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1=======
2Locking
3=======
4
5The text below describes the locking rules for VFS-related methods.
6It is (believed to be) up-to-date. *Please*, if you change anything in
7prototypes or locking protocols - update this file. And update the relevant
8instances in the tree, don't leave that to maintainers of filesystems/devices/
9etc. At the very least, put the list of dubious cases in the end of this file.
10Don't turn it into log - maintainers of out-of-the-tree code are supposed to
11be able to use diff(1).
12
13Thing currently missing here: socket operations. Alexey?
14
15dentry_operations
16=================
17
18prototypes::
19
20 int (*d_revalidate)(struct dentry *, unsigned int);
21 int (*d_weak_revalidate)(struct dentry *, unsigned int);
22 int (*d_hash)(const struct dentry *, struct qstr *);
23 int (*d_compare)(const struct dentry *,
24 unsigned int, const char *, const struct qstr *);
25 int (*d_delete)(struct dentry *);
26 int (*d_init)(struct dentry *);
27 void (*d_release)(struct dentry *);
28 void (*d_iput)(struct dentry *, struct inode *);
29 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
30 struct vfsmount *(*d_automount)(struct path *path);
31 int (*d_manage)(const struct path *, bool);
32 struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
33
34locking rules:
35
36================== =========== ======== ============== ========
37ops rename_lock ->d_lock may block rcu-walk
38================== =========== ======== ============== ========
39d_revalidate: no no yes (ref-walk) maybe
40d_weak_revalidate: no no yes no
41d_hash no no no maybe
42d_compare: yes no no maybe
43d_delete: no yes no no
44d_init: no no yes no
45d_release: no no yes no
46d_prune: no yes no no
47d_iput: no no yes no
48d_dname: no no no no
49d_automount: no no yes no
50d_manage: no no yes (ref-walk) maybe
51d_real no no yes no
52================== =========== ======== ============== ========
53
54inode_operations
55================
56
57prototypes::
58
59 int (*create) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t, bool);
60 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
61 int (*link) (struct dentry *,struct inode *,struct dentry *);
62 int (*unlink) (struct inode *,struct dentry *);
63 int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *);
64 int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);
65 int (*rmdir) (struct inode *,struct dentry *);
66 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);
67 int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *,
68 struct inode *, struct dentry *, unsigned int);
69 int (*readlink) (struct dentry *, char __user *,int);
70 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
71 void (*truncate) (struct inode *);
72 int (*permission) (struct mnt_idmap *, struct inode *, int, unsigned int);
73 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
74 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);
75 int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int);
76 ssize_t (*listxattr) (struct dentry *, char *, size_t);
77 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
78 void (*update_time)(struct inode *, struct timespec *, int);
79 int (*atomic_open)(struct inode *, struct dentry *,
80 struct file *, unsigned open_flag,
81 umode_t create_mode);
82 int (*tmpfile) (struct mnt_idmap *, struct inode *,
83 struct file *, umode_t);
84 int (*fileattr_set)(struct mnt_idmap *idmap,
85 struct dentry *dentry, struct fileattr *fa);
86 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
87 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);
88 struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
89
90locking rules:
91 all may block
92
93============== ==================================================
94ops i_rwsem(inode)
95============== ==================================================
96lookup: shared
97create: exclusive
98link: exclusive (both)
99mknod: exclusive
100symlink: exclusive
101mkdir: exclusive
102unlink: exclusive (both)
103rmdir: exclusive (both)(see below)
104rename: exclusive (both parents, some children) (see below)
105readlink: no
106get_link: no
107setattr: exclusive
108permission: no (may not block if called in rcu-walk mode)
109get_inode_acl: no
110get_acl: no
111getattr: no
112listxattr: no
113fiemap: no
114update_time: no
115atomic_open: shared (exclusive if O_CREAT is set in open flags)
116tmpfile: no
117fileattr_get: no or exclusive
118fileattr_set: exclusive
119get_offset_ctx no
120============== ==================================================
121
122
123 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
124 exclusive on victim.
125 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
126 ->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories
127 involved.
128 ->rename() has ->i_rwsem exclusive on any subdirectory that changes parent.
129
130See Documentation/filesystems/directory-locking.rst for more detailed discussion
131of the locking scheme for directory operations.
132
133xattr_handler operations
134========================
135
136prototypes::
137
138 bool (*list)(struct dentry *dentry);
139 int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
140 struct inode *inode, const char *name, void *buffer,
141 size_t size);
142 int (*set)(const struct xattr_handler *handler,
143 struct mnt_idmap *idmap,
144 struct dentry *dentry, struct inode *inode, const char *name,
145 const void *buffer, size_t size, int flags);
146
147locking rules:
148 all may block
149
150===== ==============
151ops i_rwsem(inode)
152===== ==============
153list: no
154get: no
155set: exclusive
156===== ==============
157
158super_operations
159================
160
161prototypes::
162
163 struct inode *(*alloc_inode)(struct super_block *sb);
164 void (*free_inode)(struct inode *);
165 void (*destroy_inode)(struct inode *);
166 void (*dirty_inode) (struct inode *, int flags);
167 int (*write_inode) (struct inode *, struct writeback_control *wbc);
168 int (*drop_inode) (struct inode *);
169 void (*evict_inode) (struct inode *);
170 void (*put_super) (struct super_block *);
171 int (*sync_fs)(struct super_block *sb, int wait);
172 int (*freeze_fs) (struct super_block *);
173 int (*unfreeze_fs) (struct super_block *);
174 int (*statfs) (struct dentry *, struct kstatfs *);
175 int (*remount_fs) (struct super_block *, int *, char *);
176 void (*umount_begin) (struct super_block *);
177 int (*show_options)(struct seq_file *, struct dentry *);
178 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
179 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
180
181locking rules:
182 All may block [not true, see below]
183
184====================== ============ ========================
185ops s_umount note
186====================== ============ ========================
187alloc_inode:
188free_inode: called from RCU callback
189destroy_inode:
190dirty_inode:
191write_inode:
192drop_inode: !!!inode->i_lock!!!
