2 Overview of the Linux Virtual File System
4 Original author: Richard Gooch <rgooch@atnf.csiro.au>
6 Last updated on June 24, 2007.
8 Copyright (C) 1999 Richard Gooch
9 Copyright (C) 2005 Pekka Enberg
11 This file is released under the GPLv2.
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
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.
28 Directory Entry Cache (dcache)
29 ------------------------------
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.
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
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).
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
70 Opening a file requires another operation: allocation of a file
71 structure (this is the kernel-side implementation of file
72 descriptors). The freshly allocated file structure is initialized with
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 its work. You
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.
79 Reading, writing and closing files (and other assorted VFS operations)
80 is done by using the userspace file descriptor to grab the appropriate
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.
86 Registering and Mounting a Filesystem
87 =====================================
89 To register and unregister a filesystem, use the following API
94 extern int register_filesystem(struct file_system_type *);
95 extern int unregister_filesystem(struct file_system_type *);
97 The passed struct file_system_type describes your filesystem. When a
98 request is made to mount a filesystem onto a directory in your namespace,
99 the VFS will call the appropriate mount() method for the specific
100 filesystem. New vfsmount referring to the tree returned by ->mount()
101 will be attached to the mountpoint, so that when pathname resolution
102 reaches the mountpoint it will jump into the root of that vfsmount.
104 You can see all filesystems that are registered to the kernel in the
105 file /proc/filesystems.
108 struct file_system_type
109 -----------------------
111 This describes the filesystem. As of kernel 2.6.39, the following
114 struct file_system_type {
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;
127 name: the name of the filesystem type, such as "ext2", "iso9660",
130 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
132 mount: the method to call when a new instance of this
133 filesystem should be mounted
135 kill_sb: the method to call when an instance of this filesystem
138 owner: for internal VFS use: you should initialize this to THIS_MODULE in
141 next: for internal VFS use: you should initialize this to NULL
143 s_lock_key, s_umount_key: lockdep-specific
145 The mount() method has the following arguments:
147 struct file_system_type *fs_type: describes the filesystem, partly initialized
148 by the specific filesystem code
150 int flags: mount flags
152 const char *dev_name: the device name we are mounting.
154 void *data: arbitrary mount options, usually comes as an ASCII
155 string (see "Mount Options" section)
157 The mount() method must return the root dentry of the tree requested by
158 caller. An active reference to its superblock must be grabbed and the
159 superblock must be locked. On failure it should return ERR_PTR(error).
161 The arguments match those of mount(2) and their interpretation
162 depends on filesystem type. E.g. for block filesystems, dev_name is
163 interpreted as block device name, that device is opened and if it
164 contains a suitable filesystem image the method creates and initializes
165 struct super_block accordingly, returning its root dentry to caller.
167 ->mount() may choose to return a subtree of existing filesystem - it
168 doesn't have to create a new one. The main result from the caller's
169 point of view is a reference to dentry at the root of (sub)tree to
170 be attached; creation of new superblock is a common side effect.
172 The most interesting member of the superblock structure that the
173 mount() method fills in is the "s_op" field. This is a pointer to
174 a "struct super_operations" which describes the next level of the
175 filesystem implementation.
177 Usually, a filesystem uses one of the generic mount() implementations
178 and provides a fill_super() callback instead. The generic variants are:
180 mount_bdev: mount a filesystem residing on a block device
182 mount_nodev: mount a filesystem that is not backed by a device
184 mount_single: mount a filesystem which shares the instance between
187 A fill_super() callback implementation has the following arguments:
189 struct super_block *sb: the superblock structure. The callback
190 must initialize this properly.
192 void *data: arbitrary mount options, usually comes as an ASCII
193 string (see "Mount Options" section)
195 int silent: whether or not to be silent on error
198 The Superblock Object
199 =====================
201 A superblock object represents a mounted filesystem.
204 struct super_operations
205 -----------------------
207 This describes how the VFS can manipulate the superblock of your
208 filesystem. As of kernel 2.6.22, the following members are defined:
210 struct super_operations {
211 struct inode *(*alloc_inode)(struct super_block *sb);
212 void (*destroy_inode)(struct inode *);
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 *);
227 int (*show_options)(struct seq_file *, struct dentry *);
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);
235 All methods are called without any locks being held, unless otherwise
236 noted. This means that most methods can block safely. All methods are
237 only called from a process context (i.e. not from an interrupt handler
240 alloc_inode: this method is called by inode_alloc() 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.
