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Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.

1		The text below describes the locking rules for VFS-related methods.
2	It is (believed to be) up-to-date. *Please*, if you change anything in
3	prototypes or locking protocols - update this file. And update the relevant
4	instances in the tree, don't leave that to maintainers of filesystems/devices/
5	etc. At the very least, put the list of dubious cases in the end of this file.
6	Don't turn it into log - maintainers of out-of-the-tree code are supposed to
7	be able to use diff(1).
8		Thing currently missing here: socket operations. Alexey?
9	
10	--------------------------- dentry_operations --------------------------
11	prototypes:
12		int (*d_revalidate)(struct dentry *, unsigned int);
13		int (*d_weak_revalidate)(struct dentry *, unsigned int);
14		int (*d_hash)(const struct dentry *, struct qstr *);
15		int (*d_compare)(const struct dentry *, const struct dentry *,
16				unsigned int, const char *, const struct qstr *);
17		int (*d_delete)(struct dentry *);
18		void (*d_release)(struct dentry *);
19		void (*d_iput)(struct dentry *, struct inode *);
20		char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
21		struct vfsmount *(*d_automount)(struct path *path);
22		int (*d_manage)(struct dentry *, bool);
23	
24	locking rules:
25			rename_lock	->d_lock	may block	rcu-walk
26	d_revalidate:	no		no		yes (ref-walk)	maybe
27	d_weak_revalidate:no		no		yes	 	no
28	d_hash		no		no		no		maybe
29	d_compare:	yes		no		no		maybe
30	d_delete:	no		yes		no		no
31	d_release:	no		no		yes		no
32	d_prune:        no              yes             no              no
33	d_iput:		no		no		yes		no
34	d_dname:	no		no		no		no
35	d_automount:	no		no		yes		no
36	d_manage:	no		no		yes (ref-walk)	maybe
37	
38	--------------------------- inode_operations --------------------------- 
39	prototypes:
40		int (*create) (struct inode *,struct dentry *,umode_t, bool);
41		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
42		int (*link) (struct dentry *,struct inode *,struct dentry *);
43		int (*unlink) (struct inode *,struct dentry *);
44		int (*symlink) (struct inode *,struct dentry *,const char *);
45		int (*mkdir) (struct inode *,struct dentry *,umode_t);
46		int (*rmdir) (struct inode *,struct dentry *);
47		int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
48		int (*rename) (struct inode *, struct dentry *,
49				struct inode *, struct dentry *);
50		int (*rename2) (struct inode *, struct dentry *,
51				struct inode *, struct dentry *, unsigned int);
52		int (*readlink) (struct dentry *, char __user *,int);
53		void * (*follow_link) (struct dentry *, struct nameidata *);
54		void (*put_link) (struct dentry *, struct nameidata *, void *);
55		void (*truncate) (struct inode *);
56		int (*permission) (struct inode *, int, unsigned int);
57		int (*get_acl)(struct inode *, int);
58		int (*setattr) (struct dentry *, struct iattr *);
59		int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
60		int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
61		ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
62		ssize_t (*listxattr) (struct dentry *, char *, size_t);
63		int (*removexattr) (struct dentry *, const char *);
64		int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
65		void (*update_time)(struct inode *, struct timespec *, int);
66		int (*atomic_open)(struct inode *, struct dentry *,
67					struct file *, unsigned open_flag,
68					umode_t create_mode, int *opened);
69		int (*tmpfile) (struct inode *, struct dentry *, umode_t);
70		int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
71	
72	locking rules:
73		all may block
74			i_mutex(inode)
75	lookup:		yes
76	create:		yes
77	link:		yes (both)
78	mknod:		yes
79	symlink:	yes
80	mkdir:		yes
81	unlink:		yes (both)
82	rmdir:		yes (both)	(see below)
83	rename:		yes (all)	(see below)
84	rename2:	yes (all)	(see below)
85	readlink:	no
86	follow_link:	no
87	put_link:	no
88	setattr:	yes
89	permission:	no (may not block if called in rcu-walk mode)
90	get_acl:	no
91	getattr:	no
92	setxattr:	yes
93	getxattr:	no
94	listxattr:	no
95	removexattr:	yes
96	fiemap:		no
97	update_time:	no
98	atomic_open:	yes
99	tmpfile:	no
100	dentry_open:	no
101	
102		Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
103	victim.
