method is called with the directory inode semaphore held
392
393 link: called by the link(2) system call. Only required if you want
394 to support hard links. You will probably need to call
395 d_instantiate() just as you would in the create() method
396
397 unlink: called by the unlink(2) system call. Only required if you
398 want to support deleting inodes
399
400 symlink: called by the symlink(2) system call. Only required if you
401 want to support symlinks. You will probably need to call
402 d_instantiate() just as you would in the create() method
403
404 mkdir: called by the mkdir(2) system call. Only required if you want
405 to support creating subdirectories. You will probably need to
406 call d_instantiate() just as you would in the create() method
407
408 rmdir: called by the rmdir(2) system call. Only required if you want
409 to support deleting subdirectories
410
411 mknod: called by the mknod(2) system call to create a device (char,
412 block) inode or a named pipe (FIFO) or socket. Only required
413 if you want to support creating these types of inodes. You
414 will probably need to call d_instantiate() just as you would
415 in the create() method
416
417 rename: called by the rename(2) system call to rename the object to
418 have the parent and name given by the second inode and dentry.
419
420 rename2: this has an additional flags argument compared to rename.
421 If no flags are supported by the filesystem then this method
422 need not be implemented. If some flags are supported then the
423 filesystem must return -EINVAL for any unsupported or unknown
424 flags. Currently the following flags are implemented:
425 (1) RENAME_NOREPLACE: this flag indicates that if the target
426 of the rename exists the rename should fail with -EEXIST
427 instead of replacing the target. The VFS already checks for
428 existence, so for local filesystems the RENAME_NOREPLACE
429 implementation is equivalent to plain rename.
430 (2) RENAME_EXCHANGE: exchange source and target. Both must
431 exist; this is checked by the VFS. Unlike plain rename,
432 source and target may be of different type.
433
434 readlink: called by the readlink(2) system call. Only required if
435 you want to support reading symbolic links
436
437 get_link: called by the VFS to follow a symbolic link to the
438 inode it points to. Only required if you want to support
439 symbolic links. This method returns the symlink body
440 to traverse (and possibly resets the current position with
441 nd_jump_link()). If the body won't go away until the inode
442 is gone, nothing else is needed; if it needs to be otherwise
443 pinned, arrange for its release by having get_link(..., ..., done)
444 do set_delayed_call(done, destructor, argument).
445 In that case destructor(argument) will be called once VFS is
446 done with the body you've returned.
447 May be called in RCU mode; that is indicated by NULL dentry
448 argument. If request can't be handled without leaving RCU mode,
449 have it return ERR_PTR(-ECHILD).
450
451 permission: called by the VFS to check for access rights on a POSIX-like
452 filesystem.
453
454 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
455 mode, the filesystem must check the permission without blocking or
456 storing to the inode.
457
458 If a situation is encountered that rcu-walk cannot handle, return
459 -ECHILD and it will be called again in ref-walk mode.
460
461 setattr: called by the VFS to set attributes for a file. This method
462 is called by chmod(2) and related system calls.
463
464 getattr: called by the VFS to get attributes of a file. This method
465 is called by stat(2) and related system calls.
466
467 setxattr: called by the VFS to set an extended attribute for a file.
468 Extended attribute is a name:value pair associated with an
469 inode. This method is called by setxattr(2) system call.
470
471 getxattr: called by the VFS to retrieve the value of an extended
472 attribute name. This method is called by getxattr(2) function
473 call.
474
475 listxattr: called by the VFS to list all extended attributes for a
476 given file. This method is called by listxattr(2) system call.
477
478 removexattr: called by the VFS to remove an extended attribute from
479 a file. This method is called by removexattr(2) system call.
480
481 update_time: called by the VFS to update a specific time or the i_version of
482 an inode. If this is not defined the VFS will update the inode itself
483 and call mark_inode_dirty_sync.
484
485 atomic_open: called on the last component of an open. Using this optional
486 method the filesystem can look up, possibly create and open the file in
487 one atomic operation. If it cannot perform this (e.g. the file type
488 turned out to be wrong) it may signal this by returning 1 instead of
489 usual 0 or -ve . This method is only called if the last component is
490 negative or needs lookup. Cached positive dentries are still handled by
491 f_op->open(). If the file was created, the FILE_CREATED flag should be
492 set in "opened". In case of O_EXCL the method must only succeed if the
493 file didn't exist and hence FILE_CREATED shall always be set on success.