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
251 dirty_inode: this method is called by the VFS to mark an inode dirty.
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.
257 drop_inode: called when the last access to the inode is dropped,
258 with the inode->i_lock spinlock held.
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)
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
270 delete_inode: called when the VFS wants to delete an inode
272 put_super: called when the VFS wishes to free the superblock
273 (i.e. unmount). This is called with the superblock lock held
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.
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).
283 unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
286 statfs: called when the VFS needs to get filesystem statistics.
288 remount_fs: called when the filesystem is remounted. This is called
289 with the kernel lock held
291 clear_inode: called then the VFS clears the inode. Optional
293 umount_begin: called when the VFS is unmounting a filesystem.
295 show_options: called by the VFS to show mount options for
296 /proc/<pid>/mounts. (see "Mount Options" section)
298 quota_read: called by the VFS to read from filesystem quota file.
300 quota_write: called by the VFS to write to filesystem quota file.
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.
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.
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.
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
322 Whoever sets up the inode is responsible for filling in the "i_op" field. This
323 is a pointer to a "struct inode_operations" which describes the methods that
324 can be performed on individual inodes.
330 An inode object represents an object within the filesystem.
333 struct inode_operations
334 -----------------------
336 This describes how the VFS can manipulate an inode in your
337 filesystem. As of kernel 2.6.22, the following members are defined:
339 struct 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 (*readlink) (struct dentry *, char __user *,int);
351 void * (*follow_link) (struct dentry *, struct nameidata *);
352 void (*put_link) (struct dentry *, struct nameidata *, void *);
353 void (*truncate) (struct inode *);
354 int (*permission) (struct inode *, int);
355 int (*get_acl)(struct inode *, int);
356 int (*setattr) (struct dentry *, struct iattr *);
357 int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
358 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
359 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
360 ssize_t (*listxattr) (struct dentry *, char *, size_t);
361 int (*removexattr) (struct dentry *, const char *);
362 void (*update_time)(struct inode *, struct timespec *, int);
363 int (*atomic_open)(struct inode *, struct dentry *,
364 struct file *, unsigned open_flag,
365 umode_t create_mode, int *opened);
368 Again, all methods are called without any locks being held, unless
371 create: called by the open(2) and creat(2) system calls. Only
372 required if you want to support regular files. The dentry you
373 get should not have an inode (i.e. it should be a negative
374 dentry). Here you will probably call d_instantiate() with the
375 dentry and the newly created inode
377 lookup: called when the VFS needs to look up an inode in a parent
378 directory. The name to look for is found in the dentry. This
379 method must call d_add() to insert the found inode into the
380 dentry. The "i_count" field in the inode structure should be
381 incremented. If the named inode does not exist a NULL inode
382 should be inserted into the dentry (this is called a negative
383 dentry). Returning an error code from this routine must only
384 be done on a real error, otherwise creating inodes with system
385 calls like create(2), mknod(2), mkdir(2) and so on will fail.
386 If you wish to overload the dentry methods then you should
387 initialise the "d_dop" field in the dentry; this is a pointer
388 to a struct "dentry_operations".
389 This method is called with the directory inode semaphore held
391 link: called by the link(2) system call. Only required if you want
392 to support hard links. You will probably need to call
393 d_instantiate() just as you would in the create() method
395 unlink: called by the unlink(2) system call. Only required if you
396 want to support deleting inodes
398 symlink: called by the symlink(2) system call. Only required if you
399 want to support symlinks. You will probably need to call
400 d_instantiate() just as you would in the create() method
402 mkdir: called by the mkdir(2) system call. Only required if you want
403 to support creating subdirectories. You will probably need to
404 call d_instantiate() just as you would in the create() method
406 rmdir: called by the rmdir(2) system call. Only required if you want
407 to support deleting subdirectories
409 mknod: called by the mknod(2) system call to create a device (char,
410 block) inode or a named pipe (FIFO) or socket. Only required
411 if you want to support creating these types of inodes. You
412 will probably need to call d_instantiate() just as you would
413 in the create() method
415 rename: called by the rename(2) system call to rename the object to
416 have the parent and name given by the second inode and dentry.
