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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.
14 Introduction
15 ============
17 The Virtual File System (also known as the Virtual Filesystem Switch)
18 is the software layer in the kernel that provides the filesystem
19 interface to userspace programs. It also provides an abstraction
20 within the kernel which allows different filesystem implementations to
21 coexist.
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
43 inode.
46 The Inode Object
47 ----------------
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
64 userspace.
67 The File Object
68 ---------------
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 it's 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
90 functions:
92 #include <linux/fs.h>
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 device onto a directory in your filespace,
99 the VFS will call the appropriate get_sb() method for the specific
100 filesystem. The dentry for the mount point will then be updated to
101 point to the root inode for the new filesystem.
103 You can see all filesystems that are registered to the kernel in the
104 file /proc/filesystems.
107 struct file_system_type
108 -----------------------
110 This describes the filesystem. As of kernel 2.6.22, the following
111 members are defined:
113 struct file_system_type {
114 const char *name;
115 int fs_flags;
116 int (*get_sb) (struct file_system_type *, int,
117 const char *, void *, struct vfsmount *);
118 void (*kill_sb) (struct super_block *);
119 struct module *owner;
120 struct file_system_type * next;
121 struct list_head fs_supers;
122 struct lock_class_key s_lock_key;
123 struct lock_class_key s_umount_key;
124 };
126 name: the name of the filesystem type, such as "ext2", "iso9660",
127 "msdos" and so on
129 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
131 get_sb: the method to call when a new instance of this
132 filesystem should be mounted
134 kill_sb: the method to call when an instance of this filesystem
135 should be unmounted
137 owner: for internal VFS use: you should initialize this to THIS_MODULE in
138 most cases.
140 next: for internal VFS use: you should initialize this to NULL
142 s_lock_key, s_umount_key: lockdep-specific
144 The get_sb() method has the following arguments:
146 struct file_system_type *fs_type: decribes the filesystem, partly initialized
147 by the specific filesystem code
149 int flags: mount flags
151 const char *dev_name: the device name we are mounting.
153 void *data: arbitrary mount options, usually comes as an ASCII
154 string (see "Mount Options" section)
156 struct vfsmount *mnt: a vfs-internal representation of a mount point
158 The get_sb() method must determine if the block device specified
159 in the dev_name and fs_type contains a filesystem of the type the method
160 supports. If it succeeds in opening the named block device, it initializes a
161 struct super_block descriptor for the filesystem contained by the block device.
162 On failure it returns an error.
164 The most interesting member of the superblock structure that the
165 get_sb() method fills in is the "s_op" field. This is a pointer to
166 a "struct super_operations" which describes the next level of the
167 filesystem implementation.
169 Usually, a filesystem uses one of the generic get_sb() implementations
170 and provides a fill_super() method instead. The generic methods are:
172 get_sb_bdev: mount a filesystem residing on a block device
174 get_sb_nodev: mount a filesystem that is not backed by a device
176 get_sb_single: mount a filesystem which shares the instance between
177 all mounts
179 A fill_super() method implementation has the following arguments:
181 struct super_block *sb: the superblock structure. The method fill_super()
182 must initialize this properly.
184 void *data: arbitrary mount options, usually comes as an ASCII
185 string (see "Mount Options" section)
187 int silent: whether or not to be silent on error
190 The Superblock Object
191 =====================
193 A superblock object represents a mounted filesystem.
196 struct super_operations
197 -----------------------
199 This describes how the VFS can manipulate the superblock of your
200 filesystem. As of kernel 2.6.22, the following members are defined:
202 struct super_operations {
203 struct inode *(*alloc_inode)(struct super_block *sb);
204 void (*destroy_inode)(struct inode *);
206 void (*dirty_inode) (struct inode *);
207 int (*write_inode) (struct inode *, int);
208 void (*put_inode) (struct inode *);
209 void (*drop_inode) (struct inode *);
210 void (*delete_inode) (struct inode *);
211 void (*put_super) (struct super_block *);
212 void (*write_super) (struct super_block *);
213 int (*sync_fs)(struct super_block *sb, int wait);
214 void (*write_super_lockfs) (struct super_block *);
215 void (*unlockfs) (struct super_block *);
216 int (*statfs) (struct dentry *, struct kstatfs *);
217 int (*remount_fs) (struct super_block *, int *, char *);
218 void (*clear_inode) (struct inode *);
219 void (*umount_begin) (struct super_block *);
221 int (*show_options)(struct seq_file *, struct vfsmount *);
223 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
224 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
225 };
227 All methods are called without any locks being held, unless otherwise
228 noted. This means that most methods can block safely. All methods are
229 only called from a process context (i.e. not from an interrupt handler
230 or bottom half).