193evict_inode:
194put_super: write
195sync_fs: read
196freeze_fs: write
197unfreeze_fs: write
198statfs: maybe(read) (see below)
199remount_fs: write
200umount_begin: no
201show_options: no (namespace_sem)
202quota_read: no (see below)
203quota_write: no (see below)
204====================== ============ ========================
205
206->statfs() has s_umount (shared) when called by ustat(2) (native or
207compat), but that's an accident of bad API; s_umount is used to pin
208the superblock down when we only have dev_t given us by userland to
209identify the superblock. Everything else (statfs(), fstatfs(), etc.)
210doesn't hold it when calling ->statfs() - superblock is pinned down
211by resolving the pathname passed to syscall.
212
213->quota_read() and ->quota_write() functions are both guaranteed to
214be the only ones operating on the quota file by the quota code (via
215dqio_sem) (unless an admin really wants to screw up something and
216writes to quota files with quotas on). For other details about locking
217see also dquot_operations section.
218
219file_system_type
220================
221
222prototypes::
223
224 struct dentry *(*mount) (struct file_system_type *, int,
225 const char *, void *);
226 void (*kill_sb) (struct super_block *);
227
228locking rules:
229
230======= =========
231ops may block
232======= =========
233mount yes
234kill_sb yes
235======= =========
236
237->mount() returns ERR_PTR or the root dentry; its superblock should be locked
238on return.
239
240->kill_sb() takes a write-locked superblock, does all shutdown work on it,
241unlocks and drops the reference.
242
243address_space_operations
244========================
245prototypes::
246
247 int (*writepage)(struct page *page, struct writeback_control *wbc);
248 int (*read_folio)(struct file *, struct folio *);
249 int (*writepages)(struct address_space *, struct writeback_control *);
250 bool (*dirty_folio)(struct address_space *, struct folio *folio);
251 void (*readahead)(struct readahead_control *);
252 int (*write_begin)(struct file *, struct address_space *mapping,
253 loff_t pos, unsigned len,
254 struct folio **foliop, void **fsdata);
255 int (*write_end)(struct file *, struct address_space *mapping,
256 loff_t pos, unsigned len, unsigned copied,
257 struct folio *folio, void *fsdata);
258 sector_t (*bmap)(struct address_space *, sector_t);
259 void (*invalidate_folio) (struct folio *, size_t start, size_t len);
260 bool (*release_folio)(struct folio *, gfp_t);
261 void (*free_folio)(struct folio *);
262 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
263 int (*migrate_folio)(struct address_space *, struct folio *dst,
264 struct folio *src, enum migrate_mode);
265 int (*launder_folio)(struct folio *);
266 bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count);
267 int (*error_remove_folio)(struct address_space *, struct folio *);
268 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
269 int (*swap_deactivate)(struct file *);
270 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
271
272locking rules:
273 All except dirty_folio and free_folio may block
274
275====================== ======================== ========= ===============
276ops folio locked i_rwsem invalidate_lock
277====================== ======================== ========= ===============
278writepage: yes, unlocks (see below)
279read_folio: yes, unlocks shared
280writepages:
281dirty_folio: maybe
282readahead: yes, unlocks shared
283write_begin: locks the folio exclusive
284write_end: yes, unlocks exclusive
285bmap:
286invalidate_folio: yes exclusive
287release_folio: yes
288free_folio: yes
289direct_IO:
290migrate_folio: yes (both)
291launder_folio: yes
292is_partially_uptodate: yes
293error_remove_folio: yes
294swap_activate: no
295swap_deactivate: no
296swap_rw: yes, unlocks
297====================== ======================== ========= ===============
298
299->write_begin(), ->write_end() and ->read_folio() may be called from
300the request handler (/dev/loop).
301
302->read_folio() unlocks the folio, either synchronously or via I/O
303completion.
304
305->readahead() unlocks the folios that I/O is attempted on like ->read_folio().
306
307->writepage() is used for two purposes: for "memory cleansing" and for
308"sync". These are quite different operations and the behaviour may differ
309depending upon the mode.
310
311If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
312it *must* start I/O against the page, even if that would involve
313blocking on in-progress I/O.
314
315If writepage is called for memory cleansing (sync_mode ==
316WBC_SYNC_NONE) then its role is to get as much writeout underway as
317possible. So writepage should try to avoid blocking against
318currently-in-progress I/O.
319
320If the filesystem is not called for "sync" and it determines that it
321would need to block against in-progress I/O to be able to start new I/O
322against the page the filesystem should redirty the page with
323redirty_page_for_writepage(), then unlock the page and return zero.
324This may also be done to avoid internal deadlocks, but rarely.
325
326If the filesystem is called for sync then it must wait on any
327in-progress I/O and then start new I/O.
328
329The filesystem should unlock the page synchronously, before returning to the
330caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
331value. WRITEPAGE_ACTIVATE means that page cannot really be written out
332currently, and VM should stop calling ->writepage() on this page for some
333time. VM does this by moving page to the head of the active list, hence the
334name.
335
336Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
337and return zero, writepage *must* run set_page_writeback() against the page,
338followed by unlocking it. Once set_page_writeback() has been run against the
339page, write I/O can be submitted and the write I/O completion handler must run
340end_page_writeback() once the I/O is complete. If no I/O is submitted, the
341filesystem must run end_page_writeback() against the page before returning from
342writepage.
343
344That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
345if the filesystem needs the page to be locked during writeout, that is ok, too,
346the page is allowed to be unlocked at any point in time between the calls to
347set_page_writeback() and end_page_writeback().