494
495 tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
496 atomically creating, opening and unlinking a file in given directory.
497
498The Address Space Object
499========================
500
501The address space object is used to group and manage pages in the page
502cache. It can be used to keep track of the pages in a file (or
503anything else) and also track the mapping of sections of the file into
504process address spaces.
505
506There are a number of distinct yet related services that an
507address-space can provide. These include communicating memory
508pressure, page lookup by address, and keeping track of pages tagged as
509Dirty or Writeback.
510
511The first can be used independently to the others. The VM can try to
512either write dirty pages in order to clean them, or release clean
513pages in order to reuse them. To do this it can call the ->writepage
514method on dirty pages, and ->releasepage on clean pages with
515PagePrivate set. Clean pages without PagePrivate and with no external
516references will be released without notice being given to the
517address_space.
518
519To achieve this functionality, pages need to be placed on an LRU with
520lru_cache_add and mark_page_active needs to be called whenever the
521page is used.
522
523Pages are normally kept in a radix tree index by ->index. This tree
524maintains information about the PG_Dirty and PG_Writeback status of
525each page, so that pages with either of these flags can be found
526quickly.
527
528The Dirty tag is primarily used by mpage_writepages - the default
529->writepages method. It uses the tag to find dirty pages to call
530->writepage on. If mpage_writepages is not used (i.e. the address
531provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
532almost unused. write_inode_now and sync_inode do use it (through
533__sync_single_inode) to check if ->writepages has been successful in
534writing out the whole address_space.
535
536The Writeback tag is used by filemap*wait* and sync_page* functions,
537via filemap_fdatawait_range, to wait for all writeback to
538complete. While waiting ->sync_page (if defined) will be called on
539each page that is found to require writeback.
540
541An address_space handler may attach extra information to a page,
542typically using the 'private' field in the 'struct page'. If such
543information is attached, the PG_Private flag should be set. This will
544cause various VM routines to make extra calls into the address_space
545handler to deal with that data.
546
547An address space acts as an intermediate between storage and
548application. Data is read into the address space a whole page at a
549time, and provided to the application either by copying of the page,
550or by memory-mapping the page.
551Data is written into the address space by the application, and then
552written-back to storage typically in whole pages, however the
553address_space has finer control of write sizes.
554
555The read process essentially only requires 'readpage'. The write
556process is more complicated and uses write_begin/write_end or
557set_page_dirty to write data into the address_space, and writepage,
558sync_page, and writepages to writeback data to storage.
559
560Adding and removing pages to/from an address_space is protected by the
561inode's i_mutex.
562
563When data is written to a page, the PG_Dirty flag should be set. It
564typically remains set until writepage asks for it to be written. This
565should clear PG_Dirty and set PG_Writeback. It can be actually
566written at any point after PG_Dirty is clear. Once it is known to be
567safe, PG_Writeback is cleared.
568
569Writeback makes use of a writeback_control structure...
570
571struct address_space_operations
572-------------------------------
573
574This describes how the VFS can manipulate mapping of a file to page cache in
575your filesystem. The following members are defined:
576
577struct address_space_operations {
578 int (*writepage)(struct page *page, struct writeback_control *wbc);
579 int (*readpage)(struct file *, struct page *);
580 int (*writepages)(struct address_space *, struct writeback_control *);
581 int (*set_page_dirty)(struct page *page);
582 int (*readpages)(struct file *filp, struct address_space *mapping,
583 struct list_head *pages, unsigned nr_pages);
584 int (*write_begin)(struct file *, struct address_space *mapping,
585 loff_t pos, unsigned len, unsigned flags,
586 struct page **pagep, void **fsdata);
587 int (*write_end)(struct file *, struct address_space *mapping,
588 loff_t pos, unsigned len, unsigned copied,
589 struct page *page, void *fsdata);
590 sector_t (*bmap)(struct address_space *, sector_t);
591 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
592 int (*releasepage) (struct page *, int);
593 void (*freepage)(struct page *);
594 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t offset);
595 /* migrate the contents of a page to the specified target */
596 int (*migratepage) (struct page *, struct page *);
597 int (*launder_page) (struct page *);
598 int (*is_partially_uptodate) (struct page *, unsigned long,
599 unsigned long);
600 void (*is_dirty_writeback) (struct page *, bool *, bool *);
601 int (*error_remove_page) (struct mapping *mapping, struct page *page);
602 int (*swap_activate)(struct file *);
603 int (*swap_deactivate)(struct file *);
604};
605
606 writepage: called by the VM to write a dirty page to backing store.
