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