232 alloc_inode: this method is called by inode_alloc() to allocate memory
233 for struct inode and initialize it. If this function is not
234 defined, a simple 'struct inode' is allocated. Normally
235 alloc_inode will be used to allocate a larger structure which
236 contains a 'struct inode' embedded within it.
238 destroy_inode: this method is called by destroy_inode() to release
239 resources allocated for struct inode. It is only required if
240 ->alloc_inode was defined and simply undoes anything done by
241 ->alloc_inode.
243 dirty_inode: this method is called by the VFS to mark an inode dirty.
245 write_inode: this method is called when the VFS needs to write an
246 inode to disc. The second parameter indicates whether the write
247 should be synchronous or not, not all filesystems check this flag.
249 put_inode: called when the VFS inode is removed from the inode
250 cache.
252 drop_inode: called when the last access to the inode is dropped,
253 with the inode_lock spinlock held.
255 This method should be either NULL (normal UNIX filesystem
256 semantics) or "generic_delete_inode" (for filesystems that do not
257 want to cache inodes - causing "delete_inode" to always be
258 called regardless of the value of i_nlink)
260 The "generic_delete_inode()" behavior is equivalent to the
261 old practice of using "force_delete" in the put_inode() case,
262 but does not have the races that the "force_delete()" approach
263 had.
265 delete_inode: called when the VFS wants to delete an inode
267 put_super: called when the VFS wishes to free the superblock
268 (i.e. unmount). This is called with the superblock lock held
270 write_super: called when the VFS superblock needs to be written to
271 disc. This method is optional
273 sync_fs: called when VFS is writing out all dirty data associated with
274 a superblock. The second parameter indicates whether the method
275 should wait until the write out has been completed. Optional.
277 write_super_lockfs: called when VFS is locking a filesystem and
278 forcing it into a consistent state. This method is currently
279 used by the Logical Volume Manager (LVM).
281 unlockfs: called when VFS is unlocking a filesystem and making it writable
282 again.
284 statfs: called when the VFS needs to get filesystem statistics. This
285 is called with the kernel lock held
287 remount_fs: called when the filesystem is remounted. This is called
288 with the kernel lock held
290 clear_inode: called then the VFS clears the inode. Optional
292 umount_begin: called when the VFS is unmounting a filesystem.
294 show_options: called by the VFS to show mount options for
295 /proc/<pid>/mounts. (see "Mount Options" section)
297 quota_read: called by the VFS to read from filesystem quota file.
299 quota_write: called by the VFS to write to filesystem quota file.
301 Whoever sets up the inode is responsible for filling in the "i_op" field. This
302 is a pointer to a "struct inode_operations" which describes the methods that
303 can be performed on individual inodes.
306 The Inode Object
307 ================
309 An inode object represents an object within the filesystem.
312 struct inode_operations
313 -----------------------
315 This describes how the VFS can manipulate an inode in your
316 filesystem. As of kernel 2.6.22, the following members are defined:
318 struct inode_operations {
319 int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
320 struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
321 int (*link) (struct dentry *,struct inode *,struct dentry *);
322 int (*unlink) (struct inode *,struct dentry *);
323 int (*symlink) (struct inode *,struct dentry *,const char *);
324 int (*mkdir) (struct inode *,struct dentry *,int);
325 int (*rmdir) (struct inode *,struct dentry *);
326 int (*mknod) (struct inode *,struct dentry *,int,dev_t);
327 int (*rename) (struct inode *, struct dentry *,
328 struct inode *, struct dentry *);
329 int (*readlink) (struct dentry *, char __user *,int);
330 void * (*follow_link) (struct dentry *, struct nameidata *);
331 void (*put_link) (struct dentry *, struct nameidata *, void *);
332 void (*truncate) (struct inode *);
333 int (*permission) (struct inode *, int, struct nameidata *);
334 int (*setattr) (struct dentry *, struct iattr *);
335 int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
336 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
337 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
338 ssize_t (*listxattr) (struct dentry *, char *, size_t);
339 int (*removexattr) (struct dentry *, const char *);
340 void (*truncate_range)(struct inode *, loff_t, loff_t);
341 };
343 Again, all methods are called without any locks being held, unless
344 otherwise noted.
