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
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 mnt_idmap *idmap,
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)
697