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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_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 inode *, struct dentry *, umode_t);
83	int (*fileattr_set)(struct user_namespace *mnt_userns,
84			    struct dentry *dentry, struct fileattr *fa);
85	int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
86
87locking rules:
88	all may block
89
90=============	=============================================
91ops		i_rwsem(inode)
92=============	=============================================
93lookup:		shared
94create:		exclusive
95link:		exclusive (both)
96mknod:		exclusive
97symlink:	exclusive
98mkdir:		exclusive
99unlink:		exclusive (both)
100rmdir:		exclusive (both)(see below)
101rename:		exclusive (both parents, some children)	(see below)
102readlink:	no
103get_link:	no
104setattr:	exclusive
105permission:	no (may not block if called in rcu-walk mode)
106get_acl:	no
107getattr:	no
108listxattr:	no
109fiemap:		no
110update_time:	no
111atomic_open:	shared (exclusive if O_CREAT is set in open flags)
112tmpfile:	no
113fileattr_get:	no or exclusive
114fileattr_set:	exclusive
115=============	=============================================
116
117
118	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
119	exclusive on victim.
120	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
121	->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories
122	involved.
123	->rename() has ->i_rwsem exclusive on any subdirectory that changes parent.
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	int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
176
177locking rules:
178	All may block [not true, see below]
179
180======================	============	========================
181ops			s_umount	note
182======================	============	========================
183alloc_inode:
184free_inode:				called from RCU callback
185destroy_inode:
186dirty_inode:
187write_inode:
188drop_inode:				!!!inode->i_lock!!!
189evict_inode:
190put_super:		write
191sync_fs:		read
192freeze_fs:		write
193unfreeze_fs:		write
194statfs:			maybe(read)	(see below)
195remount_fs:		write
196umount_begin:		no
197show_options:		no		(namespace_sem)
198quota_read:		no		(see below)
199quota_write:		no		(see below)
200bdev_try_to_free_page:	no		(see below)
201======================	============	========================
202
203->statfs() has s_umount (shared) when called by ustat(2) (native or
204compat), but that's an accident of bad API; s_umount is used to pin
205the superblock down when we only have dev_t given us by userland to
206identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
207doesn't hold it when calling ->statfs() - superblock is pinned down
208by resolving the pathname passed to syscall.
209
210->quota_read() and ->quota_write() functions are both guaranteed to
211be the only ones operating on the quota file by the quota code (via
212dqio_sem) (unless an admin really wants to screw up something and
213writes to quota files with quotas on). For other details about locking
214see also dquot_operations section.
215
216->bdev_try_to_free_page is called from the ->releasepage handler of
217the block device inode.  See there for more details.
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 (*readpage)(struct file *, struct page *);
249	int (*writepages)(struct address_space *, struct writeback_control *);
250	int (*set_page_dirty)(struct page *page);
251	void (*readahead)(struct readahead_control *);
252	int (*readpages)(struct file *filp, struct address_space *mapping,
253			struct list_head *pages, unsigned nr_pages);
254	int (*write_begin)(struct file *, struct address_space *mapping,
255				loff_t pos, unsigned len, unsigned flags,
256				struct page **pagep, void **fsdata);
257	int (*write_end)(struct file *, struct address_space *mapping,
258				loff_t pos, unsigned len, unsigned copied,
259				struct page *page, void *fsdata);
260	sector_t (*bmap)(struct address_space *, sector_t);
261	void (*invalidatepage) (struct page *, unsigned int, unsigned int);
262	int (*releasepage) (struct page *, int);
263	void (*freepage)(struct page *);
264	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
265	bool (*isolate_page) (struct page *, isolate_mode_t);
266	int (*migratepage)(struct address_space *, struct page *, struct page *);
267	void (*putback_page) (struct page *);
268	int (*launder_page)(struct page *);
269	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
270	int (*error_remove_page)(struct address_space *, struct page *);
271	int (*swap_activate)(struct file *);
272	int (*swap_deactivate)(struct file *);
273
274locking rules:
275	All except set_page_dirty and freepage may block
276
277======================	======================== =========	===============
278ops			PageLocked(page)	 i_rwsem	invalidate_lock
279======================	======================== =========	===============
280writepage:		yes, unlocks (see below)
281readpage:		yes, unlocks				shared
282writepages:
283set_page_dirty		no
284readahead:		yes, unlocks				shared
285readpages:		no					shared
286write_begin:		locks the page		 exclusive
287write_end:		yes, unlocks		 exclusive
288bmap:
289invalidatepage:		yes					exclusive
290releasepage:		yes
291freepage:		yes
292direct_IO:
293isolate_page:		yes
294migratepage:		yes (both)
295putback_page:		yes
296launder_page:		yes
297is_partially_uptodate:	yes
298error_remove_page:	yes
299swap_activate:		no
300swap_deactivate:	no
301======================	======================== =========	===============
302
303->write_begin(), ->write_end() and ->readpage() may be called from
304the request handler (/dev/loop).