346 create: called by the open(2) and creat(2) system calls. Only
347 required if you want to support regular files. The dentry you
348 get should not have an inode (i.e. it should be a negative
349 dentry). Here you will probably call d_instantiate() with the
350 dentry and the newly created inode
352 lookup: called when the VFS needs to look up an inode in a parent
353 directory. The name to look for is found in the dentry. This
354 method must call d_add() to insert the found inode into the
355 dentry. The "i_count" field in the inode structure should be
356 incremented. If the named inode does not exist a NULL inode
357 should be inserted into the dentry (this is called a negative
358 dentry). Returning an error code from this routine must only
359 be done on a real error, otherwise creating inodes with system
360 calls like create(2), mknod(2), mkdir(2) and so on will fail.
361 If you wish to overload the dentry methods then you should
362 initialise the "d_dop" field in the dentry; this is a pointer
363 to a struct "dentry_operations".
364 This method is called with the directory inode semaphore held
366 link: called by the link(2) system call. Only required if you want
367 to support hard links. You will probably need to call
368 d_instantiate() just as you would in the create() method
370 unlink: called by the unlink(2) system call. Only required if you
371 want to support deleting inodes
373 symlink: called by the symlink(2) system call. Only required if you
374 want to support symlinks. You will probably need to call
375 d_instantiate() just as you would in the create() method
377 mkdir: called by the mkdir(2) system call. Only required if you want
378 to support creating subdirectories. You will probably need to
379 call d_instantiate() just as you would in the create() method
381 rmdir: called by the rmdir(2) system call. Only required if you want
382 to support deleting subdirectories
384 mknod: called by the mknod(2) system call to create a device (char,
385 block) inode or a named pipe (FIFO) or socket. Only required
386 if you want to support creating these types of inodes. You
387 will probably need to call d_instantiate() just as you would
388 in the create() method
390 rename: called by the rename(2) system call to rename the object to
391 have the parent and name given by the second inode and dentry.
393 readlink: called by the readlink(2) system call. Only required if
394 you want to support reading symbolic links
396 follow_link: called by the VFS to follow a symbolic link to the
397 inode it points to. Only required if you want to support
398 symbolic links. This method returns a void pointer cookie
399 that is passed to put_link().
401 put_link: called by the VFS to release resources allocated by
402 follow_link(). The cookie returned by follow_link() is passed
403 to this method as the last parameter. It is used by
404 filesystems such as NFS where page cache is not stable
405 (i.e. page that was installed when the symbolic link walk
406 started might not be in the page cache at the end of the
407 walk).
409 truncate: called by the VFS to change the size of a file. The
410 i_size field of the inode is set to the desired size by the
411 VFS before this method is called. This method is called by
412 the truncate(2) system call and related functionality.
414 permission: called by the VFS to check for access rights on a POSIX-like
415 filesystem.
417 setattr: called by the VFS to set attributes for a file. This method
418 is called by chmod(2) and related system calls.
420 getattr: called by the VFS to get attributes of a file. This method
421 is called by stat(2) and related system calls.
423 setxattr: called by the VFS to set an extended attribute for a file.
424 Extended attribute is a name:value pair associated with an
425 inode. This method is called by setxattr(2) system call.
427 getxattr: called by the VFS to retrieve the value of an extended
428 attribute name. This method is called by getxattr(2) function
429 call.
431 listxattr: called by the VFS to list all extended attributes for a
432 given file. This method is called by listxattr(2) system call.
434 removexattr: called by the VFS to remove an extended attribute from
435 a file. This method is called by removexattr(2) system call.
437 truncate_range: a method provided by the underlying filesystem to truncate a
438 range of blocks , i.e. punch a hole somewhere in a file.
441 The Address Space Object
442 ========================
444 The address space object is used to group and manage pages in the page
445 cache. It can be used to keep track of the pages in a file (or
446 anything else) and also track the mapping of sections of the file into
447 process address spaces.