104		cross-directory ->rename() and rename2() has (per-superblock)
105	->s_vfs_rename_sem.
106	
107	See Documentation/filesystems/directory-locking for more detailed discussion
108	of the locking scheme for directory operations.
109	
110	--------------------------- super_operations ---------------------------
111	prototypes:
112		struct inode *(*alloc_inode)(struct super_block *sb);
113		void (*destroy_inode)(struct inode *);
114		void (*dirty_inode) (struct inode *, int flags);
115		int (*write_inode) (struct inode *, struct writeback_control *wbc);
116		int (*drop_inode) (struct inode *);
117		void (*evict_inode) (struct inode *);
118		void (*put_super) (struct super_block *);
119		int (*sync_fs)(struct super_block *sb, int wait);
120		int (*freeze_fs) (struct super_block *);
121		int (*unfreeze_fs) (struct super_block *);
122		int (*statfs) (struct dentry *, struct kstatfs *);
123		int (*remount_fs) (struct super_block *, int *, char *);
124		void (*umount_begin) (struct super_block *);
125		int (*show_options)(struct seq_file *, struct dentry *);
126		ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
127		ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
128		int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
129	
130	locking rules:
131		All may block [not true, see below]
132				s_umount
133	alloc_inode:
134	destroy_inode:
135	dirty_inode:
136	write_inode:
137	drop_inode:				!!!inode->i_lock!!!
138	evict_inode:
139	put_super:		write
140	sync_fs:		read
141	freeze_fs:		write
142	unfreeze_fs:		write
143	statfs:			maybe(read)	(see below)
144	remount_fs:		write
145	umount_begin:		no
146	show_options:		no		(namespace_sem)
147	quota_read:		no		(see below)
148	quota_write:		no		(see below)
149	bdev_try_to_free_page:	no		(see below)
150	
151	->statfs() has s_umount (shared) when called by ustat(2) (native or
152	compat), but that's an accident of bad API; s_umount is used to pin
153	the superblock down when we only have dev_t given us by userland to
154	identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
155	doesn't hold it when calling ->statfs() - superblock is pinned down
156	by resolving the pathname passed to syscall.
157	->quota_read() and ->quota_write() functions are both guaranteed to
158	be the only ones operating on the quota file by the quota code (via
159	dqio_sem) (unless an admin really wants to screw up something and
160	writes to quota files with quotas on). For other details about locking
161	see also dquot_operations section.
162	->bdev_try_to_free_page is called from the ->releasepage handler of
163	the block device inode.  See there for more details.
164	
165	--------------------------- file_system_type ---------------------------
166	prototypes:
167		struct dentry *(*mount) (struct file_system_type *, int,
168			       const char *, void *);
169		void (*kill_sb) (struct super_block *);
170	locking rules:
171			may block
172	mount		yes
173	kill_sb		yes
174	
175	->mount() returns ERR_PTR or the root dentry; its superblock should be locked
176	on return.
177	->kill_sb() takes a write-locked superblock, does all shutdown work on it,
178	unlocks and drops the reference.