418 readlink: called by the readlink(2) system call. Only required if
419 you want to support reading symbolic links
421 follow_link: called by the VFS to follow a symbolic link to the
422 inode it points to. Only required if you want to support
423 symbolic links. This method returns a void pointer cookie
424 that is passed to put_link().
426 put_link: called by the VFS to release resources allocated by
427 follow_link(). The cookie returned by follow_link() is passed
428 to this method as the last parameter. It is used by
429 filesystems such as NFS where page cache is not stable
430 (i.e. page that was installed when the symbolic link walk
431 started might not be in the page cache at the end of the
434 truncate: Deprecated. This will not be called if ->setsize is defined.
435 Called by the VFS to change the size of a file. The
436 i_size field of the inode is set to the desired size by the
437 VFS before this method is called. This method is called by
438 the truncate(2) system call and related functionality.
440 Note: ->truncate and vmtruncate are deprecated. Do not add new
441 instances/calls of these. Filesystems should be converted to do their
442 truncate sequence via ->setattr().
444 permission: called by the VFS to check for access rights on a POSIX-like
447 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
448 mode, the filesystem must check the permission without blocking or
449 storing to the inode.
451 If a situation is encountered that rcu-walk cannot handle, return
452 -ECHILD and it will be called again in ref-walk mode.
454 setattr: called by the VFS to set attributes for a file. This method
455 is called by chmod(2) and related system calls.
457 getattr: called by the VFS to get attributes of a file. This method
458 is called by stat(2) and related system calls.
460 setxattr: called by the VFS to set an extended attribute for a file.
461 Extended attribute is a name:value pair associated with an
462 inode. This method is called by setxattr(2) system call.
464 getxattr: called by the VFS to retrieve the value of an extended
465 attribute name. This method is called by getxattr(2) function
468 listxattr: called by the VFS to list all extended attributes for a
469 given file. This method is called by listxattr(2) system call.
471 removexattr: called by the VFS to remove an extended attribute from
472 a file. This method is called by removexattr(2) system call.
474 update_time: called by the VFS to update a specific time or the i_version of
475 an inode. If this is not defined the VFS will update the inode itself
476 and call mark_inode_dirty_sync.
478 atomic_open: called on the last component of an open. Using this optional
479 method the filesystem can look up, possibly create and open the file in
480 one atomic operation. If it cannot perform this (e.g. the file type
481 turned out to be wrong) it may signal this by returning 1 instead of
482 usual 0 or -ve . This method is only called if the last
483 component is negative or needs lookup. Cached positive dentries are
484 still handled by f_op->open().
486 The Address Space Object
487 ========================
489 The address space object is used to group and manage pages in the page
490 cache. It can be used to keep track of the pages in a file (or
491 anything else) and also track the mapping of sections of the file into
492 process address spaces.
494 There are a number of distinct yet related services that an
495 address-space can provide. These include communicating memory
496 pressure, page lookup by address, and keeping track of pages tagged as
499 The first can be used independently to the others. The VM can try to
500 either write dirty pages in order to clean them, or release clean
501 pages in order to reuse them. To do this it can call the ->writepage
502 method on dirty pages, and ->releasepage on clean pages with
503 PagePrivate set. Clean pages without PagePrivate and with no external
504 references will be released without notice being given to the
507 To achieve this functionality, pages need to be placed on an LRU with
508 lru_cache_add and mark_page_active needs to be called whenever the
511 Pages are normally kept in a radix tree index by ->index. This tree
512 maintains information about the PG_Dirty and PG_Writeback status of
513 each page, so that pages with either of these flags can be found
516 The Dirty tag is primarily used by mpage_writepages - the default
517 ->writepages method. It uses the tag to find dirty pages to call
518 ->writepage on. If mpage_writepages is not used (i.e. the address
519 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
520 almost unused. write_inode_now and sync_inode do use it (through
521 __sync_single_inode) to check if ->writepages has been successful in
522 writing out the whole address_space.
524 The Writeback tag is used by filemap*wait* and sync_page* functions,
525 via filemap_fdatawait_range, to wait for all writeback to
526 complete. While waiting ->sync_page (if defined) will be called on
527 each page that is found to require writeback.
529 An address_space handler may attach extra information to a page,
530 typically using the 'private' field in the 'struct page'. If such
531 information is attached, the PG_Private flag should be set. This will
532 cause various VM routines to make extra calls into the address_space
533 handler to deal with that data.