449 There are a number of distinct yet related services that an
450 address-space can provide. These include communicating memory
451 pressure, page lookup by address, and keeping track of pages tagged as
452 Dirty or Writeback.
454 The first can be used independently to the others. The VM can try to
455 either write dirty pages in order to clean them, or release clean
456 pages in order to reuse them. To do this it can call the ->writepage
457 method on dirty pages, and ->releasepage on clean pages with
458 PagePrivate set. Clean pages without PagePrivate and with no external
459 references will be released without notice being given to the
460 address_space.
462 To achieve this functionality, pages need to be placed on an LRU with
463 lru_cache_add and mark_page_active needs to be called whenever the
464 page is used.
466 Pages are normally kept in a radix tree index by ->index. This tree
467 maintains information about the PG_Dirty and PG_Writeback status of
468 each page, so that pages with either of these flags can be found
469 quickly.
471 The Dirty tag is primarily used by mpage_writepages - the default
472 ->writepages method. It uses the tag to find dirty pages to call
473 ->writepage on. If mpage_writepages is not used (i.e. the address
474 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
475 almost unused. write_inode_now and sync_inode do use it (through
476 __sync_single_inode) to check if ->writepages has been successful in
477 writing out the whole address_space.
479 The Writeback tag is used by filemap*wait* and sync_page* functions,
480 via wait_on_page_writeback_range, to wait for all writeback to
481 complete. While waiting ->sync_page (if defined) will be called on
482 each page that is found to require writeback.
484 An address_space handler may attach extra information to a page,
485 typically using the 'private' field in the 'struct page'. If such
486 information is attached, the PG_Private flag should be set. This will
487 cause various VM routines to make extra calls into the address_space
488 handler to deal with that data.
490 An address space acts as an intermediate between storage and
491 application. Data is read into the address space a whole page at a
492 time, and provided to the application either by copying of the page,
493 or by memory-mapping the page.
494 Data is written into the address space by the application, and then
495 written-back to storage typically in whole pages, however the
496 address_space has finer control of write sizes.
498 The read process essentially only requires 'readpage'. The write
499 process is more complicated and uses prepare_write/commit_write or
500 set_page_dirty to write data into the address_space, and writepage,
501 sync_page, and writepages to writeback data to storage.
503 Adding and removing pages to/from an address_space is protected by the
504 inode's i_mutex.
506 When data is written to a page, the PG_Dirty flag should be set. It
507 typically remains set until writepage asks for it to be written. This
508 should clear PG_Dirty and set PG_Writeback. It can be actually
509 written at any point after PG_Dirty is clear. Once it is known to be
510 safe, PG_Writeback is cleared.
512 Writeback makes use of a writeback_control structure...
514 struct address_space_operations
515 -------------------------------
517 This describes how the VFS can manipulate mapping of a file to page cache in
518 your filesystem. As of kernel 2.6.22, the following members are defined:
520 struct address_space_operations {
521 int (*writepage)(struct page *page, struct writeback_control *wbc);
522 int (*readpage)(struct file *, struct page *);
523 int (*sync_page)(struct page *);
524 int (*writepages)(struct address_space *, struct writeback_control *);
525 int (*set_page_dirty)(struct page *page);
526 int (*readpages)(struct file *filp, struct address_space *mapping,
527 struct list_head *pages, unsigned nr_pages);
528 int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
529 int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
530 int (*write_begin)(struct file *, struct address_space *mapping,
531 loff_t pos, unsigned len, unsigned flags,
532 struct page **pagep, void **fsdata);
533 int (*write_end)(struct file *, struct address_space *mapping,
534 loff_t pos, unsigned len, unsigned copied,
535 struct page *page, void *fsdata);
536 sector_t (*bmap)(struct address_space *, sector_t);
537 int (*invalidatepage) (struct page *, unsigned long);
538 int (*releasepage) (struct page *, int);
539 ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
540 loff_t offset, unsigned long nr_segs);
541 struct page* (*get_xip_page)(struct address_space *, sector_t,
542 int);
543 /* migrate the contents of a page to the specified target */
544 int (*migratepage) (struct page *, struct page *);
545 int (*launder_page) (struct page *);
546 };
548 writepage: called by the VM to write a dirty page to backing store.