607 This may happen for data integrity reasons (i.e. 'sync'), or
608 to free up memory (flush). The difference can be seen in
609 wbc->sync_mode.
610 The PG_Dirty flag has been cleared and PageLocked is true.
611 writepage should start writeout, should set PG_Writeback,
612 and should make sure the page is unlocked, either synchronously
613 or asynchronously when the write operation completes.
614
615 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
616 try too hard if there are problems, and may choose to write out
617 other pages from the mapping if that is easier (e.g. due to
618 internal dependencies). If it chooses not to start writeout, it
619 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
620 calling ->writepage on that page.
621
622 See the file "Locking" for more details.
623
624 readpage: called by the VM to read a page from backing store.
625 The page will be Locked when readpage is called, and should be
626 unlocked and marked uptodate once the read completes.
627 If ->readpage discovers that it needs to unlock the page for
628 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
629 In this case, the page will be relocated, relocked and if
630 that all succeeds, ->readpage will be called again.
631
632 writepages: called by the VM to write out pages associated with the
633 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
634 the writeback_control will specify a range of pages that must be
635 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
636 and that many pages should be written if possible.
637 If no ->writepages is given, then mpage_writepages is used
638 instead. This will choose pages from the address space that are
639 tagged as DIRTY and will pass them to ->writepage.
640
641 set_page_dirty: called by the VM to set a page dirty.
642 This is particularly needed if an address space attaches
643 private data to a page, and that data needs to be updated when
644 a page is dirtied. This is called, for example, when a memory
645 mapped page gets modified.
646 If defined, it should set the PageDirty flag, and the
647 PAGECACHE_TAG_DIRTY tag in the radix tree.
648
649 readpages: called by the VM to read pages associated with the address_space
650 object. This is essentially just a vector version of
651 readpage. Instead of just one page, several pages are
652 requested.
653 readpages is only used for read-ahead, so read errors are
654 ignored. If anything goes wrong, feel free to give up.
655
656 write_begin:
657 Called by the generic buffered write code to ask the filesystem to
658 prepare to write len bytes at the given offset in the file. The
659 address_space should check that the write will be able to complete,
660 by allocating space if necessary and doing any other internal
661 housekeeping. If the write will update parts of any basic-blocks on
662 storage, then those blocks should be pre-read (if they haven't been
663 read already) so that the updated blocks can be written out properly.
664
665 The filesystem must return the locked pagecache page for the specified
666 offset, in *pagep, for the caller to write into.
667
668 It must be able to cope with short writes (where the length passed to
669 write_begin is greater than the number of bytes copied into the page).
670
671 flags is a field for AOP_FLAG_xxx flags, described in
672 include/linux/fs.h.
673
674 A void * may be returned in fsdata, which then gets passed into
675 write_end.
676
677 Returns 0 on success; < 0 on failure (which is the error code), in
678 which case write_end is not called.
679
680 write_end: After a successful write_begin, and data copy, write_end must
681 be called. len is the original len passed to write_begin, and copied
682 is the amount that was able to be copied (copied == len is always true
683 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
684
685 The filesystem must take care of unlocking the page and releasing it
686 refcount, and updating i_size.
687
688 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
689 that were able to be copied into pagecache.
690
691 bmap: called by the VFS to map a logical block offset within object to
692 physical block number. This method is used by the FIBMAP
693 ioctl and for working with swap-files. To be able to swap to
694 a file, the file must have a stable mapping to a block
695 device. The swap system does not go through the filesystem
696 but instead uses bmap to find out where the blocks in the file
697 are and uses those addresses directly.
698
699 dentry_open: *WARNING: probably going away soon, do not use!* This is an
700 alternative to f_op->open(), the difference is that this method may open
701 a file not necessarily originating from the same filesystem as the one
702 i_op->open() was called on. It may be useful for stacking filesystems
703 which want to allow native I/O directly on underlying files.
704
705
706 invalidatepage: If a page has PagePrivate set, then invalidatepage
707 will be called when part or all of the page is to be removed
708 from the address space. This generally corresponds to either a
709 truncation, punch hole or a complete invalidation of the address
710 space (in the latter case 'offset' will always be 0 and 'length'
711 will be PAGE_SIZE). Any private data associated with the page
712 should be updated to reflect this truncation. If offset is 0 and
713 length is PAGE_SIZE, then the private data should be released,
714 because the page must be able to be completely discarded. This may
715 be done by calling the ->releasepage function, but in this case the
716 release MUST succeed.