535 An address space acts as an intermediate between storage and
536 application. Data is read into the address space a whole page at a
537 time, and provided to the application either by copying of the page,
538 or by memory-mapping the page.
539 Data is written into the address space by the application, and then
540 written-back to storage typically in whole pages, however the
541 address_space has finer control of write sizes.
543 The read process essentially only requires 'readpage'. The write
544 process is more complicated and uses write_begin/write_end or
545 set_page_dirty to write data into the address_space, and writepage,
546 sync_page, and writepages to writeback data to storage.
548 Adding and removing pages to/from an address_space is protected by the
551 When data is written to a page, the PG_Dirty flag should be set. It
552 typically remains set until writepage asks for it to be written. This
553 should clear PG_Dirty and set PG_Writeback. It can be actually
554 written at any point after PG_Dirty is clear. Once it is known to be
555 safe, PG_Writeback is cleared.
557 Writeback makes use of a writeback_control structure...
559 struct address_space_operations
560 -------------------------------
562 This describes how the VFS can manipulate mapping of a file to page cache in
563 your filesystem. As of kernel 2.6.22, the following members are defined:
565 struct address_space_operations {
566 int (*writepage)(struct page *page, struct writeback_control *wbc);
567 int (*readpage)(struct file *, struct page *);
568 int (*sync_page)(struct page *);
569 int (*writepages)(struct address_space *, struct writeback_control *);
570 int (*set_page_dirty)(struct page *page);
571 int (*readpages)(struct file *filp, struct address_space *mapping,
572 struct list_head *pages, unsigned nr_pages);
573 int (*write_begin)(struct file *, struct address_space *mapping,
574 loff_t pos, unsigned len, unsigned flags,
575 struct page **pagep, void **fsdata);
576 int (*write_end)(struct file *, struct address_space *mapping,
577 loff_t pos, unsigned len, unsigned copied,
578 struct page *page, void *fsdata);
579 sector_t (*bmap)(struct address_space *, sector_t);
580 int (*invalidatepage) (struct page *, unsigned long);
581 int (*releasepage) (struct page *, int);
582 void (*freepage)(struct page *);
583 ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
584 loff_t offset, unsigned long nr_segs);
585 struct page* (*get_xip_page)(struct address_space *, sector_t,
587 /* migrate the contents of a page to the specified target */
588 int (*migratepage) (struct page *, struct page *);
589 int (*launder_page) (struct page *);
590 int (*error_remove_page) (struct mapping *mapping, struct page *page);
591 int (*swap_activate)(struct file *);
592 int (*swap_deactivate)(struct file *);
595 writepage: called by the VM to write a dirty page to backing store.
596 This may happen for data integrity reasons (i.e. 'sync'), or
597 to free up memory (flush). The difference can be seen in
599 The PG_Dirty flag has been cleared and PageLocked is true.
600 writepage should start writeout, should set PG_Writeback,
601 and should make sure the page is unlocked, either synchronously
602 or asynchronously when the write operation completes.
604 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
605 try too hard if there are problems, and may choose to write out
606 other pages from the mapping if that is easier (e.g. due to
607 internal dependencies). If it chooses not to start writeout, it
608 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
609 calling ->writepage on that page.
611 See the file "Locking" for more details.
613 readpage: called by the VM to read a page from backing store.
614 The page will be Locked when readpage is called, and should be
615 unlocked and marked uptodate once the read completes.
616 If ->readpage discovers that it needs to unlock the page for
617 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
618 In this case, the page will be relocated, relocked and if
619 that all succeeds, ->readpage will be called again.
621 sync_page: called by the VM to notify the backing store to perform all
622 queued I/O operations for a page. I/O operations for other pages
623 associated with this address_space object may also be performed.
625 This function is optional and is called only for pages with
626 PG_Writeback set while waiting for the writeback to complete.
628 writepages: called by the VM to write out pages associated with the
629 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
630 the writeback_control will specify a range of pages that must be
631 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
632 and that many pages should be written if possible.
633 If no ->writepages is given, then mpage_writepages is used
634 instead. This will choose pages from the address space that are
635 tagged as DIRTY and will pass them to ->writepage.
637 set_page_dirty: called by the VM to set a page dirty.
638 This is particularly needed if an address space attaches
639 private data to a page, and that data needs to be updated when
640 a page is dirtied. This is called, for example, when a memory
641 mapped page gets modified.