348
349Note, failure to run either redirty_page_for_writepage() or the combination of
350set_page_writeback()/end_page_writeback() on a page submitted to writepage
351will leave the page itself marked clean but it will be tagged as dirty in the
352radix tree. This incoherency can lead to all sorts of hard-to-debug problems
353in the filesystem like having dirty inodes at umount and losing written data.
354
355->writepages() is used for periodic writeback and for syscall-initiated
356sync operations. The address_space should start I/O against at least
357``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page
358which is written. The address_space implementation may write more (or less)
359pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
360If nr_to_write is NULL, all dirty pages must be written.
361
362writepages should _only_ write pages which are present on
363mapping->io_pages.
364
365->dirty_folio() is called from various places in the kernel when
366the target folio is marked as needing writeback. The folio cannot be
367truncated because either the caller holds the folio lock, or the caller
368has found the folio while holding the page table lock which will block
369truncation.
370
371->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
372filesystems and by the swapper. The latter will eventually go away. Please,
373keep it that way and don't breed new callers.
374
375->invalidate_folio() is called when the filesystem must attempt to drop
376some or all of the buffers from the page when it is being truncated. It
377returns zero on success. The filesystem must exclusively acquire
378invalidate_lock before invalidating page cache in truncate / hole punch
379path (and thus calling into ->invalidate_folio) to block races between page
380cache invalidation and page cache filling functions (fault, read, ...).
381
382->release_folio() is called when the MM wants to make a change to the
383folio that would invalidate the filesystem's private data. For example,
384it may be about to be removed from the address_space or split. The folio
385is locked and not under writeback. It may be dirty. The gfp parameter
386is not usually used for allocation, but rather to indicate what the
387filesystem may do to attempt to free the private data. The filesystem may
388return false to indicate that the folio's private data cannot be freed.
389If it returns true, it should have already removed the private data from
390the folio. If a filesystem does not provide a ->release_folio method,
391the pagecache will assume that private data is buffer_heads and call
392try_to_free_buffers().
393
394->free_folio() is called when the kernel has dropped the folio
395from the page cache.
396
397->launder_folio() may be called prior to releasing a folio if
398it is still found to be dirty. It returns zero if the folio was successfully
399cleaned, or an error value if not. Note that in order to prevent the folio
400getting mapped back in and redirtied, it needs to be kept locked
401across the entire operation.
402
403->swap_activate() will be called to prepare the given file for swap. It
404should perform any validation and preparation necessary to ensure that
405writes can be performed with minimal memory allocation. It should call
406add_swap_extent(), or the helper iomap_swapfile_activate(), and return
407the number of extents added. If IO should be submitted through
408->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted
409directly to the block device ``sis->bdev``.
410
411->swap_deactivate() will be called in the sys_swapoff()
412path after ->swap_activate() returned success.
413
414->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate().
415
416file_lock_operations
417====================
418
419prototypes::
420
421 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
422 void (*fl_release_private)(struct file_lock *);
423
424
425locking rules:
426
427=================== ============= =========
428ops inode->i_lock may block
429=================== ============= =========
430fl_copy_lock: yes no
431fl_release_private: maybe maybe[1]_
432=================== ============= =========
433
434.. [1]:
435 ->fl_release_private for flock or POSIX locks is currently allowed
436 to block. Leases however can still be freed while the i_lock is held and
437 so fl_release_private called on a lease should not block.
438
439lock_manager_operations
440=======================
441
442prototypes::
443
444 void (*lm_notify)(struct file_lock *); /* unblock callback */
445 int (*lm_grant)(struct file_lock *, struct file_lock *, int);
446 void (*lm_break)(struct file_lock *); /* break_lease callback */
447 int (*lm_change)(struct file_lock **, int);
448 bool (*lm_breaker_owns_lease)(struct file_lock *);
449 bool (*lm_lock_expirable)(struct file_lock *);
450 void (*lm_expire_lock)(void);
451
452locking rules:
453
454====================== ============= ================= =========
455ops flc_lock blocked_lock_lock may block
456====================== ============= ================= =========
457lm_notify: no yes no
458lm_grant: no no no
459lm_break: yes no no
460lm_change yes no no
461lm_breaker_owns_lease: yes no no
462lm_lock_expirable yes no no
463lm_expire_lock no no yes
464====================== ============= ================= =========
465
466buffer_head
467===========
468
469prototypes::
470
471 void (*b_end_io)(struct buffer_head *bh, int uptodate);
472
473locking rules:
474
475called from interrupts. In other words, extreme care is needed here.
476bh is locked, but that's all warranties we have here. Currently only RAID1,
477highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
478call this method upon the IO completion.
479
480block_device_operations
481=======================
482prototypes::
483
484 int (*open) (struct block_device *, fmode_t);
485 int (*release) (struct gendisk *, fmode_t);
486 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
487 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
488 int (*direct_access) (struct block_device *, sector_t, void **,
489 unsigned long *);
490 void (*unlock_native_capacity) (struct gendisk *);
491 int (*getgeo)(struct block_device *, struct hd_geometry *);
492 void (*swap_slot_free_notify) (struct block_device *, unsigned long);
493
494locking rules:
495
496======================= ===================
497ops open_mutex
498======================= ===================
499open: yes
500release: yes
501ioctl: no
502compat_ioctl: no
503direct_access: no
504unlock_native_capacity: no
505getgeo: no
506swap_slot_free_notify: no (see below)
507======================= ===================
508
509swap_slot_free_notify is called with swap_lock and sometimes the page lock
510held.