179	
180	--------------------------- address_space_operations --------------------------
181	prototypes:
182		int (*writepage)(struct page *page, struct writeback_control *wbc);
183		int (*readpage)(struct file *, struct page *);
184		int (*sync_page)(struct page *);
185		int (*writepages)(struct address_space *, struct writeback_control *);
186		int (*set_page_dirty)(struct page *page);
187		int (*readpages)(struct file *filp, struct address_space *mapping,
188				struct list_head *pages, unsigned nr_pages);
189		int (*write_begin)(struct file *, struct address_space *mapping,
190					loff_t pos, unsigned len, unsigned flags,
191					struct page **pagep, void **fsdata);
192		int (*write_end)(struct file *, struct address_space *mapping,
193					loff_t pos, unsigned len, unsigned copied,
194					struct page *page, void *fsdata);
195		sector_t (*bmap)(struct address_space *, sector_t);
196		void (*invalidatepage) (struct page *, unsigned int, unsigned int);
197		int (*releasepage) (struct page *, int);
198		void (*freepage)(struct page *);
199		int (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t offset);
200		int (*migratepage)(struct address_space *, struct page *, struct page *);
201		int (*launder_page)(struct page *);
202		int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
203		int (*error_remove_page)(struct address_space *, struct page *);
204		int (*swap_activate)(struct file *);
205		int (*swap_deactivate)(struct file *);
206	
207	locking rules:
208		All except set_page_dirty and freepage may block
209	
210				PageLocked(page)	i_mutex
211	writepage:		yes, unlocks (see below)
212	readpage:		yes, unlocks
213	sync_page:		maybe
214	writepages:
215	set_page_dirty		no
216	readpages:
217	write_begin:		locks the page		yes
218	write_end:		yes, unlocks		yes
219	bmap:
220	invalidatepage:		yes
221	releasepage:		yes
222	freepage:		yes
223	direct_IO:
224	migratepage:		yes (both)
225	launder_page:		yes
226	is_partially_uptodate:	yes
227	error_remove_page:	yes
228	swap_activate:		no
229	swap_deactivate:	no
230	
231		->write_begin(), ->write_end(), ->sync_page() and ->readpage()
232	may be called from the request handler (/dev/loop).
233	
234		->readpage() unlocks the page, either synchronously or via I/O
235	completion.
236	
237		->readpages() populates the pagecache with the passed pages and starts
238	I/O against them.  They come unlocked upon I/O completion.
239	
240		->writepage() is used for two purposes: for "memory cleansing" and for
241	"sync".  These are quite different operations and the behaviour may differ
242	depending upon the mode.
243	
244	If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
245	it *must* start I/O against the page, even if that would involve
246	blocking on in-progress I/O.
247	
248	If writepage is called for memory cleansing (sync_mode ==
249	WBC_SYNC_NONE) then its role is to get as much writeout underway as
250	possible.  So writepage should try to avoid blocking against
251	currently-in-progress I/O.
252	
253	If the filesystem is not called for "sync" and it determines that it
254	would need to block against in-progress I/O to be able to start new I/O
255	against the page the filesystem should redirty the page with
256	redirty_page_for_writepage(), then unlock the page and return zero.
257	This may also be done to avoid internal deadlocks, but rarely.
258	
259	If the filesystem is called for sync then it must wait on any
260	in-progress I/O and then start new I/O.
261	
262	The filesystem should unlock the page synchronously, before returning to the
263	caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
264	value. WRITEPAGE_ACTIVATE means that page cannot really be written out
265	currently, and VM should stop calling ->writepage() on this page for some
266	time. VM does this by moving page to the head of the active list, hence the
267	name.
268	
269	Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
270	and return zero, writepage *must* run set_page_writeback() against the page,
271	followed by unlocking it.  Once set_page_writeback() has been run against the
272	page, write I/O can be submitted and the write I/O completion handler must run
273	end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
274	filesystem must run end_page_writeback() against the page before returning from
275	writepage.
276	
277	That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
278	if the filesystem needs the page to be locked during writeout, that is ok, too,
279	the page is allowed to be unlocked at any point in time between the calls to
280	set_page_writeback() and end_page_writeback().
281	
282	Note, failure to run either redirty_page_for_writepage() or the combination of
283	set_page_writeback()/end_page_writeback() on a page submitted to writepage
284	will leave the page itself marked clean but it will be tagged as dirty in the
285	radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
286	in the filesystem like having dirty inodes at umount and losing written data.