642 If defined, it should set the PageDirty flag, and the
643 PAGECACHE_TAG_DIRTY tag in the radix tree.
645 readpages: called by the VM to read pages associated with the address_space
646 object. This is essentially just a vector version of
647 readpage. Instead of just one page, several pages are
649 readpages is only used for read-ahead, so read errors are
650 ignored. If anything goes wrong, feel free to give up.
653 Called by the generic buffered write code to ask the filesystem to
654 prepare to write len bytes at the given offset in the file. The
655 address_space should check that the write will be able to complete,
656 by allocating space if necessary and doing any other internal
657 housekeeping. If the write will update parts of any basic-blocks on
658 storage, then those blocks should be pre-read (if they haven't been
659 read already) so that the updated blocks can be written out properly.
661 The filesystem must return the locked pagecache page for the specified
662 offset, in *pagep, for the caller to write into.
664 It must be able to cope with short writes (where the length passed to
665 write_begin is greater than the number of bytes copied into the page).
667 flags is a field for AOP_FLAG_xxx flags, described in
670 A void * may be returned in fsdata, which then gets passed into
673 Returns 0 on success; < 0 on failure (which is the error code), in
674 which case write_end is not called.
676 write_end: After a successful write_begin, and data copy, write_end must
677 be called. len is the original len passed to write_begin, and copied
678 is the amount that was able to be copied (copied == len is always true
679 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
681 The filesystem must take care of unlocking the page and releasing it
682 refcount, and updating i_size.
684 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
685 that were able to be copied into pagecache.
687 bmap: called by the VFS to map a logical block offset within object to
688 physical block number. This method is used by the FIBMAP
689 ioctl and for working with swap-files. To be able to swap to
690 a file, the file must have a stable mapping to a block
691 device. The swap system does not go through the filesystem
692 but instead uses bmap to find out where the blocks in the file
693 are and uses those addresses directly.
696 invalidatepage: If a page has PagePrivate set, then invalidatepage
697 will be called when part or all of the page is to be removed
698 from the address space. This generally corresponds to either a
699 truncation or a complete invalidation of the address space
700 (in the latter case 'offset' will always be 0).
701 Any private data associated with the page should be updated
702 to reflect this truncation. If offset is 0, then
703 the private data should be released, because the page
704 must be able to be completely discarded. This may be done by
705 calling the ->releasepage function, but in this case the
706 release MUST succeed.
708 releasepage: releasepage is called on PagePrivate pages to indicate
709 that the page should be freed if possible. ->releasepage
710 should remove any private data from the page and clear the
711 PagePrivate flag. If releasepage() fails for some reason, it must
712 indicate failure with a 0 return value.
713 releasepage() is used in two distinct though related cases. The
714 first is when the VM finds a clean page with no active users and
715 wants to make it a free page. If ->releasepage succeeds, the
716 page will be removed from the address_space and become free.
718 The second case is when a request has been made to invalidate
719 some or all pages in an address_space. This can happen
720 through the fadvice(POSIX_FADV_DONTNEED) system call or by the
721 filesystem explicitly requesting it as nfs and 9fs do (when
722 they believe the cache may be out of date with storage) by
723 calling invalidate_inode_pages2().
724 If the filesystem makes such a call, and needs to be certain
725 that all pages are invalidated, then its releasepage will
726 need to ensure this. Possibly it can clear the PageUptodate
727 bit if it cannot free private data yet.
729 freepage: freepage is called once the page is no longer visible in
730 the page cache in order to allow the cleanup of any private
731 data. Since it may be called by the memory reclaimer, it
732 should not assume that the original address_space mapping still
733 exists, and it should not block.
735 direct_IO: called by the generic read/write routines to perform
736 direct_IO - that is IO requests which bypass the page cache
737 and transfer data directly between the storage and the
738 application's address space.
740 get_xip_page: called by the VM to translate a block number to a page.
741 The page is valid until the corresponding filesystem is unmounted.
742 Filesystems that want to use execute-in-place (XIP) need to implement
743 it. An example implementation can be found in fs/ext2/xip.c.
745 migrate_page: This is used to compact the physical memory usage.
746 If the VM wants to relocate a page (maybe off a memory card
747 that is signalling imminent failure) it will pass a new page
748 and an old page to this function. migrate_page should
749 transfer any private data across and update any references
750 that it has to the page.