511
512
513file_operations
514===============
515
516prototypes::
517
518 loff_t (*llseek) (struct file *, loff_t, int);
519 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
520 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
521 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
522 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
523 int (*iopoll) (struct kiocb *kiocb, bool spin);
524 int (*iterate_shared) (struct file *, struct dir_context *);
525 __poll_t (*poll) (struct file *, struct poll_table_struct *);
526 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
527 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
528 int (*mmap) (struct file *, struct vm_area_struct *);
529 int (*open) (struct inode *, struct file *);
530 int (*flush) (struct file *);
531 int (*release) (struct inode *, struct file *);
532 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
533 int (*fasync) (int, struct file *, int);
534 int (*lock) (struct file *, int, struct file_lock *);
535 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
536 unsigned long, unsigned long, unsigned long);
537 int (*check_flags)(int);
538 int (*flock) (struct file *, int, struct file_lock *);
539 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
540 size_t, unsigned int);
541 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
542 size_t, unsigned int);
543 int (*setlease)(struct file *, long, struct file_lock **, void **);
544 long (*fallocate)(struct file *, int, loff_t, loff_t);
545 void (*show_fdinfo)(struct seq_file *m, struct file *f);
546 unsigned (*mmap_capabilities)(struct file *);
547 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
548 loff_t, size_t, unsigned int);
549 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
550 struct file *file_out, loff_t pos_out,
551 loff_t len, unsigned int remap_flags);
552 int (*fadvise)(struct file *, loff_t, loff_t, int);
553
554locking rules:
555 All may block.
556
557->llseek() locking has moved from llseek to the individual llseek
558implementations. If your fs is not using generic_file_llseek, you
559need to acquire and release the appropriate locks in your ->llseek().
560For many filesystems, it is probably safe to acquire the inode
561mutex or just to use i_size_read() instead.
562Note: this does not protect the file->f_pos against concurrent modifications
563since this is something the userspace has to take care about.
564
565->iterate_shared() is called with i_rwsem held for reading, and with the
566file f_pos_lock held exclusively
567
568->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
569Most instances call fasync_helper(), which does that maintenance, so it's
570not normally something one needs to worry about. Return values > 0 will be
571mapped to zero in the VFS layer.
572
573->readdir() and ->ioctl() on directories must be changed. Ideally we would
574move ->readdir() to inode_operations and use a separate method for directory
575->ioctl() or kill the latter completely. One of the problems is that for
576anything that resembles union-mount we won't have a struct file for all
577components. And there are other reasons why the current interface is a mess...
578
579->read on directories probably must go away - we should just enforce -EISDIR
580in sys_read() and friends.
581
582->setlease operations should call generic_setlease() before or after setting
583the lease within the individual filesystem to record the result of the
584operation
585
586->fallocate implementation must be really careful to maintain page cache
587consistency when punching holes or performing other operations that invalidate
588page cache contents. Usually the filesystem needs to call
589truncate_inode_pages_range() to invalidate relevant range of the page cache.
590However the filesystem usually also needs to update its internal (and on disk)
591view of file offset -> disk block mapping. Until this update is finished, the
592filesystem needs to block page faults and reads from reloading now-stale page
593cache contents from the disk. Since VFS acquires mapping->invalidate_lock in
594shared mode when loading pages from disk (filemap_fault(), filemap_read(),
595readahead paths), the fallocate implementation must take the invalidate_lock to
596prevent reloading.
597
598->copy_file_range and ->remap_file_range implementations need to serialize
599against modifications of file data while the operation is running. For
600blocking changes through write(2) and similar operations inode->i_rwsem can be
601used. To block changes to file contents via a memory mapping during the
602operation, the filesystem must take mapping->invalidate_lock to coordinate
603with ->page_mkwrite.
604
605dquot_operations
606================
607
608prototypes::
609
610 int (*write_dquot) (struct dquot *);
611 int (*acquire_dquot) (struct dquot *);
612 int (*release_dquot) (struct dquot *);
613 int (*mark_dirty) (struct dquot *);
614 int (*write_info) (struct super_block *, int);
615
616These operations are intended to be more or less wrapping functions that ensure
617a proper locking wrt the filesystem and call the generic quota operations.
618
619What filesystem should expect from the generic quota functions:
620
621============== ============ =========================
622ops FS recursion Held locks when called
623============== ============ =========================
624write_dquot: yes dqonoff_sem or dqptr_sem
625acquire_dquot: yes dqonoff_sem or dqptr_sem
626release_dquot: yes dqonoff_sem or dqptr_sem
627mark_dirty: no -
628write_info: yes dqonoff_sem
629============== ============ =========================
630
631FS recursion means calling ->quota_read() and ->quota_write() from superblock
632operations.
633
634More details about quota locking can be found in fs/dquot.c.
635
636vm_operations_struct
637====================
638
639prototypes::
640
641 void (*open)(struct vm_area_struct *);
642 void (*close)(struct vm_area_struct *);
643 vm_fault_t (*fault)(struct vm_fault *);
644 vm_fault_t (*huge_fault)(struct vm_fault *, unsigned int order);
645 vm_fault_t (*map_pages)(struct vm_fault *, pgoff_t start, pgoff_t end);
646 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
647 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
648 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
649
650locking rules:
651
652============= ========== ===========================
653ops mmap_lock PageLocked(page)
654============= ========== ===========================
655open: write
656close: read/write
657fault: read can return with page locked
658huge_fault: maybe-read
659map_pages: maybe-read
660page_mkwrite: read can return with page locked
661pfn_mkwrite: read
662access: read
663============= ========== ===========================
664
665->fault() is called when a previously not present pte is about to be faulted
666in. The filesystem must find and return the page associated with the passed in
667"pgoff" in the vm_fault structure. If it is possible that the page may be
668truncated and/or invalidated, then the filesystem must lock invalidate_lock,
669then ensure the page is not already truncated (invalidate_lock will block
670subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
671locked. The VM will unlock the page.
672
673->huge_fault() is called when there is no PUD or PMD entry present. This
674gives the filesystem the opportunity to install a PUD or PMD sized page.
675Filesystems can also use the ->fault method to return a PMD sized page,
676so implementing this function may not be necessary. In particular,
677filesystems should not call filemap_fault() from ->huge_fault().