287	
288		->sync_page() locking rules are not well-defined - usually it is called
289	with lock on page, but that is not guaranteed. Considering the currently
290	existing instances of this method ->sync_page() itself doesn't look
291	well-defined...
292	
293		->writepages() is used for periodic writeback and for syscall-initiated
294	sync operations.  The address_space should start I/O against at least
295	*nr_to_write pages.  *nr_to_write must be decremented for each page which is
296	written.  The address_space implementation may write more (or less) pages
297	than *nr_to_write asks for, but it should try to be reasonably close.  If
298	nr_to_write is NULL, all dirty pages must be written.
299	
300	writepages should _only_ write pages which are present on
301	mapping->io_pages.
302	
303		->set_page_dirty() is called from various places in the kernel
304	when the target page is marked as needing writeback.  It may be called
305	under spinlock (it cannot block) and is sometimes called with the page
306	not locked.
307	
308		->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
309	filesystems and by the swapper. The latter will eventually go away.  Please,
310	keep it that way and don't breed new callers.
311	
312		->invalidatepage() is called when the filesystem must attempt to drop
313	some or all of the buffers from the page when it is being truncated. It
314	returns zero on success. If ->invalidatepage is zero, the kernel uses
315	block_invalidatepage() instead.
316	
317		->releasepage() is called when the kernel is about to try to drop the
318	buffers from the page in preparation for freeing it.  It returns zero to
319	indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
320	the kernel assumes that the fs has no private interest in the buffers.
321	
322		->freepage() is called when the kernel is done dropping the page
323	from the page cache.
324	
325		->launder_page() may be called prior to releasing a page if
326	it is still found to be dirty. It returns zero if the page was successfully
327	cleaned, or an error value if not. Note that in order to prevent the page
328	getting mapped back in and redirtied, it needs to be kept locked
329	across the entire operation.
330	
331		->swap_activate will be called with a non-zero argument on
332	files backing (non block device backed) swapfiles. A return value
333	of zero indicates success, in which case this file can be used for
334	backing swapspace. The swapspace operations will be proxied to the
335	address space operations.
336	
337		->swap_deactivate() will be called in the sys_swapoff()
338	path after ->swap_activate() returned success.
339	
340	----------------------- file_lock_operations ------------------------------
341	prototypes:
342		void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
343		void (*fl_release_private)(struct file_lock *);
344	
345	
346	locking rules:
347				inode->i_lock	may block
348	fl_copy_lock:		yes		no
349	fl_release_private:	maybe		maybe[1]
350	
351	[1]:	->fl_release_private for flock or POSIX locks is currently allowed
352	to block. Leases however can still be freed while the i_lock is held and
353	so fl_release_private called on a lease should not block.
354	
355	----------------------- lock_manager_operations ---------------------------
356	prototypes:
357		int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
358		unsigned long (*lm_owner_key)(struct file_lock *);
359		void (*lm_notify)(struct file_lock *);  /* unblock callback */
360		int (*lm_grant)(struct file_lock *, struct file_lock *, int);
361		void (*lm_break)(struct file_lock *); /* break_lease callback */
362		int (*lm_change)(struct file_lock **, int);
363	
364	locking rules:
365	
366				inode->i_lock	blocked_lock_lock	may block
367	lm_compare_owner:	yes[1]		maybe			no
368	lm_owner_key		yes[1]		yes			no
369	lm_notify:		yes		yes			no
370	lm_grant:		no		no			no
371	lm_break:		yes		no			no
372	lm_change		yes		no			no
373	
374	[1]:	->lm_compare_owner and ->lm_owner_key are generally called with
375	*an* inode->i_lock held. It may not be the i_lock of the inode
376	associated with either file_lock argument! This is the case with deadlock
377	detection, since the code has to chase down the owners of locks that may
378	be entirely unrelated to the one on which the lock is being acquired.