752 launder_page: Called before freeing a page - it writes back the dirty page. To
753 prevent redirtying the page, it is kept locked during the whole
756 error_remove_page: normally set to generic_error_remove_page if truncation
757 is ok for this address space. Used for memory failure handling.
758 Setting this implies you deal with pages going away under you,
759 unless you have them locked or reference counts increased.
761 swap_activate: Called when swapon is used on a file to allocate
762 space if necessary and pin the block lookup information in
763 memory. A return value of zero indicates success,
764 in which case this file can be used to back swapspace. The
765 swapspace operations will be proxied to this address space's
766 ->swap_{out,in} methods.
768 swap_deactivate: Called during swapoff on files where swap_activate
775 A file object represents a file opened by a process.
778 struct file_operations
779 ----------------------
781 This describes how the VFS can manipulate an open file. As of kernel
782 3.5, the following members are defined:
784 struct file_operations {
785 struct module *owner;
786 loff_t (*llseek) (struct file *, loff_t, int);
787 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
788 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
789 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
790 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
791 int (*readdir) (struct file *, void *, filldir_t);
792 unsigned int (*poll) (struct file *, struct poll_table_struct *);
793 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
794 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
795 int (*mmap) (struct file *, struct vm_area_struct *);
796 int (*open) (struct inode *, struct file *);
797 int (*flush) (struct file *);
798 int (*release) (struct inode *, struct file *);
799 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
800 int (*aio_fsync) (struct kiocb *, int datasync);
801 int (*fasync) (int, struct file *, int);
802 int (*lock) (struct file *, int, struct file_lock *);
803 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
804 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
805 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
806 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
807 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
808 int (*check_flags)(int);
809 int (*flock) (struct file *, int, struct file_lock *);
810 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
811 ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
812 int (*setlease)(struct file *, long arg, struct file_lock **);
813 long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
816 Again, all methods are called without any locks being held, unless
819 llseek: called when the VFS needs to move the file position index
821 read: called by read(2) and related system calls
823 aio_read: called by io_submit(2) and other asynchronous I/O operations
825 write: called by write(2) and related system calls
827 aio_write: called by io_submit(2) and other asynchronous I/O operations
829 readdir: called when the VFS needs to read the directory contents
831 poll: called by the VFS when a process wants to check if there is
832 activity on this file and (optionally) go to sleep until there
833 is activity. Called by the select(2) and poll(2) system calls
835 unlocked_ioctl: called by the ioctl(2) system call.
837 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
838 are used on 64 bit kernels.
840 mmap: called by the mmap(2) system call
842 open: called by the VFS when an inode should be opened. When the VFS
843 opens a file, it creates a new "struct file". It then calls the
844 open method for the newly allocated file structure. You might
845 think that the open method really belongs in
846 "struct inode_operations", and you may be right. I think it's
847 done the way it is because it makes filesystems simpler to
848 implement. The open() method is a good place to initialize the
849 "private_data" member in the file structure if you want to point
850 to a device structure
852 flush: called by the close(2) system call to flush a file
854 release: called when the last reference to an open file is closed
856 fsync: called by the fsync(2) system call
858 fasync: called by the fcntl(2) system call when asynchronous
859 (non-blocking) mode is enabled for a file
861 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
864 readv: called by the readv(2) system call
866 writev: called by the writev(2) system call
868 sendfile: called by the sendfile(2) system call
870 get_unmapped_area: called by the mmap(2) system call
872 check_flags: called by the fcntl(2) system call for F_SETFL command
874 flock: called by the flock(2) system call
876 splice_write: called by the VFS to splice data from a pipe to a file. This
877 method is used by the splice(2) system call
879 splice_read: called by the VFS to splice data from file to a pipe. This
880 method is used by the splice(2) system call
882 setlease: called by the VFS to set or release a file lock lease.
883 setlease has the file_lock_lock held and must not sleep.
885 fallocate: called by the VFS to preallocate blocks or punch a hole.