305
306->readpage() unlocks the page, either synchronously or via I/O
307completion.
308
309->readahead() unlocks the pages that I/O is attempted on like ->readpage().
310
311->readpages() populates the pagecache with the passed pages and starts
312I/O against them.  They come unlocked upon I/O completion.
313
314->writepage() is used for two purposes: for "memory cleansing" and for
315"sync".  These are quite different operations and the behaviour may differ
316depending upon the mode.
317
318If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
319it *must* start I/O against the page, even if that would involve
320blocking on in-progress I/O.
321
322If writepage is called for memory cleansing (sync_mode ==
323WBC_SYNC_NONE) then its role is to get as much writeout underway as
324possible.  So writepage should try to avoid blocking against
325currently-in-progress I/O.
326
327If the filesystem is not called for "sync" and it determines that it
328would need to block against in-progress I/O to be able to start new I/O
329against the page the filesystem should redirty the page with
330redirty_page_for_writepage(), then unlock the page and return zero.
331This may also be done to avoid internal deadlocks, but rarely.
332
333If the filesystem is called for sync then it must wait on any
334in-progress I/O and then start new I/O.
335
336The filesystem should unlock the page synchronously, before returning to the
337caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
338value. WRITEPAGE_ACTIVATE means that page cannot really be written out
339currently, and VM should stop calling ->writepage() on this page for some
340time. VM does this by moving page to the head of the active list, hence the
341name.
342
343Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
344and return zero, writepage *must* run set_page_writeback() against the page,
345followed by unlocking it.  Once set_page_writeback() has been run against the
346page, write I/O can be submitted and the write I/O completion handler must run
347end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
348filesystem must run end_page_writeback() against the page before returning from
349writepage.
350
351That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
352if the filesystem needs the page to be locked during writeout, that is ok, too,
353the page is allowed to be unlocked at any point in time between the calls to
354set_page_writeback() and end_page_writeback().
355
356Note, failure to run either redirty_page_for_writepage() or the combination of
357set_page_writeback()/end_page_writeback() on a page submitted to writepage
358will leave the page itself marked clean but it will be tagged as dirty in the
359radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
360in the filesystem like having dirty inodes at umount and losing written data.
361
362->writepages() is used for periodic writeback and for syscall-initiated
363sync operations.  The address_space should start I/O against at least
364``*nr_to_write`` pages.  ``*nr_to_write`` must be decremented for each page
365which is written.  The address_space implementation may write more (or less)
366pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
367If nr_to_write is NULL, all dirty pages must be written.
368
369writepages should _only_ write pages which are present on
370mapping->io_pages.
371
372->set_page_dirty() is called from various places in the kernel
373when the target page is marked as needing writeback.  It may be called
374under spinlock (it cannot block) and is sometimes called with the page
375not locked.
376
377->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
378filesystems and by the swapper. The latter will eventually go away.  Please,
379keep it that way and don't breed new callers.
380
381->invalidatepage() is called when the filesystem must attempt to drop
382some or all of the buffers from the page when it is being truncated. It
383returns zero on success. If ->invalidatepage is zero, the kernel uses
384block_invalidatepage() instead. The filesystem must exclusively acquire
385invalidate_lock before invalidating page cache in truncate / hole punch path
386(and thus calling into ->invalidatepage) to block races between page cache
387invalidation and page cache filling functions (fault, read, ...).
388
389->releasepage() is called when the kernel is about to try to drop the
390buffers from the page in preparation for freeing it.  It returns zero to
391indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
392the kernel assumes that the fs has no private interest in the buffers.
393
394->freepage() is called when the kernel is done dropping the page
395from the page cache.
396
397->launder_page() may be called prior to releasing a page if
398it is still found to be dirty. It returns zero if the page was successfully
399cleaned, or an error value if not. Note that in order to prevent the page
400getting mapped back in and redirtied, it needs to be kept locked
401across the entire operation.
402
403->swap_activate will be called with a non-zero argument on
404files backing (non block device backed) swapfiles. A return value
405of zero indicates success, in which case this file can be used for
406backing swapspace. The swapspace operations will be proxied to the
407address space operations.
408
409->swap_deactivate() will be called in the sys_swapoff()
410path after ->swap_activate() returned success.
411
412file_lock_operations
413====================
414
415prototypes::
416
417	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
418	void (*fl_release_private)(struct file_lock *);
419
420
421locking rules:
422
423===================	=============	=========
424ops			inode->i_lock	may block
425===================	=============	=========
426fl_copy_lock:		yes		no
427fl_release_private:	maybe		maybe[1]_
428===================	=============	=========
429
430.. [1]:
431   ->fl_release_private for flock or POSIX locks is currently allowed
432   to block. Leases however can still be freed while the i_lock is held and
433   so fl_release_private called on a lease should not block.