379	For deadlock detection however, the blocked_lock_lock is also held. The
380	fact that these locks are held ensures that the file_locks do not
381	disappear out from under you while doing the comparison or generating an
382	owner key.
383	
384	--------------------------- buffer_head -----------------------------------
385	prototypes:
386		void (*b_end_io)(struct buffer_head *bh, int uptodate);
387	
388	locking rules:
389		called from interrupts. In other words, extreme care is needed here.
390	bh is locked, but that's all warranties we have here. Currently only RAID1,
391	highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
392	call this method upon the IO completion.
393	
394	--------------------------- block_device_operations -----------------------
395	prototypes:
396		int (*open) (struct block_device *, fmode_t);
397		int (*release) (struct gendisk *, fmode_t);
398		int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
399		int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
400		int (*direct_access) (struct block_device *, sector_t, void **, unsigned long *);
401		int (*media_changed) (struct gendisk *);
402		void (*unlock_native_capacity) (struct gendisk *);
403		int (*revalidate_disk) (struct gendisk *);
404		int (*getgeo)(struct block_device *, struct hd_geometry *);
405		void (*swap_slot_free_notify) (struct block_device *, unsigned long);
406	
407	locking rules:
408				bd_mutex
409	open:			yes
410	release:		yes
411	ioctl:			no
412	compat_ioctl:		no
413	direct_access:		no
414	media_changed:		no
415	unlock_native_capacity:	no
416	revalidate_disk:	no
417	getgeo:			no
418	swap_slot_free_notify:	no	(see below)
419	
420	media_changed, unlock_native_capacity and revalidate_disk are called only from
421	check_disk_change().
422	
423	swap_slot_free_notify is called with swap_lock and sometimes the page lock
424	held.
425	
426	
427	--------------------------- file_operations -------------------------------
428	prototypes:
429		loff_t (*llseek) (struct file *, loff_t, int);
430		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
431		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
432		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
433		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
434		int (*iterate) (struct file *, struct dir_context *);
435		unsigned int (*poll) (struct file *, struct poll_table_struct *);
436		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
437		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
438		int (*mmap) (struct file *, struct vm_area_struct *);
439		int (*open) (struct inode *, struct file *);
440		int (*flush) (struct file *);
441		int (*release) (struct inode *, struct file *);
442		int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
443		int (*aio_fsync) (struct kiocb *, int datasync);
444		int (*fasync) (int, struct file *, int);
445		int (*lock) (struct file *, int, struct file_lock *);
446		ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
447				loff_t *);
448		ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
449				loff_t *);
450		ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
451				void __user *);
452		ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
453				loff_t *, int);
454		unsigned long (*get_unmapped_area)(struct file *, unsigned long,
455				unsigned long, unsigned long, unsigned long);
456		int (*check_flags)(int);
457		int (*flock) (struct file *, int, struct file_lock *);
458		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
459				size_t, unsigned int);
460		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
461				size_t, unsigned int);
462		int (*setlease)(struct file *, long, struct file_lock **, void **);
463		long (*fallocate)(struct file *, int, loff_t, loff_t);
464	};
465	
466	locking rules:
467		All may block.
468	
469	->llseek() locking has moved from llseek to the individual llseek
470	implementations.  If your fs is not using generic_file_llseek, you
471	need to acquire and release the appropriate locks in your ->llseek().
472	For many filesystems, it is probably safe to acquire the inode
473	mutex or just to use i_size_read() instead.
474	Note: this does not protect the file->f_pos against concurrent modifications
475	since this is something the userspace has to take care about.
476	
477	->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
478	Most instances call fasync_helper(), which does that maintenance, so it's
479	not normally something one needs to worry about.  Return values > 0 will be
480	mapped to zero in the VFS layer.