549 This may happen for data integrity reasons (i.e. 'sync'), or
550 to free up memory (flush). The difference can be seen in
551 wbc->sync_mode.
552 The PG_Dirty flag has been cleared and PageLocked is true.
553 writepage should start writeout, should set PG_Writeback,
554 and should make sure the page is unlocked, either synchronously
555 or asynchronously when the write operation completes.
557 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
558 try too hard if there are problems, and may choose to write out
559 other pages from the mapping if that is easier (e.g. due to
560 internal dependencies). If it chooses not to start writeout, it
561 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
562 calling ->writepage on that page.
564 See the file "Locking" for more details.
566 readpage: called by the VM to read a page from backing store.
567 The page will be Locked when readpage is called, and should be
568 unlocked and marked uptodate once the read completes.
569 If ->readpage discovers that it needs to unlock the page for
570 some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
571 In this case, the page will be relocated, relocked and if
572 that all succeeds, ->readpage will be called again.
574 sync_page: called by the VM to notify the backing store to perform all
575 queued I/O operations for a page. I/O operations for other pages
576 associated with this address_space object may also be performed.
578 This function is optional and is called only for pages with
579 PG_Writeback set while waiting for the writeback to complete.
581 writepages: called by the VM to write out pages associated with the
582 address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
583 the writeback_control will specify a range of pages that must be
584 written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
585 and that many pages should be written if possible.
586 If no ->writepages is given, then mpage_writepages is used
587 instead. This will choose pages from the address space that are
588 tagged as DIRTY and will pass them to ->writepage.
590 set_page_dirty: called by the VM to set a page dirty.
591 This is particularly needed if an address space attaches
592 private data to a page, and that data needs to be updated when
593 a page is dirtied. This is called, for example, when a memory
594 mapped page gets modified.
595 If defined, it should set the PageDirty flag, and the
596 PAGECACHE_TAG_DIRTY tag in the radix tree.
598 readpages: called by the VM to read pages associated with the address_space
599 object. This is essentially just a vector version of
600 readpage. Instead of just one page, several pages are
601 requested.
602 readpages is only used for read-ahead, so read errors are
603 ignored. If anything goes wrong, feel free to give up.
605 prepare_write: called by the generic write path in VM to set up a write
606 request for a page. This indicates to the address space that
607 the given range of bytes is about to be written. The
608 address_space should check that the write will be able to
609 complete, by allocating space if necessary and doing any other
610 internal housekeeping. If the write will update parts of
611 any basic-blocks on storage, then those blocks should be
612 pre-read (if they haven't been read already) so that the
613 updated blocks can be written out properly.
614 The page will be locked.
616 Note: the page _must not_ be marked uptodate in this function
617 (or anywhere else) unless it actually is uptodate right now. As
618 soon as a page is marked uptodate, it is possible for a concurrent
619 read(2) to copy it to userspace.
621 commit_write: If prepare_write succeeds, new data will be copied
622 into the page and then commit_write will be called. It will
623 typically update the size of the file (if appropriate) and
624 mark the inode as dirty, and do any other related housekeeping
625 operations. It should avoid returning an error if possible -
626 errors should have been handled by prepare_write.
628 write_begin: This is intended as a replacement for prepare_write. The
629 key differences being that:
630 - it returns a locked page (in *pagep) rather than being
631 given a pre locked page;
632 - it must be able to cope with short writes (where the
633 length passed to write_begin is greater than the number
634 of bytes copied into the page).
636 Called by the generic buffered write code to ask the filesystem to
637 prepare to write len bytes at the given offset in the file. The
638 address_space should check that the write will be able to complete,
639 by allocating space if necessary and doing any other internal
640 housekeeping. If the write will update parts of any basic-blocks on
641 storage, then those blocks should be pre-read (if they haven't been
642 read already) so that the updated blocks can be written out properly.
644 The filesystem must return the locked pagecache page for the specified
645 offset, in *pagep, for the caller to write into.
647 flags is a field for AOP_FLAG_xxx flags, described in
648 include/linux/fs.h.
650 A void * may be returned in fsdata, which then gets passed into
651 write_end.
653 Returns 0 on success; < 0 on failure (which is the error code), in
654 which case write_end is not called.