434
435lock_manager_operations
436=======================
437
438prototypes::
439
440	void (*lm_notify)(struct file_lock *);  /* unblock callback */
441	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
442	void (*lm_break)(struct file_lock *); /* break_lease callback */
443	int (*lm_change)(struct file_lock **, int);
444	bool (*lm_breaker_owns_lease)(struct file_lock *);
445
446locking rules:
447
448======================	=============	=================	=========
449ops			inode->i_lock	blocked_lock_lock	may block
450======================	=============	=================	=========
451lm_notify:		yes		yes			no
452lm_grant:		no		no			no
453lm_break:		yes		no			no
454lm_change		yes		no			no
455lm_breaker_owns_lease:	no		no			no
456======================	=============	=================	=========
457
458buffer_head
459===========
460
461prototypes::
462
463	void (*b_end_io)(struct buffer_head *bh, int uptodate);
464
465locking rules:
466
467called from interrupts. In other words, extreme care is needed here.
468bh is locked, but that's all warranties we have here. Currently only RAID1,
469highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
470call this method upon the IO completion.
471
472block_device_operations
473=======================
474prototypes::
475
476	int (*open) (struct block_device *, fmode_t);
477	int (*release) (struct gendisk *, fmode_t);
478	int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
479	int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
480	int (*direct_access) (struct block_device *, sector_t, void **,
481				unsigned long *);
482	void (*unlock_native_capacity) (struct gendisk *);
483	int (*getgeo)(struct block_device *, struct hd_geometry *);
484	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
485
486locking rules:
487
488======================= ===================
489ops			open_mutex
490======================= ===================
491open:			yes
492release:		yes
493ioctl:			no
494compat_ioctl:		no
495direct_access:		no
496unlock_native_capacity:	no
497getgeo:			no
498swap_slot_free_notify:	no	(see below)
499======================= ===================
500
501swap_slot_free_notify is called with swap_lock and sometimes the page lock
502held.
503
504
505file_operations
506===============
507
508prototypes::
509
510	loff_t (*llseek) (struct file *, loff_t, int);
511	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
512	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
513	ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
514	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
515	int (*iopoll) (struct kiocb *kiocb, bool spin);
516	int (*iterate) (struct file *, struct dir_context *);
517	int (*iterate_shared) (struct file *, struct dir_context *);
518	__poll_t (*poll) (struct file *, struct poll_table_struct *);
519	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
520	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
521	int (*mmap) (struct file *, struct vm_area_struct *);
522	int (*open) (struct inode *, struct file *);
523	int (*flush) (struct file *);
524	int (*release) (struct inode *, struct file *);
525	int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
526	int (*fasync) (int, struct file *, int);
527	int (*lock) (struct file *, int, struct file_lock *);
528	ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
529			loff_t *, int);
530	unsigned long (*get_unmapped_area)(struct file *, unsigned long,
531			unsigned long, unsigned long, unsigned long);
532	int (*check_flags)(int);
533	int (*flock) (struct file *, int, struct file_lock *);
534	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
535			size_t, unsigned int);
536	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
537			size_t, unsigned int);
538	int (*setlease)(struct file *, long, struct file_lock **, void **);
539	long (*fallocate)(struct file *, int, loff_t, loff_t);
540	void (*show_fdinfo)(struct seq_file *m, struct file *f);
541	unsigned (*mmap_capabilities)(struct file *);
542	ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
543			loff_t, size_t, unsigned int);
544	loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
545			struct file *file_out, loff_t pos_out,
546			loff_t len, unsigned int remap_flags);
547	int (*fadvise)(struct file *, loff_t, loff_t, int);
548
549locking rules:
550	All may block.
551
552->llseek() locking has moved from llseek to the individual llseek
553implementations.  If your fs is not using generic_file_llseek, you
554need to acquire and release the appropriate locks in your ->llseek().
555For many filesystems, it is probably safe to acquire the inode
556mutex or just to use i_size_read() instead.
557Note: this does not protect the file->f_pos against concurrent modifications
558since this is something the userspace has to take care about.
559
560->iterate() is called with i_rwsem exclusive.
561
562->iterate_shared() is called with i_rwsem at least shared.
563
564->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
565Most instances call fasync_helper(), which does that maintenance, so it's
566not normally something one needs to worry about.  Return values > 0 will be
567mapped to zero in the VFS layer.
568
569->readdir() and ->ioctl() on directories must be changed. Ideally we would
570move ->readdir() to inode_operations and use a separate method for directory
571->ioctl() or kill the latter completely. One of the problems is that for
572anything that resembles union-mount we won't have a struct file for all
573components. And there are other reasons why the current interface is a mess...