717
718 releasepage: releasepage is called on PagePrivate pages to indicate
719 that the page should be freed if possible. ->releasepage
720 should remove any private data from the page and clear the
721 PagePrivate flag. If releasepage() fails for some reason, it must
722 indicate failure with a 0 return value.
723 releasepage() is used in two distinct though related cases. The
724 first is when the VM finds a clean page with no active users and
725 wants to make it a free page. If ->releasepage succeeds, the
726 page will be removed from the address_space and become free.
727
728 The second case is when a request has been made to invalidate
729 some or all pages in an address_space. This can happen
730 through the fadvice(POSIX_FADV_DONTNEED) system call or by the
731 filesystem explicitly requesting it as nfs and 9fs do (when
732 they believe the cache may be out of date with storage) by
733 calling invalidate_inode_pages2().
734 If the filesystem makes such a call, and needs to be certain
735 that all pages are invalidated, then its releasepage will
736 need to ensure this. Possibly it can clear the PageUptodate
737 bit if it cannot free private data yet.
738
739 freepage: freepage is called once the page is no longer visible in
740 the page cache in order to allow the cleanup of any private
741 data. Since it may be called by the memory reclaimer, it
742 should not assume that the original address_space mapping still
743 exists, and it should not block.
744
745 direct_IO: called by the generic read/write routines to perform
746 direct_IO - that is IO requests which bypass the page cache
747 and transfer data directly between the storage and the
748 application's address space.
749
750 migrate_page: This is used to compact the physical memory usage.
751 If the VM wants to relocate a page (maybe off a memory card
752 that is signalling imminent failure) it will pass a new page
753 and an old page to this function. migrate_page should
754 transfer any private data across and update any references
755 that it has to the page.
756
757 launder_page: Called before freeing a page - it writes back the dirty page. To
758 prevent redirtying the page, it is kept locked during the whole
759 operation.
760
761 is_partially_uptodate: Called by the VM when reading a file through the
762 pagecache when the underlying blocksize != pagesize. If the required
763 block is up to date then the read can complete without needing the IO
764 to bring the whole page up to date.
765
766 is_dirty_writeback: Called by the VM when attempting to reclaim a page.
767 The VM uses dirty and writeback information to determine if it needs
768 to stall to allow flushers a chance to complete some IO. Ordinarily
769 it can use PageDirty and PageWriteback but some filesystems have
770 more complex state (unstable pages in NFS prevent reclaim) or
771 do not set those flags due to locking problems. This callback
772 allows a filesystem to indicate to the VM if a page should be
773 treated as dirty or writeback for the purposes of stalling.
774
775 error_remove_page: normally set to generic_error_remove_page if truncation
776 is ok for this address space. Used for memory failure handling.
777 Setting this implies you deal with pages going away under you,
778 unless you have them locked or reference counts increased.
779
780 swap_activate: Called when swapon is used on a file to allocate
781 space if necessary and pin the block lookup information in
782 memory. A return value of zero indicates success,
783 in which case this file can be used to back swapspace. The
784 swapspace operations will be proxied to this address space's
785 ->swap_{out,in} methods.
786
787 swap_deactivate: Called during swapoff on files where swap_activate
788 was successful.
789
790
791The File Object
792===============
793
794A file object represents a file opened by a process.
795
796
797struct file_operations
798----------------------
799
800This describes how the VFS can manipulate an open file. As of kernel
8014.1, the following members are defined:
802
803struct file_operations {
804 struct module *owner;
805 loff_t (*llseek) (struct file *, loff_t, int);
806 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
807 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
808 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
809 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
810 int (*iterate) (struct file *, struct dir_context *);
811 unsigned int (*poll) (struct file *, struct poll_table_struct *);
812 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
813 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
814 int (*mmap) (struct file *, struct vm_area_struct *);
815 int (*mremap)(struct file *, struct vm_area_struct *);
816 int (*open) (struct inode *, struct file *);
817 int (*flush) (struct file *, fl_owner_t id);
818 int (*release) (struct inode *, struct file *);
819 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
820 int (*aio_fsync) (struct kiocb *, int datasync);
821 int (*fasync) (int, struct file *, int);
822 int (*lock) (struct file *, int, struct file_lock *);
823 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
824 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
825 int (*check_flags)(int);
826 int (*flock) (struct file *, int, struct file_lock *);
827 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
828 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
829 int (*setlease)(struct file *, long, struct file_lock **, void **);
830 long (*fallocate)(struct file *file, int mode, loff_t offset,
831 loff_t len);
832 void (*show_fdinfo)(struct seq_file *m, struct file *f);
833#ifndef CONFIG_MMU
834 unsigned (*mmap_capabilities)(struct file *);
835#endif
836};
837
838Again, all methods are called without any locks being held, unless
839otherwise noted.