656 write_end: After a successful write_begin, and data copy, write_end must
657 be called. len is the original len passed to write_begin, and copied
658 is the amount that was able to be copied (copied == len is always true
659 if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
661 The filesystem must take care of unlocking the page and releasing it
662 refcount, and updating i_size.
664 Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
665 that were able to be copied into pagecache.
667 bmap: called by the VFS to map a logical block offset within object to
668 physical block number. This method is used by the FIBMAP
669 ioctl and for working with swap-files. To be able to swap to
670 a file, the file must have a stable mapping to a block
671 device. The swap system does not go through the filesystem
672 but instead uses bmap to find out where the blocks in the file
673 are and uses those addresses directly.
676 invalidatepage: If a page has PagePrivate set, then invalidatepage
677 will be called when part or all of the page is to be removed
678 from the address space. This generally corresponds to either a
679 truncation or a complete invalidation of the address space
680 (in the latter case 'offset' will always be 0).
681 Any private data associated with the page should be updated
682 to reflect this truncation. If offset is 0, then
683 the private data should be released, because the page
684 must be able to be completely discarded. This may be done by
685 calling the ->releasepage function, but in this case the
686 release MUST succeed.
688 releasepage: releasepage is called on PagePrivate pages to indicate
689 that the page should be freed if possible. ->releasepage
690 should remove any private data from the page and clear the
691 PagePrivate flag. It may also remove the page from the
692 address_space. If this fails for some reason, it may indicate
693 failure with a 0 return value.
694 This is used in two distinct though related cases. The first
695 is when the VM finds a clean page with no active users and
696 wants to make it a free page. If ->releasepage succeeds, the
697 page will be removed from the address_space and become free.
699 The second case is when a request has been made to invalidate
700 some or all pages in an address_space. This can happen
701 through the fadvice(POSIX_FADV_DONTNEED) system call or by the
702 filesystem explicitly requesting it as nfs and 9fs do (when
703 they believe the cache may be out of date with storage) by
704 calling invalidate_inode_pages2().
705 If the filesystem makes such a call, and needs to be certain
706 that all pages are invalidated, then its releasepage will
707 need to ensure this. Possibly it can clear the PageUptodate
708 bit if it cannot free private data yet.
710 direct_IO: called by the generic read/write routines to perform
711 direct_IO - that is IO requests which bypass the page cache
712 and transfer data directly between the storage and the
713 application's address space.
715 get_xip_page: called by the VM to translate a block number to a page.
716 The page is valid until the corresponding filesystem is unmounted.
717 Filesystems that want to use execute-in-place (XIP) need to implement
718 it. An example implementation can be found in fs/ext2/xip.c.
720 migrate_page: This is used to compact the physical memory usage.
721 If the VM wants to relocate a page (maybe off a memory card
722 that is signalling imminent failure) it will pass a new page
723 and an old page to this function. migrate_page should
724 transfer any private data across and update any references
725 that it has to the page.
727 launder_page: Called before freeing a page - it writes back the dirty page. To
728 prevent redirtying the page, it is kept locked during the whole
729 operation.
731 The File Object
732 ===============
734 A file object represents a file opened by a process.
737 struct file_operations
738 ----------------------
740 This describes how the VFS can manipulate an open file. As of kernel
741 2.6.22, the following members are defined:
743 struct file_operations {
744 struct module *owner;
745 loff_t (*llseek) (struct file *, loff_t, int);
746 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
747 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
748 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
749 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
750 int (*readdir) (struct file *, void *, filldir_t);
751 unsigned int (*poll) (struct file *, struct poll_table_struct *);
752 int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
753 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
754 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
755 int (*mmap) (struct file *, struct vm_area_struct *);
756 int (*open) (struct inode *, struct file *);
757 int (*flush) (struct file *);
758 int (*release) (struct inode *, struct file *);
759 int (*fsync) (struct file *, struct dentry *, int datasync);
760 int (*aio_fsync) (struct kiocb *, int datasync);
761 int (*fasync) (int, struct file *, int);
762 int (*lock) (struct file *, int, struct file_lock *);
763 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
764 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
765 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
766 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
767 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
768 int (*check_flags)(int);
769 int (*dir_notify)(struct file *filp, unsigned long arg);
770 int (*flock) (struct file *, int, struct file_lock *);
771 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
772 ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
773 };
775 Again, all methods are called without any locks being held, unless
776 otherwise noted.