887 Note that the file operations are implemented by the specific
888 filesystem in which the inode resides. When opening a device node
889 (character or block special) most filesystems will call special
890 support routines in the VFS which will locate the required device
891 driver information. These support routines replace the filesystem file
892 operations with those for the device driver, and then proceed to call
893 the new open() method for the file. This is how opening a device file
894 in the filesystem eventually ends up calling the device driver open()
898 Directory Entry Cache (dcache)
899 ==============================
902 struct dentry_operations
903 ------------------------
905 This describes how a filesystem can overload the standard dentry
906 operations. Dentries and the dcache are the domain of the VFS and the
907 individual filesystem implementations. Device drivers have no business
908 here. These methods may be set to NULL, as they are either optional or
909 the VFS uses a default. As of kernel 2.6.22, the following members are
912 struct dentry_operations {
913 int (*d_revalidate)(struct dentry *, unsigned int);
914 int (*d_hash)(const struct dentry *, const struct inode *,
916 int (*d_compare)(const struct dentry *, const struct inode *,
917 const struct dentry *, const struct inode *,
918 unsigned int, const char *, const struct qstr *);
919 int (*d_delete)(const struct dentry *);
920 void (*d_release)(struct dentry *);
921 void (*d_iput)(struct dentry *, struct inode *);
922 char *(*d_dname)(struct dentry *, char *, int);
923 struct vfsmount *(*d_automount)(struct path *);
924 int (*d_manage)(struct dentry *, bool);
927 d_revalidate: called when the VFS needs to revalidate a dentry. This
928 is called whenever a name look-up finds a dentry in the
929 dcache. Most filesystems leave this as NULL, because all their
930 dentries in the dcache are valid
932 d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
933 If in rcu-walk mode, the filesystem must revalidate the dentry without
934 blocking or storing to the dentry, d_parent and d_inode should not be
935 used without care (because they can change and, in d_inode case, even
936 become NULL under us).
938 If a situation is encountered that rcu-walk cannot handle, return
939 -ECHILD and it will be called again in ref-walk mode.
941 d_hash: called when the VFS adds a dentry to the hash table. The first
942 dentry passed to d_hash is the parent directory that the name is
943 to be hashed into. The inode is the dentry's inode.
945 Same locking and synchronisation rules as d_compare regarding
946 what is safe to dereference etc.
948 d_compare: called to compare a dentry name with a given name. The first
949 dentry is the parent of the dentry to be compared, the second is
950 the parent's inode, then the dentry and inode (may be NULL) of the
951 child dentry. len and name string are properties of the dentry to be
952 compared. qstr is the name to compare it with.
954 Must be constant and idempotent, and should not take locks if
955 possible, and should not or store into the dentry or inodes.
956 Should not dereference pointers outside the dentry or inodes without
957 lots of care (eg. d_parent, d_inode, d_name should not be used).
959 However, our vfsmount is pinned, and RCU held, so the dentries and
960 inodes won't disappear, neither will our sb or filesystem module.
961 ->i_sb and ->d_sb may be used.
963 It is a tricky calling convention because it needs to be called under
964 "rcu-walk", ie. without any locks or references on things.
966 d_delete: called when the last reference to a dentry is dropped and the
967 dcache is deciding whether or not to cache it. Return 1 to delete
968 immediately, or 0 to cache the dentry. Default is NULL which means to
969 always cache a reachable dentry. d_delete must be constant and
972 d_release: called when a dentry is really deallocated
974 d_iput: called when a dentry loses its inode (just prior to its
975 being deallocated). The default when this is NULL is that the
976 VFS calls iput(). If you define this method, you must call
979 d_dname: called when the pathname of a dentry should be generated.
980 Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
981 pathname generation. (Instead of doing it when dentry is created,
982 it's done only when the path is needed.). Real filesystems probably
983 dont want to use it, because their dentries are present in global
984 dcache hash, so their hash should be an invariant. As no lock is
985 held, d_dname() should not try to modify the dentry itself, unless
986 appropriate SMP safety is used. CAUTION : d_path() logic is quite
987 tricky. The correct way to return for example "Hello" is to put it
988 at the end of the buffer, and returns a pointer to the first char.
989 dynamic_dname() helper function is provided to take care of this.
991 d_automount: called when an automount dentry is to be traversed (optional).
992 This should create a new VFS mount record and return the record to the
993 caller. The caller is supplied with a path parameter giving the
994 automount directory to describe the automount target and the parent
995 VFS mount record to provide inheritable mount parameters. NULL should
996 be returned if someone else managed to make the automount first. If
997 the vfsmount creation failed, then an error code should be returned.
998 If -EISDIR is returned, then the directory will be treated as an
999 ordinary directory and returned to pathwalk to continue walking.