840
841 llseek: called when the VFS needs to move the file position index
842
843 read: called by read(2) and related system calls
844
845 read_iter: possibly asynchronous read with iov_iter as destination
846
847 write: called by write(2) and related system calls
848
849 write_iter: possibly asynchronous write with iov_iter as source
850
851 iterate: called when the VFS needs to read the directory contents
852
853 poll: called by the VFS when a process wants to check if there is
854 activity on this file and (optionally) go to sleep until there
855 is activity. Called by the select(2) and poll(2) system calls
856
857 unlocked_ioctl: called by the ioctl(2) system call.
858
859 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
860 are used on 64 bit kernels.
861
862 mmap: called by the mmap(2) system call
863
864 open: called by the VFS when an inode should be opened. When the VFS
865 opens a file, it creates a new "struct file". It then calls the
866 open method for the newly allocated file structure. You might
867 think that the open method really belongs in
868 "struct inode_operations", and you may be right. I think it's
869 done the way it is because it makes filesystems simpler to
870 implement. The open() method is a good place to initialize the
871 "private_data" member in the file structure if you want to point
872 to a device structure
873
874 flush: called by the close(2) system call to flush a file
875
876 release: called when the last reference to an open file is closed
877
878 fsync: called by the fsync(2) system call
879
880 fasync: called by the fcntl(2) system call when asynchronous
881 (non-blocking) mode is enabled for a file
882
883 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
884 commands
885
886 get_unmapped_area: called by the mmap(2) system call
887
888 check_flags: called by the fcntl(2) system call for F_SETFL command
889
890 flock: called by the flock(2) system call
891
892 splice_write: called by the VFS to splice data from a pipe to a file. This
893 method is used by the splice(2) system call
894
895 splice_read: called by the VFS to splice data from file to a pipe. This
896 method is used by the splice(2) system call
897
898 setlease: called by the VFS to set or release a file lock lease. setlease
899 implementations should call generic_setlease to record or remove
900 the lease in the inode after setting it.
901
902 fallocate: called by the VFS to preallocate blocks or punch a hole.
903
904Note that the file operations are implemented by the specific
905filesystem in which the inode resides. When opening a device node
906(character or block special) most filesystems will call special
907support routines in the VFS which will locate the required device
908driver information. These support routines replace the filesystem file
909operations with those for the device driver, and then proceed to call
910the new open() method for the file. This is how opening a device file
911in the filesystem eventually ends up calling the device driver open()
912method.
913
914
915Directory Entry Cache (dcache)
916==============================
917
918
919struct dentry_operations
920------------------------
921
922This describes how a filesystem can overload the standard dentry
923operations. Dentries and the dcache are the domain of the VFS and the
924individual filesystem implementations. Device drivers have no business
925here. These methods may be set to NULL, as they are either optional or
926the VFS uses a default. As of kernel 2.6.22, the following members are
927defined:
928
929struct dentry_operations {
930 int (*d_revalidate)(struct dentry *, unsigned int);
931 int (*d_weak_revalidate)(struct dentry *, unsigned int);
932 int (*d_hash)(const struct dentry *, struct qstr *);
933 int (*d_compare)(const struct dentry *, const struct dentry *,
934 unsigned int, const char *, const struct qstr *);
935 int (*d_delete)(const struct dentry *);
936 void (*d_release)(struct dentry *);
937 void (*d_iput)(struct dentry *, struct inode *);
938 char *(*d_dname)(struct dentry *, char *, int);
939 struct vfsmount *(*d_automount)(struct path *);
940 int (*d_manage)(struct dentry *, bool);
941};
942
943 d_revalidate: called when the VFS needs to revalidate a dentry. This
944 is called whenever a name look-up finds a dentry in the
945 dcache. Most local filesystems leave this as NULL, because all their
946 dentries in the dcache are valid. Network filesystems are different
947 since things can change on the server without the client necessarily
948 being aware of it.