778 llseek: called when the VFS needs to move the file position index
780 read: called by read(2) and related system calls
782 aio_read: called by io_submit(2) and other asynchronous I/O operations
784 write: called by write(2) and related system calls
786 aio_write: called by io_submit(2) and other asynchronous I/O operations
788 readdir: called when the VFS needs to read the directory contents
790 poll: called by the VFS when a process wants to check if there is
791 activity on this file and (optionally) go to sleep until there
792 is activity. Called by the select(2) and poll(2) system calls
794 ioctl: called by the ioctl(2) system call
796 unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not
797 require the BKL should use this method instead of the ioctl() above.
799 compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
800 are used on 64 bit kernels.
802 mmap: called by the mmap(2) system call
804 open: called by the VFS when an inode should be opened. When the VFS
805 opens a file, it creates a new "struct file". It then calls the
806 open method for the newly allocated file structure. You might
807 think that the open method really belongs in
808 "struct inode_operations", and you may be right. I think it's
809 done the way it is because it makes filesystems simpler to
810 implement. The open() method is a good place to initialize the
811 "private_data" member in the file structure if you want to point
812 to a device structure
814 flush: called by the close(2) system call to flush a file
816 release: called when the last reference to an open file is closed
818 fsync: called by the fsync(2) system call
820 fasync: called by the fcntl(2) system call when asynchronous
821 (non-blocking) mode is enabled for a file
823 lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
824 commands
826 readv: called by the readv(2) system call
828 writev: called by the writev(2) system call
830 sendfile: called by the sendfile(2) system call
832 get_unmapped_area: called by the mmap(2) system call
834 check_flags: called by the fcntl(2) system call for F_SETFL command
836 dir_notify: called by the fcntl(2) system call for F_NOTIFY command
838 flock: called by the flock(2) system call
840 splice_write: called by the VFS to splice data from a pipe to a file. This
841 method is used by the splice(2) system call
843 splice_read: called by the VFS to splice data from file to a pipe. This
844 method is used by the splice(2) system call
846 Note that the file operations are implemented by the specific
847 filesystem in which the inode resides. When opening a device node
848 (character or block special) most filesystems will call special
849 support routines in the VFS which will locate the required device
850 driver information. These support routines replace the filesystem file
851 operations with those for the device driver, and then proceed to call
852 the new open() method for the file. This is how opening a device file
853 in the filesystem eventually ends up calling the device driver open()
854 method.
857 Directory Entry Cache (dcache)
858 ==============================
861 struct dentry_operations
862 ------------------------
864 This describes how a filesystem can overload the standard dentry
865 operations. Dentries and the dcache are the domain of the VFS and the
866 individual filesystem implementations. Device drivers have no business
867 here. These methods may be set to NULL, as they are either optional or
868 the VFS uses a default. As of kernel 2.6.22, the following members are
869 defined:
871 struct dentry_operations {
872 int (*d_revalidate)(struct dentry *, struct nameidata *);
873 int (*d_hash) (struct dentry *, struct qstr *);
874 int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
875 int (*d_delete)(struct dentry *);
876 void (*d_release)(struct dentry *);
877 void (*d_iput)(struct dentry *, struct inode *);
878 char *(*d_dname)(struct dentry *, char *, int);
879 };
881 d_revalidate: called when the VFS needs to revalidate a dentry. This
882 is called whenever a name look-up finds a dentry in the
883 dcache. Most filesystems leave this as NULL, because all their
884 dentries in the dcache are valid
886 d_hash: called when the VFS adds a dentry to the hash table
888 d_compare: called when a dentry should be compared with another
890 d_delete: called when the last reference to a dentry is
891 deleted. This means no-one is using the dentry, however it is
892 still valid and in the dcache
894 d_release: called when a dentry is really deallocated
896 d_iput: called when a dentry loses its inode (just prior to its
897 being deallocated). The default when this is NULL is that the
898 VFS calls iput(). If you define this method, you must call
899 iput() yourself
901 d_dname: called when the pathname of a dentry should be generated.
