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 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 page **pagep, void **fsdata);
255	int (*write_end)(struct file *, struct address_space *mapping,
256				loff_t pos, unsigned len, unsigned copied,
257				struct page *page, 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_page)(struct address_space *, struct page *);
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 page		 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_page:	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)
715