481	
482	->readdir() and ->ioctl() on directories must be changed. Ideally we would
483	move ->readdir() to inode_operations and use a separate method for directory
484	->ioctl() or kill the latter completely. One of the problems is that for
485	anything that resembles union-mount we won't have a struct file for all
486	components. And there are other reasons why the current interface is a mess...
487	
488	->read on directories probably must go away - we should just enforce -EISDIR
489	in sys_read() and friends.
490	
491	->setlease operations should call generic_setlease() before or after setting
492	the lease within the individual filesystem to record the result of the
493	operation
494	
495	--------------------------- dquot_operations -------------------------------
496	prototypes:
497		int (*write_dquot) (struct dquot *);
498		int (*acquire_dquot) (struct dquot *);
499		int (*release_dquot) (struct dquot *);
500		int (*mark_dirty) (struct dquot *);
501		int (*write_info) (struct super_block *, int);
502	
503	These operations are intended to be more or less wrapping functions that ensure
504	a proper locking wrt the filesystem and call the generic quota operations.
505	
506	What filesystem should expect from the generic quota functions:
507	
508			FS recursion	Held locks when called
509	write_dquot:	yes		dqonoff_sem or dqptr_sem
510	acquire_dquot:	yes		dqonoff_sem or dqptr_sem
511	release_dquot:	yes		dqonoff_sem or dqptr_sem
512	mark_dirty:	no		-
513	write_info:	yes		dqonoff_sem
514	
515	FS recursion means calling ->quota_read() and ->quota_write() from superblock
516	operations.
517	
518	More details about quota locking can be found in fs/dquot.c.
519	
520	--------------------------- vm_operations_struct -----------------------------
521	prototypes:
522		void (*open)(struct vm_area_struct*);
523		void (*close)(struct vm_area_struct*);
524		int (*fault)(struct vm_area_struct*, struct vm_fault *);
525		int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
526		int (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
527		int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
528	
529	locking rules:
530			mmap_sem	PageLocked(page)
531	open:		yes
532	close:		yes
533	fault:		yes		can return with page locked
534	map_pages:	yes
535	page_mkwrite:	yes		can return with page locked
536	pfn_mkwrite:	yes
537	access:		yes
538	
539		->fault() is called when a previously not present pte is about
540	to be faulted in. The filesystem must find and return the page associated
541	with the passed in "pgoff" in the vm_fault structure. If it is possible that
542	the page may be truncated and/or invalidated, then the filesystem must lock
543	the page, then ensure it is not already truncated (the page lock will block
544	subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
545	locked. The VM will unlock the page.
546	
547		->map_pages() is called when VM asks to map easy accessible pages.
548	Filesystem should find and map pages associated with offsets from "pgoff"
549	till "max_pgoff". ->map_pages() is called with page table locked and must
550	not block.  If it's not possible to reach a page without blocking,
551	filesystem should skip it. Filesystem should use do_set_pte() to setup
552	page table entry. Pointer to entry associated with offset "pgoff" is
553	passed in "pte" field in vm_fault structure. Pointers to entries for other
554	offsets should be calculated relative to "pte".
555	
556		->page_mkwrite() is called when a previously read-only pte is
557	about to become writeable. The filesystem again must ensure that there are
558	no truncate/invalidate races, and then return with the page locked. If
559	the page has been truncated, the filesystem should not look up a new page
560	like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
561	will cause the VM to retry the fault.
562	
563		->pfn_mkwrite() is the same as page_mkwrite but when the pte is
564	VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
565	VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
566	after this call is to make the pte read-write, unless pfn_mkwrite returns
567	an error.
568	
569		->access() is called when get_user_pages() fails in
570	access_process_vm(), typically used to debug a process through
571	/proc/pid/mem or ptrace.  This function is needed only for
572	VM_IO | VM_PFNMAP VMAs.
573	
574	================================================================================
575				Dubious stuff
576	
577	(if you break something or notice that it is broken and do not fix it yourself
578	- at least put it here)
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