902 Usefull for some pseudo filesystems (sockfs, pipefs, ...) to delay
903 pathname generation. (Instead of doing it when dentry is created,
904 its done only when the path is needed.). Real filesystems probably
905 dont want to use it, because their dentries are present in global
906 dcache hash, so their hash should be an invariant. As no lock is
907 held, d_dname() should not try to modify the dentry itself, unless
908 appropriate SMP safety is used. CAUTION : d_path() logic is quite
909 tricky. The correct way to return for example "Hello" is to put it
910 at the end of the buffer, and returns a pointer to the first char.
911 dynamic_dname() helper function is provided to take care of this.
913 Example :
915 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
916 {
917 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
918 dentry->d_inode->i_ino);
919 }
921 Each dentry has a pointer to its parent dentry, as well as a hash list
922 of child dentries. Child dentries are basically like files in a
923 directory.
926 Directory Entry Cache API
927 --------------------------
929 There are a number of functions defined which permit a filesystem to
930 manipulate dentries:
932 dget: open a new handle for an existing dentry (this just increments
933 the usage count)
935 dput: close a handle for a dentry (decrements the usage count). If
936 the usage count drops to 0, the "d_delete" method is called
937 and the dentry is placed on the unused list if the dentry is
938 still in its parents hash list. Putting the dentry on the
939 unused list just means that if the system needs some RAM, it
940 goes through the unused list of dentries and deallocates them.
941 If the dentry has already been unhashed and the usage count
942 drops to 0, in this case the dentry is deallocated after the
943 "d_delete" method is called
945 d_drop: this unhashes a dentry from its parents hash list. A
946 subsequent call to dput() will deallocate the dentry if its
947 usage count drops to 0
949 d_delete: delete a dentry. If there are no other open references to
950 the dentry then the dentry is turned into a negative dentry
951 (the d_iput() method is called). If there are other
952 references, then d_drop() is called instead
954 d_add: add a dentry to its parents hash list and then calls
955 d_instantiate()
957 d_instantiate: add a dentry to the alias hash list for the inode and
958 updates the "d_inode" member. The "i_count" member in the
959 inode structure should be set/incremented. If the inode
960 pointer is NULL, the dentry is called a "negative
961 dentry". This function is commonly called when an inode is
962 created for an existing negative dentry
964 d_lookup: look up a dentry given its parent and path name component
965 It looks up the child of that given name from the dcache
966 hash table. If it is found, the reference count is incremented
967 and the dentry is returned. The caller must use d_put()
968 to free the dentry when it finishes using it.
970 For further information on dentry locking, please refer to the document
971 Documentation/filesystems/dentry-locking.txt.
973 Mount Options
974 =============
976 Parsing options
977 ---------------
979 On mount and remount the filesystem is passed a string containing a
980 comma separated list of mount options. The options can have either of
981 these forms:
983 option
984 option=value
986 The <linux/parser.h> header defines an API that helps parse these
987 options. There are plenty of examples on how to use it in existing
988 filesystems.
990 Showing options
991 ---------------
993 If a filesystem accepts mount options, it must define show_options()
994 to show all the currently active options. The rules are:
996 - options MUST be shown which are not default or their values differ
997 from the default
999 - options MAY be shown which are enabled by default or have their
1000 default value
1002 Options used only internally between a mount helper and the kernel
1003 (such as file descriptors), or which only have an effect during the
1004 mounting (such as ones controlling the creation of a journal) are exempt
1005 from the above rules.
1007 The underlying reason for the above rules is to make sure, that a
1008 mount can be accurately replicated (e.g. umounting and mounting again)
1009 based on the information found in /proc/mounts.
1011 A simple method of saving options at mount/remount time and showing
1012 them is provided with the save_mount_options() and
1013 generic_show_options() helper functions. Please note, that using
1014 these may have drawbacks. For more info see header comments for these
1015 functions in fs/namespace.c.
1017 Resources
1018 =========
1020 (Note some of these resources are not up-to-date with the latest kernel
1021 version.)
1023 Creating Linux virtual filesystems. 2002
1024 <http://lwn.net/Articles/13325/>
1026 The Linux Virtual File-system Layer by Neil Brown. 1999
1027 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1029 A tour of the Linux VFS by Michael K. Johnson. 1996
1030 <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1032 A small trail through the Linux kernel by Andries Brouwer. 2001
1033 <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>