574
575->read on directories probably must go away - we should just enforce -EISDIR
576in sys_read() and friends.
577
578->setlease operations should call generic_setlease() before or after setting
579the lease within the individual filesystem to record the result of the
580operation
581
582->fallocate implementation must be really careful to maintain page cache
583consistency when punching holes or performing other operations that invalidate
584page cache contents. Usually the filesystem needs to call
585truncate_inode_pages_range() to invalidate relevant range of the page cache.
586However the filesystem usually also needs to update its internal (and on disk)
587view of file offset -> disk block mapping. Until this update is finished, the
588filesystem needs to block page faults and reads from reloading now-stale page
589cache contents from the disk. Since VFS acquires mapping->invalidate_lock in
590shared mode when loading pages from disk (filemap_fault(), filemap_read(),
591readahead paths), the fallocate implementation must take the invalidate_lock to
592prevent reloading.
593
594->copy_file_range and ->remap_file_range implementations need to serialize
595against modifications of file data while the operation is running. For
596blocking changes through write(2) and similar operations inode->i_rwsem can be
597used. To block changes to file contents via a memory mapping during the
598operation, the filesystem must take mapping->invalidate_lock to coordinate
599with ->page_mkwrite.
600
601dquot_operations
602================
603
604prototypes::
605
606	int (*write_dquot) (struct dquot *);
607	int (*acquire_dquot) (struct dquot *);
608	int (*release_dquot) (struct dquot *);
609	int (*mark_dirty) (struct dquot *);
610	int (*write_info) (struct super_block *, int);
611
612These operations are intended to be more or less wrapping functions that ensure
613a proper locking wrt the filesystem and call the generic quota operations.
614
615What filesystem should expect from the generic quota functions:
616
617==============	============	=========================
618ops		FS recursion	Held locks when called
619==============	============	=========================
620write_dquot:	yes		dqonoff_sem or dqptr_sem
621acquire_dquot:	yes		dqonoff_sem or dqptr_sem
622release_dquot:	yes		dqonoff_sem or dqptr_sem
623mark_dirty:	no		-
624write_info:	yes		dqonoff_sem
625==============	============	=========================
626
627FS recursion means calling ->quota_read() and ->quota_write() from superblock
628operations.
629
630More details about quota locking can be found in fs/dquot.c.
631
632vm_operations_struct
633====================
634
635prototypes::
636
637	void (*open)(struct vm_area_struct*);
638	void (*close)(struct vm_area_struct*);
639	vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
640	vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
641	vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
642	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
643
644locking rules:
645
646=============	=========	===========================
647ops		mmap_lock	PageLocked(page)
648=============	=========	===========================
649open:		yes
650close:		yes
651fault:		yes		can return with page locked
652map_pages:	yes
653page_mkwrite:	yes		can return with page locked
654pfn_mkwrite:	yes
655access:		yes
656=============	=========	===========================
657
658->fault() is called when a previously not present pte is about to be faulted
659in. The filesystem must find and return the page associated with the passed in
660"pgoff" in the vm_fault structure. If it is possible that the page may be
661truncated and/or invalidated, then the filesystem must lock invalidate_lock,
662then ensure the page is not already truncated (invalidate_lock will block
663subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
664locked. The VM will unlock the page.
665
666->map_pages() is called when VM asks to map easy accessible pages.
667Filesystem should find and map pages associated with offsets from "start_pgoff"
668till "end_pgoff". ->map_pages() is called with page table locked and must
669not block.  If it's not possible to reach a page without blocking,
670filesystem should skip it. Filesystem should use do_set_pte() to setup
671page table entry. Pointer to entry associated with the page is passed in
672"pte" field in vm_fault structure. Pointers to entries for other offsets
673should be calculated relative to "pte".
674
675->page_mkwrite() is called when a previously read-only pte is about to become
676writeable. The filesystem again must ensure that there are no
677truncate/invalidate races or races with operations such as ->remap_file_range
678or ->copy_file_range, and then return with the page locked. Usually
679mapping->invalidate_lock is suitable for proper serialization. If the page has
680been truncated, the filesystem should not look up a new page like the ->fault()
681handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to
682retry the fault.
683
684->pfn_mkwrite() is the same as page_mkwrite but when the pte is
685VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
686VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
687after this call is to make the pte read-write, unless pfn_mkwrite returns
688an error.
689
690->access() is called when get_user_pages() fails in
691access_process_vm(), typically used to debug a process through
692/proc/pid/mem or ptrace.  This function is needed only for
693VM_IO | VM_PFNMAP VMAs.
694
695--------------------------------------------------------------------------------
696
697			Dubious stuff
698
699(if you break something or notice that it is broken and do not fix it yourself
700- at least put it here)
701