678The mmap_lock may not be held when this method is called.
679
680->map_pages() is called when VM asks to map easy accessible pages.
681Filesystem should find and map pages associated with offsets from "start_pgoff"
682till "end_pgoff". ->map_pages() is called with the RCU lock held and must
683not block. If it's not possible to reach a page without blocking,
684filesystem should skip it. Filesystem should use set_pte_range() to setup
685page table entry. Pointer to entry associated with the page is passed in
686"pte" field in vm_fault structure. Pointers to entries for other offsets
687should be calculated relative to "pte".
688
689->page_mkwrite() is called when a previously read-only pte is about to become
690writeable. The filesystem again must ensure that there are no
691truncate/invalidate races or races with operations such as ->remap_file_range
692or ->copy_file_range, and then return with the page locked. Usually
693mapping->invalidate_lock is suitable for proper serialization. If the page has
694been truncated, the filesystem should not look up a new page like the ->fault()
695handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to
696retry the fault.
697
698->pfn_mkwrite() is the same as page_mkwrite but when the pte is
699VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
700VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
701after this call is to make the pte read-write, unless pfn_mkwrite returns
702an error.
703
704->access() is called when get_user_pages() fails in
705access_process_vm(), typically used to debug a process through
706/proc/pid/mem or ptrace. This function is needed only for
707VM_IO | VM_PFNMAP VMAs.
708
709--------------------------------------------------------------------------------
710
711 Dubious stuff
712
713(if you break something or notice that it is broken and do not fix it yourself
714- at least put it here)
1=======
2Locking
3=======
4
5The text below describes the locking rules for VFS-related methods.
6It is (believed to be) up-to-date. *Please*, if you change anything in
7prototypes or locking protocols - update this file. And update the relevant
8instances in the tree, don't leave that to maintainers of filesystems/devices/
9etc. At the very least, put the list of dubious cases in the end of this file.
10Don't turn it into log - maintainers of out-of-the-tree code are supposed to
11be able to use diff(1).
12
13Thing currently missing here: socket operations. Alexey?
14
15dentry_operations
16=================
17
18prototypes::
19
20 int (*d_revalidate)(struct dentry *, unsigned int);
21 int (*d_weak_revalidate)(struct dentry *, unsigned int);
22 int (*d_hash)(const struct dentry *, struct qstr *);
23 int (*d_compare)(const struct dentry *,
24 unsigned int, const char *, const struct qstr *);
25 int (*d_delete)(struct dentry *);
26 int (*d_init)(struct dentry *);
27 void (*d_release)(struct dentry *);
28 void (*d_iput)(struct dentry *, struct inode *);
29 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
30 struct vfsmount *(*d_automount)(struct path *path);
31 int (*d_manage)(const struct path *, bool);
32 struct dentry *(*d_real)(struct dentry *, const struct inode *);
33
34locking rules:
35
36================== =========== ======== ============== ========
37ops rename_lock ->d_lock may block rcu-walk
38================== =========== ======== ============== ========
39d_revalidate: no no yes (ref-walk) maybe
40d_weak_revalidate: no no yes no
41d_hash no no no maybe
42d_compare: yes no no maybe
43d_delete: no yes no no
44d_init: no no yes no
45d_release: no no yes no
46d_prune: no yes no no
47d_iput: no no yes no
48d_dname: no no no no
49d_automount: no no yes no
50d_manage: no no yes (ref-walk) maybe
51d_real no no yes no
52================== =========== ======== ============== ========
53
54inode_operations
55================
56
57prototypes::
58
59 int (*create) (struct inode *,struct dentry *,umode_t, bool);
60 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
61 int (*link) (struct dentry *,struct inode *,struct dentry *);
62 int (*unlink) (struct inode *,struct dentry *);
63 int (*symlink) (struct inode *,struct dentry *,const char *);
64 int (*mkdir) (struct inode *,struct dentry *,umode_t);
65 int (*rmdir) (struct inode *,struct dentry *);
66 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
67 int (*rename) (struct inode *, struct dentry *,
68 struct inode *, struct dentry *, unsigned int);
69 int (*readlink) (struct dentry *, char __user *,int);
70 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
71 void (*truncate) (struct inode *);
72 int (*permission) (struct inode *, int, unsigned int);
73 int (*get_acl)(struct inode *, int);
74 int (*setattr) (struct dentry *, struct iattr *);
75 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
76 ssize_t (*listxattr) (struct dentry *, char *, size_t);
77 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
78 void (*update_time)(struct inode *, struct timespec *, int);
79 int (*atomic_open)(struct inode *, struct dentry *,
80 struct file *, unsigned open_flag,
81 umode_t create_mode);
82 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
83
84locking rules:
85 all may block
86
87============ =============================================
88ops i_rwsem(inode)
89============ =============================================
90lookup: shared
91create: exclusive
92link: exclusive (both)
93mknod: exclusive
94symlink: exclusive
95mkdir: exclusive
96unlink: exclusive (both)
97rmdir: exclusive (both)(see below)
98rename: exclusive (all) (see below)
99readlink: no
100get_link: no
101setattr: exclusive
102permission: no (may not block if called in rcu-walk mode)
103get_acl: no
104getattr: no
105listxattr: no
106fiemap: no
107update_time: no
108atomic_open: exclusive
109tmpfile: no
110============ =============================================
111
112
113 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
114 exclusive on victim.
115 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
116
117See Documentation/filesystems/directory-locking.rst for more detailed discussion
118of the locking scheme for directory operations.