949
950 This function should return a positive value if the dentry is still
951 valid, and zero or a negative error code if it isn't.
952
953 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
954 If in rcu-walk mode, the filesystem must revalidate the dentry without
955 blocking or storing to the dentry, d_parent and d_inode should not be
956 used without care (because they can change and, in d_inode case, even
957 become NULL under us).
958
959 If a situation is encountered that rcu-walk cannot handle, return
960 -ECHILD and it will be called again in ref-walk mode.
961
962 d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
963 This is called when a path-walk ends at dentry that was not acquired by
964 doing a lookup in the parent directory. This includes "/", "." and "..",
965 as well as procfs-style symlinks and mountpoint traversal.
966
967 In this case, we are less concerned with whether the dentry is still
968 fully correct, but rather that the inode is still valid. As with
969 d_revalidate, most local filesystems will set this to NULL since their
970 dcache entries are always valid.
971
972 This function has the same return code semantics as d_revalidate.
973
974 d_weak_revalidate is only called after leaving rcu-walk mode.
975
976 d_hash: called when the VFS adds a dentry to the hash table. The first
977 dentry passed to d_hash is the parent directory that the name is
978 to be hashed into.
979
980 Same locking and synchronisation rules as d_compare regarding
981 what is safe to dereference etc.
982
983 d_compare: called to compare a dentry name with a given name. The first
984 dentry is the parent of the dentry to be compared, the second is
985 the child dentry. len and name string are properties of the dentry
986 to be compared. qstr is the name to compare it with.
987
988 Must be constant and idempotent, and should not take locks if
989 possible, and should not or store into the dentry.
990 Should not dereference pointers outside the dentry without
991 lots of care (eg. d_parent, d_inode, d_name should not be used).
992
993 However, our vfsmount is pinned, and RCU held, so the dentries and
994 inodes won't disappear, neither will our sb or filesystem module.
995 ->d_sb may be used.
996
997 It is a tricky calling convention because it needs to be called under
998 "rcu-walk", ie. without any locks or references on things.
999
1000 d_delete: called when the last reference to a dentry is dropped and the
1001 dcache is deciding whether or not to cache it. Return 1 to delete
1002 immediately, or 0 to cache the dentry. Default is NULL which means to
1003 always cache a reachable dentry. d_delete must be constant and
1004 idempotent.
1005
1006 d_release: called when a dentry is really deallocated
1007
1008 d_iput: called when a dentry loses its inode (just prior to its
1009 being deallocated). The default when this is NULL is that the
1010 VFS calls iput(). If you define this method, you must call
1011 iput() yourself
1012
1013 d_dname: called when the pathname of a dentry should be generated.
1014 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
1015 pathname generation. (Instead of doing it when dentry is created,
1016 it's done only when the path is needed.). Real filesystems probably
1017 dont want to use it, because their dentries are present in global
1018 dcache hash, so their hash should be an invariant. As no lock is
1019 held, d_dname() should not try to modify the dentry itself, unless
1020 appropriate SMP safety is used. CAUTION : d_path() logic is quite
1021 tricky. The correct way to return for example "Hello" is to put it
1022 at the end of the buffer, and returns a pointer to the first char.
1023 dynamic_dname() helper function is provided to take care of this.
1024
1025 d_automount: called when an automount dentry is to be traversed (optional).
1026 This should create a new VFS mount record and return the record to the
1027 caller. The caller is supplied with a path parameter giving the
1028 automount directory to describe the automount target and the parent
1029 VFS mount record to provide inheritable mount parameters. NULL should
1030 be returned if someone else managed to make the automount first. If
1031 the vfsmount creation failed, then an error code should be returned.
1032 If -EISDIR is returned, then the directory will be treated as an
1033 ordinary directory and returned to pathwalk to continue walking.
1034
1035 If a vfsmount is returned, the caller will attempt to mount it on the
1036 mountpoint and will remove the vfsmount from its expiration list in
1037 the case of failure. The vfsmount should be returned with 2 refs on
1038 it to prevent automatic expiration - the caller will clean up the
1039 additional ref.
1040
1041 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1042 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1043 inode being added.