1001 If a vfsmount is returned, the caller will attempt to mount it on the
1002 mountpoint and will remove the vfsmount from its expiration list in
1003 the case of failure. The vfsmount should be returned with 2 refs on
1004 it to prevent automatic expiration - the caller will clean up the
1007 This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
1008 dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
1011 d_manage: called to allow the filesystem to manage the transition from a
1012 dentry (optional). This allows autofs, for example, to hold up clients
1013 waiting to explore behind a 'mountpoint' whilst letting the daemon go
1014 past and construct the subtree there. 0 should be returned to let the
1015 calling process continue. -EISDIR can be returned to tell pathwalk to
1016 use this directory as an ordinary directory and to ignore anything
1017 mounted on it and not to check the automount flag. Any other error
1018 code will abort pathwalk completely.
1020 If the 'rcu_walk' parameter is true, then the caller is doing a
1021 pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
1022 and the caller can be asked to leave it and call again by returning
1025 This function is only used if DCACHE_MANAGE_TRANSIT is set on the
1026 dentry being transited from.
1030 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1032 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1033 dentry->d_inode->i_ino);
1036 Each dentry has a pointer to its parent dentry, as well as a hash list
1037 of child dentries. Child dentries are basically like files in a
1041 Directory Entry Cache API
1042 --------------------------
1044 There are a number of functions defined which permit a filesystem to
1045 manipulate dentries:
1047 dget: open a new handle for an existing dentry (this just increments
1050 dput: close a handle for a dentry (decrements the usage count). If
1051 the usage count drops to 0, and the dentry is still in its
1052 parent's hash, the "d_delete" method is called to check whether
1053 it should be cached. If it should not be cached, or if the dentry
1054 is not hashed, it is deleted. Otherwise cached dentries are put
1055 into an LRU list to be reclaimed on memory shortage.
1057 d_drop: this unhashes a dentry from its parents hash list. A
1058 subsequent call to dput() will deallocate the dentry if its
1059 usage count drops to 0
1061 d_delete: delete a dentry. If there are no other open references to
1062 the dentry then the dentry is turned into a negative dentry
1063 (the d_iput() method is called). If there are other
1064 references, then d_drop() is called instead
1066 d_add: add a dentry to its parents hash list and then calls
1069 d_instantiate: add a dentry to the alias hash list for the inode and
1070 updates the "d_inode" member. The "i_count" member in the
1071 inode structure should be set/incremented. If the inode
1072 pointer is NULL, the dentry is called a "negative
1073 dentry". This function is commonly called when an inode is
1074 created for an existing negative dentry
1076 d_lookup: look up a dentry given its parent and path name component
1077 It looks up the child of that given name from the dcache
1078 hash table. If it is found, the reference count is incremented
1079 and the dentry is returned. The caller must use dput()
1080 to free the dentry when it finishes using it.
1088 On mount and remount the filesystem is passed a string containing a
1089 comma separated list of mount options. The options can have either of
1095 The <linux/parser.h> header defines an API that helps parse these
1096 options. There are plenty of examples on how to use it in existing
1102 If a filesystem accepts mount options, it must define show_options()
1103 to show all the currently active options. The rules are:
1105 - options MUST be shown which are not default or their values differ
1108 - options MAY be shown which are enabled by default or have their
1111 Options used only internally between a mount helper and the kernel
1112 (such as file descriptors), or which only have an effect during the
1113 mounting (such as ones controlling the creation of a journal) are exempt
1114 from the above rules.
1116 The underlying reason for the above rules is to make sure, that a
1117 mount can be accurately replicated (e.g. umounting and mounting again)
1118 based on the information found in /proc/mounts.
1120 A simple method of saving options at mount/remount time and showing
1121 them is provided with the save_mount_options() and
1122 generic_show_options() helper functions. Please note, that using
1123 these may have drawbacks. For more info see header comments for these
1124 functions in fs/namespace.c.
1129 (Note some of these resources are not up-to-date with the latest kernel
1132 Creating Linux virtual filesystems. 2002
1133 <http://lwn.net/Articles/13325/>
1135 The Linux Virtual File-system Layer by Neil Brown. 1999
1136 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1138 A tour of the Linux VFS by Michael K. Johnson. 1996
1139 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1141 A small trail through the Linux kernel by Andries Brouwer. 2001
1142 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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