119
120xattr_handler operations
121========================
122
123prototypes::
124
125 bool (*list)(struct dentry *dentry);
126 int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
127 struct inode *inode, const char *name, void *buffer,
128 size_t size);
129 int (*set)(const struct xattr_handler *handler, struct dentry *dentry,
130 struct inode *inode, const char *name, const void *buffer,
131 size_t size, int flags);
132
133locking rules:
134 all may block
135
136===== ==============
137ops i_rwsem(inode)
138===== ==============
139list: no
140get: no
141set: exclusive
142===== ==============
143
144super_operations
145================
146
147prototypes::
148
149 struct inode *(*alloc_inode)(struct super_block *sb);
150 void (*free_inode)(struct inode *);
151 void (*destroy_inode)(struct inode *);
152 void (*dirty_inode) (struct inode *, int flags);
153 int (*write_inode) (struct inode *, struct writeback_control *wbc);
154 int (*drop_inode) (struct inode *);
155 void (*evict_inode) (struct inode *);
156 void (*put_super) (struct super_block *);
157 int (*sync_fs)(struct super_block *sb, int wait);
158 int (*freeze_fs) (struct super_block *);
159 int (*unfreeze_fs) (struct super_block *);
160 int (*statfs) (struct dentry *, struct kstatfs *);
161 int (*remount_fs) (struct super_block *, int *, char *);
162 void (*umount_begin) (struct super_block *);
163 int (*show_options)(struct seq_file *, struct dentry *);
164 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
165 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
166 int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
167
168locking rules:
169 All may block [not true, see below]
170
171====================== ============ ========================
172ops s_umount note
173====================== ============ ========================
174alloc_inode:
175free_inode: called from RCU callback
176destroy_inode:
177dirty_inode:
178write_inode:
179drop_inode: !!!inode->i_lock!!!
180evict_inode:
181put_super: write
182sync_fs: read
183freeze_fs: write
184unfreeze_fs: write
185statfs: maybe(read) (see below)
186remount_fs: write
187umount_begin: no
188show_options: no (namespace_sem)
189quota_read: no (see below)
190quota_write: no (see below)
191bdev_try_to_free_page: no (see below)
192====================== ============ ========================
193
194->statfs() has s_umount (shared) when called by ustat(2) (native or
195compat), but that's an accident of bad API; s_umount is used to pin
196the superblock down when we only have dev_t given us by userland to
197identify the superblock. Everything else (statfs(), fstatfs(), etc.)
198doesn't hold it when calling ->statfs() - superblock is pinned down
199by resolving the pathname passed to syscall.
200
201->quota_read() and ->quota_write() functions are both guaranteed to
202be the only ones operating on the quota file by the quota code (via
203dqio_sem) (unless an admin really wants to screw up something and
204writes to quota files with quotas on). For other details about locking
205see also dquot_operations section.
206
207->bdev_try_to_free_page is called from the ->releasepage handler of
208the block device inode. See there for more details.
209
210file_system_type
211================
212
213prototypes::
214
215 struct dentry *(*mount) (struct file_system_type *, int,
216 const char *, void *);
217 void (*kill_sb) (struct super_block *);
218
219locking rules:
220
221======= =========
222ops may block
223======= =========
224mount yes
225kill_sb yes
226======= =========
227
228->mount() returns ERR_PTR or the root dentry; its superblock should be locked
229on return.
230
231->kill_sb() takes a write-locked superblock, does all shutdown work on it,
232unlocks and drops the reference.
233
234address_space_operations
235========================
236prototypes::
237
238 int (*writepage)(struct page *page, struct writeback_control *wbc);
239 int (*readpage)(struct file *, struct page *);
240 int (*writepages)(struct address_space *, struct writeback_control *);
241 int (*set_page_dirty)(struct page *page);
242 int (*readpages)(struct file *filp, struct address_space *mapping,
243 struct list_head *pages, unsigned nr_pages);
244 int (*write_begin)(struct file *, struct address_space *mapping,
245 loff_t pos, unsigned len, unsigned flags,
246 struct page **pagep, void **fsdata);
247 int (*write_end)(struct file *, struct address_space *mapping,
248 loff_t pos, unsigned len, unsigned copied,
249 struct page *page, void *fsdata);
250 sector_t (*bmap)(struct address_space *, sector_t);
251 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
252 int (*releasepage) (struct page *, int);
253 void (*freepage)(struct page *);
254 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
255 bool (*isolate_page) (struct page *, isolate_mode_t);
256 int (*migratepage)(struct address_space *, struct page *, struct page *);
257 void (*putback_page) (struct page *);
258 int (*launder_page)(struct page *);
259 int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
260 int (*error_remove_page)(struct address_space *, struct page *);
261 int (*swap_activate)(struct file *);
262 int (*swap_deactivate)(struct file *);
263
264locking rules:
265 All except set_page_dirty and freepage may block
266
267====================== ======================== =========
268ops PageLocked(page) i_rwsem
269====================== ======================== =========
270writepage: yes, unlocks (see below)
271readpage: yes, unlocks
272writepages:
273set_page_dirty no
274readpages:
275write_begin: locks the page exclusive
276write_end: yes, unlocks exclusive
277bmap:
278invalidatepage: yes
279releasepage: yes
280freepage: yes
281direct_IO:
282isolate_page: yes
283migratepage: yes (both)
284putback_page: yes
285launder_page: yes
286is_partially_uptodate: yes
287error_remove_page: yes
288swap_activate: no
289swap_deactivate: no
290====================== ======================== =========
291
292->write_begin(), ->write_end() and ->readpage() may be called from
293the request handler (/dev/loop).
294
295->readpage() unlocks the page, either synchronously or via I/O
296completion.
297
298->readpages() populates the pagecache with the passed pages and starts
299I/O against them. They come unlocked upon I/O completion.
300
301->writepage() is used for two purposes: for "memory cleansing" and for
302"sync". These are quite different operations and the behaviour may differ
303depending upon the mode.
304
305If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
306it *must* start I/O against the page, even if that would involve
307blocking on in-progress I/O.
308
309If writepage is called for memory cleansing (sync_mode ==
310WBC_SYNC_NONE) then its role is to get as much writeout underway as
311possible. So writepage should try to avoid blocking against
312currently-in-progress I/O.
