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