1044
1045 d_manage: called to allow the filesystem to manage the transition from a
1046 dentry (optional). This allows autofs, for example, to hold up clients
1047 waiting to explore behind a 'mountpoint' whilst letting the daemon go
1048 past and construct the subtree there. 0 should be returned to let the
1049 calling process continue. -EISDIR can be returned to tell pathwalk to
1050 use this directory as an ordinary directory and to ignore anything
1051 mounted on it and not to check the automount flag. Any other error
1052 code will abort pathwalk completely.
1053
1054 If the 'rcu_walk' parameter is true, then the caller is doing a
1055 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
1056 and the caller can be asked to leave it and call again by returning
1057 -ECHILD. -EISDIR may also be returned to tell pathwalk to
1058 ignore d_automount or any mounts.
1059
1060 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1061 dentry being transited from.
1062
1063Example :
1064
1065static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1066{
1067 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1068 dentry->d_inode->i_ino);
1069}
1070
1071Each dentry has a pointer to its parent dentry, as well as a hash list
1072of child dentries. Child dentries are basically like files in a
1073directory.
1074
1075
1076Directory Entry Cache API
1077--------------------------
1078
1079There are a number of functions defined which permit a filesystem to
1080manipulate dentries:
1081
1082 dget: open a new handle for an existing dentry (this just increments
1083 the usage count)
1084
1085 dput: close a handle for a dentry (decrements the usage count). If
1086 the usage count drops to 0, and the dentry is still in its
1087 parent's hash, the "d_delete" method is called to check whether
1088 it should be cached. If it should not be cached, or if the dentry
1089 is not hashed, it is deleted. Otherwise cached dentries are put
1090 into an LRU list to be reclaimed on memory shortage.
1091
1092 d_drop: this unhashes a dentry from its parents hash list. A
1093 subsequent call to dput() will deallocate the dentry if its
1094 usage count drops to 0
1095
1096 d_delete: delete a dentry. If there are no other open references to
1097 the dentry then the dentry is turned into a negative dentry
1098 (the d_iput() method is called). If there are other
1099 references, then d_drop() is called instead
1100
1101 d_add: add a dentry to its parents hash list and then calls
1102 d_instantiate()
1103
1104 d_instantiate: add a dentry to the alias hash list for the inode and
1105 updates the "d_inode" member. The "i_count" member in the
1106 inode structure should be set/incremented. If the inode
1107 pointer is NULL, the dentry is called a "negative
1108 dentry". This function is commonly called when an inode is
1109 created for an existing negative dentry
1110
1111 d_lookup: look up a dentry given its parent and path name component
1112 It looks up the child of that given name from the dcache
1113 hash table. If it is found, the reference count is incremented
1114 and the dentry is returned. The caller must use dput()
1115 to free the dentry when it finishes using it.
1116
1117Mount Options
1118=============
1119
1120Parsing options
1121---------------
1122
1123On mount and remount the filesystem is passed a string containing a
1124comma separated list of mount options. The options can have either of
1125these forms:
1126
1127 option
1128 option=value
1129
1130The <linux/parser.h> header defines an API that helps parse these
1131options. There are plenty of examples on how to use it in existing
1132filesystems.
1133
1134Showing options
1135---------------
1136
1137If a filesystem accepts mount options, it must define show_options()
1138to show all the currently active options. The rules are:
1139
1140 - options MUST be shown which are not default or their values differ
1141 from the default
1142
1143 - options MAY be shown which are enabled by default or have their
1144 default value
1145
1146Options used only internally between a mount helper and the kernel
1147(such as file descriptors), or which only have an effect during the
1148mounting (such as ones controlling the creation of a journal) are exempt
1149from the above rules.
1150
1151The underlying reason for the above rules is to make sure, that a
1152mount can be accurately replicated (e.g. umounting and mounting again)
1153based on the information found in /proc/mounts.
1154
1155A simple method of saving options at mount/remount time and showing
1156them is provided with the save_mount_options() and
1157generic_show_options() helper functions. Please note, that using
1158these may have drawbacks. For more info see header comments for these
1159functions in fs/namespace.c.
1160
1161Resources
1162=========
1163
1164(Note some of these resources are not up-to-date with the latest kernel
1165 version.)
1166
1167Creating Linux virtual filesystems. 2002
1168 <http://lwn.net/Articles/13325/>
1169
1170The Linux Virtual File-system Layer by Neil Brown. 1999
1171 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1172
1173A tour of the Linux VFS by Michael K. Johnson. 1996
1174 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1175
1176A small trail through the Linux kernel by Andries Brouwer. 2001
1177 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>