313
314If the filesystem is not called for "sync" and it determines that it
315would need to block against in-progress I/O to be able to start new I/O
316against the page the filesystem should redirty the page with
317redirty_page_for_writepage(), then unlock the page and return zero.
318This may also be done to avoid internal deadlocks, but rarely.
319
320If the filesystem is called for sync then it must wait on any
321in-progress I/O and then start new I/O.
322
323The filesystem should unlock the page synchronously, before returning to the
324caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
325value. WRITEPAGE_ACTIVATE means that page cannot really be written out
326currently, and VM should stop calling ->writepage() on this page for some
327time. VM does this by moving page to the head of the active list, hence the
328name.
329
330Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
331and return zero, writepage *must* run set_page_writeback() against the page,
332followed by unlocking it. Once set_page_writeback() has been run against the
333page, write I/O can be submitted and the write I/O completion handler must run
334end_page_writeback() once the I/O is complete. If no I/O is submitted, the
335filesystem must run end_page_writeback() against the page before returning from
336writepage.
337
338That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
339if the filesystem needs the page to be locked during writeout, that is ok, too,
340the page is allowed to be unlocked at any point in time between the calls to
341set_page_writeback() and end_page_writeback().
342
343Note, failure to run either redirty_page_for_writepage() or the combination of
344set_page_writeback()/end_page_writeback() on a page submitted to writepage
345will leave the page itself marked clean but it will be tagged as dirty in the
346radix tree. This incoherency can lead to all sorts of hard-to-debug problems
347in the filesystem like having dirty inodes at umount and losing written data.
348
349->writepages() is used for periodic writeback and for syscall-initiated
350sync operations. The address_space should start I/O against at least
351``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page
352which is written. The address_space implementation may write more (or less)
353pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
354If nr_to_write is NULL, all dirty pages must be written.
355
356writepages should _only_ write pages which are present on
357mapping->io_pages.
358
359->set_page_dirty() is called from various places in the kernel
360when the target page is marked as needing writeback. It may be called
361under spinlock (it cannot block) and is sometimes called with the page
362not locked.
363
364->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
365filesystems and by the swapper. The latter will eventually go away. Please,
366keep it that way and don't breed new callers.
367
368->invalidatepage() is called when the filesystem must attempt to drop
369some or all of the buffers from the page when it is being truncated. It
370returns zero on success. If ->invalidatepage is zero, the kernel uses
371block_invalidatepage() instead.
372
373->releasepage() is called when the kernel is about to try to drop the
374buffers from the page in preparation for freeing it. It returns zero to
375indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
376the kernel assumes that the fs has no private interest in the buffers.
377
378->freepage() is called when the kernel is done dropping the page
379from the page cache.
380
381->launder_page() may be called prior to releasing a page if
382it is still found to be dirty. It returns zero if the page was successfully
383cleaned, or an error value if not. Note that in order to prevent the page
384getting mapped back in and redirtied, it needs to be kept locked
385across the entire operation.
386
387->swap_activate will be called with a non-zero argument on
388files backing (non block device backed) swapfiles. A return value
389of zero indicates success, in which case this file can be used for
390backing swapspace. The swapspace operations will be proxied to the
391address space operations.
392
393->swap_deactivate() will be called in the sys_swapoff()
394path after ->swap_activate() returned success.
395
396file_lock_operations
397====================
398
399prototypes::
400
401 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
402 void (*fl_release_private)(struct file_lock *);
403
404
405locking rules:
406
407=================== ============= =========
408ops inode->i_lock may block
409=================== ============= =========
410fl_copy_lock: yes no
411fl_release_private: maybe maybe[1]_
412=================== ============= =========
413
414.. [1]:
415 ->fl_release_private for flock or POSIX locks is currently allowed
416 to block. Leases however can still be freed while the i_lock is held and
417 so fl_release_private called on a lease should not block.
418
419lock_manager_operations
420=======================
421
422prototypes::
423
424 void (*lm_notify)(struct file_lock *); /* unblock callback */
425 int (*lm_grant)(struct file_lock *, struct file_lock *, int);
426 void (*lm_break)(struct file_lock *); /* break_lease callback */
427 int (*lm_change)(struct file_lock **, int);
428
429locking rules:
430
431========== ============= ================= =========
432ops inode->i_lock blocked_lock_lock may block
433========== ============= ================= =========
434lm_notify: yes yes no
435lm_grant: no no no
436lm_break: yes no no
437lm_change yes no no
438========== ============= ================= =========
439
440buffer_head
441===========
442
443prototypes::
444
445 void (*b_end_io)(struct buffer_head *bh, int uptodate);
446
447locking rules:
448
449called from interrupts. In other words, extreme care is needed here.
450bh is locked, but that's all warranties we have here. Currently only RAID1,
451highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
452call this method upon the IO completion.
453
454block_device_operations
455=======================
456prototypes::
457
458 int (*open) (struct block_device *, fmode_t);
459 int (*release) (struct gendisk *, fmode_t);
460 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
461 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
462 int (*direct_access) (struct block_device *, sector_t, void **,
463 unsigned long *);
464 int (*media_changed) (struct gendisk *);
465 void (*unlock_native_capacity) (struct gendisk *);
466 int (*revalidate_disk) (struct gendisk *);
467 int (*getgeo)(struct block_device *, struct hd_geometry *);
468 void (*swap_slot_free_notify) (struct block_device *, unsigned long);
469
470locking rules:
471
472======================= ===================
473ops bd_mutex
474======================= ===================
475open: yes
476release: yes
477ioctl: no
478compat_ioctl: no
479direct_access: no
480media_changed: no
481unlock_native_capacity: no
482revalidate_disk: no
483getgeo: no
484swap_slot_free_notify: no (see below)
485======================= ===================
486
487media_changed, unlock_native_capacity and revalidate_disk are called only from
488check_disk_change().
489
490swap_slot_free_notify is called with swap_lock and sometimes the page lock
491held.
492
493
494file_operations
495===============
496
497prototypes::
498
499 loff_t (*llseek) (struct file *, loff_t, int);
500 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
501 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
502 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
503 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
504 int (*iterate) (struct file *, struct dir_context *);
505 int (*iterate_shared) (struct file *, struct dir_context *);
506 __poll_t (*poll) (struct file *, struct poll_table_struct *);
507 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
508 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
509 int (*mmap) (struct file *, struct vm_area_struct *);
510 int (*open) (struct inode *, struct file *);
511 int (*flush) (struct file *);
512 int (*release) (struct inode *, struct file *);
513 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
514 int (*fasync) (int, struct file *, int);
515 int (*lock) (struct file *, int, struct file_lock *);
516 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
517 loff_t *);
518 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
519 loff_t *);
520 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
521 void __user *);
522 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
523 loff_t *, int);
524 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
525 unsigned long, unsigned long, unsigned long);
526 int (*check_flags)(int);
527 int (*flock) (struct file *, int, struct file_lock *);
528 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
529 size_t, unsigned int);
530 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
531 size_t, unsigned int);
532 int (*setlease)(struct file *, long, struct file_lock **, void **);
533 long (*fallocate)(struct file *, int, loff_t, loff_t);
534
535locking rules:
536 All may block.
537
538->llseek() locking has moved from llseek to the individual llseek
539implementations. If your fs is not using generic_file_llseek, you
540need to acquire and release the appropriate locks in your ->llseek().
541For many filesystems, it is probably safe to acquire the inode
542mutex or just to use i_size_read() instead.
543Note: this does not protect the file->f_pos against concurrent modifications
544since this is something the userspace has to take care about.
545
546->iterate() is called with i_rwsem exclusive.
547
548->iterate_shared() is called with i_rwsem at least shared.
549
550->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
551Most instances call fasync_helper(), which does that maintenance, so it's
552not normally something one needs to worry about. Return values > 0 will be
553mapped to zero in the VFS layer.
554
555->readdir() and ->ioctl() on directories must be changed. Ideally we would
556move ->readdir() to inode_operations and use a separate method for directory
557->ioctl() or kill the latter completely. One of the problems is that for
558anything that resembles union-mount we won't have a struct file for all
559components. And there are other reasons why the current interface is a mess...
560
561->read on directories probably must go away - we should just enforce -EISDIR
562in sys_read() and friends.
563
564->setlease operations should call generic_setlease() before or after setting
565the lease within the individual filesystem to record the result of the
566operation
567
568dquot_operations
569================
570
571prototypes::
572
573 int (*write_dquot) (struct dquot *);
574 int (*acquire_dquot) (struct dquot *);
575 int (*release_dquot) (struct dquot *);
576 int (*mark_dirty) (struct dquot *);
577 int (*write_info) (struct super_block *, int);
578
579These operations are intended to be more or less wrapping functions that ensure
580a proper locking wrt the filesystem and call the generic quota operations.
581
582What filesystem should expect from the generic quota functions:
583
584============== ============ =========================
585ops FS recursion Held locks when called
586============== ============ =========================
587write_dquot: yes dqonoff_sem or dqptr_sem
588acquire_dquot: yes dqonoff_sem or dqptr_sem
589release_dquot: yes dqonoff_sem or dqptr_sem
590mark_dirty: no -
591write_info: yes dqonoff_sem
592============== ============ =========================
593
594FS recursion means calling ->quota_read() and ->quota_write() from superblock
595operations.
596
597More details about quota locking can be found in fs/dquot.c.
598
599vm_operations_struct
600====================
601
602prototypes::
603
604 void (*open)(struct vm_area_struct*);
605 void (*close)(struct vm_area_struct*);
606 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
607 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
608 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
609 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
610
611locking rules:
612
613============= ======== ===========================
614ops mmap_sem PageLocked(page)
615============= ======== ===========================
616open: yes
617close: yes
618fault: yes can return with page locked
619map_pages: yes
620page_mkwrite: yes can return with page locked
621pfn_mkwrite: yes
622access: yes
623============= ======== ===========================
624
625->fault() is called when a previously not present pte is about
626to be faulted in. The filesystem must find and return the page associated
627with the passed in "pgoff" in the vm_fault structure. If it is possible that
628the page may be truncated and/or invalidated, then the filesystem must lock
629the page, then ensure it is not already truncated (the page lock will block
630subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
631locked. The VM will unlock the page.
632
633->map_pages() is called when VM asks to map easy accessible pages.
634Filesystem should find and map pages associated with offsets from "start_pgoff"
635till "end_pgoff". ->map_pages() is called with page table locked and must
636not block. If it's not possible to reach a page without blocking,
637filesystem should skip it. Filesystem should use do_set_pte() to setup
638page table entry. Pointer to entry associated with the page is passed in
639"pte" field in vm_fault structure. Pointers to entries for other offsets
640should be calculated relative to "pte".
641
642->page_mkwrite() is called when a previously read-only pte is
643about to become writeable. The filesystem again must ensure that there are
644no truncate/invalidate races, and then return with the page locked. If
645the page has been truncated, the filesystem should not look up a new page
646like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
647will cause the VM to retry the fault.
648
649->pfn_mkwrite() is the same as page_mkwrite but when the pte is
650VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
651VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
652after this call is to make the pte read-write, unless pfn_mkwrite returns
653an error.
654
655->access() is called when get_user_pages() fails in
656access_process_vm(), typically used to debug a process through
657/proc/pid/mem or ptrace. This function is needed only for
658VM_IO | VM_PFNMAP VMAs.
659
660--------------------------------------------------------------------------------
661
662 Dubious stuff
663
664(if you break something or notice that it is broken and do not fix it yourself
665- at least put it here)