1	The text below describes the locking rules for VFS-related methods.
  2It is (believed to be) up-to-date. *Please*, if you change anything in
  3prototypes or locking protocols - update this file. And update the relevant
  4instances in the tree, don't leave that to maintainers of filesystems/devices/
  5etc. At the very least, put the list of dubious cases in the end of this file.
  6Don't turn it into log - maintainers of out-of-the-tree code are supposed to
  7be able to use diff(1).
  8	Thing currently missing here: socket operations. Alexey?
  9
 10--------------------------- dentry_operations --------------------------
 11prototypes:
 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 *,
 16			unsigned int, const char *, const struct qstr *);
 17	int (*d_delete)(struct dentry *);
 18	int (*d_init)(struct dentry *);
 19	void (*d_release)(struct dentry *);
 20	void (*d_iput)(struct dentry *, struct inode *);
 21	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 22	struct vfsmount *(*d_automount)(struct path *path);
 23	int (*d_manage)(const struct path *, bool);
 24	struct dentry *(*d_real)(struct dentry *, const struct inode *,
 25				 unsigned int, unsigned int);
 26
 27locking rules:
 28		rename_lock	->d_lock	may block	rcu-walk
 29d_revalidate:	no		no		yes (ref-walk)	maybe
 30d_weak_revalidate:no		no		yes	 	no
 31d_hash		no		no		no		maybe
 32d_compare:	yes		no		no		maybe
 33d_delete:	no		yes		no		no
 34d_init:	no		no		yes		no
 35d_release:	no		no		yes		no
 36d_prune:        no              yes             no              no
 37d_iput:		no		no		yes		no
 38d_dname:	no		no		no		no
 39d_automount:	no		no		yes		no
 40d_manage:	no		no		yes (ref-walk)	maybe
 41d_real		no		no		yes 		no
 42
 43--------------------------- inode_operations --------------------------- 
 44prototypes:
 45	int (*create) (struct inode *,struct dentry *,umode_t, bool);
 46	struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
 47	int (*link) (struct dentry *,struct inode *,struct dentry *);
 48	int (*unlink) (struct inode *,struct dentry *);
 49	int (*symlink) (struct inode *,struct dentry *,const char *);
 50	int (*mkdir) (struct inode *,struct dentry *,umode_t);
 51	int (*rmdir) (struct inode *,struct dentry *);
 52	int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
 53	int (*rename) (struct inode *, struct dentry *,
 54			struct inode *, struct dentry *, unsigned int);
 55	int (*readlink) (struct dentry *, char __user *,int);
 56	const char *(*get_link) (struct dentry *, struct inode *, void **);
 57	void (*truncate) (struct inode *);
 58	int (*permission) (struct inode *, int, unsigned int);
 59	int (*get_acl)(struct inode *, int);
 60	int (*setattr) (struct dentry *, struct iattr *);
 61	int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
 62	ssize_t (*listxattr) (struct dentry *, char *, size_t);
 63	int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
 64	void (*update_time)(struct inode *, struct timespec *, int);
 65	int (*atomic_open)(struct inode *, struct dentry *,
 66				struct file *, unsigned open_flag,
 67				umode_t create_mode, int *opened);
 68	int (*tmpfile) (struct inode *, struct dentry *, umode_t);
 69
 70locking rules:
 71	all may block
 72		i_mutex(inode)
 73lookup:		yes
 74create:		yes
 75link:		yes (both)
 76mknod:		yes
 77symlink:	yes
 78mkdir:		yes
 79unlink:		yes (both)
 80rmdir:		yes (both)	(see below)
 81rename:	yes (all)	(see below)
 82readlink:	no
 83get_link:	no
 84setattr:	yes
 85permission:	no (may not block if called in rcu-walk mode)
 86get_acl:	no
 87getattr:	no
 88listxattr:	no
 89fiemap:		no
 90update_time:	no
 91atomic_open:	yes
 92tmpfile:	no
 93
 94
 95	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
 96victim.
 97	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
 98
 99See Documentation/filesystems/directory-locking for more detailed discussion
100of the locking scheme for directory operations.
101
102----------------------- xattr_handler operations -----------------------
103prototypes:
104	bool (*list)(struct dentry *dentry);
105	int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
106		   struct inode *inode, const char *name, void *buffer,
107		   size_t size);
108	int (*set)(const struct xattr_handler *handler, struct dentry *dentry,
109		   struct inode *inode, const char *name, const void *buffer,
110		   size_t size, int flags);
111
112locking rules:
113	all may block
114		i_mutex(inode)
115list:		no
116get:		no
117set:		yes
118
119--------------------------- super_operations ---------------------------
120prototypes:
121	struct inode *(*alloc_inode)(struct super_block *sb);
122	void (*destroy_inode)(struct inode *);
123	void (*dirty_inode) (struct inode *, int flags);
124	int (*write_inode) (struct inode *, struct writeback_control *wbc);
125	int (*drop_inode) (struct inode *);
126	void (*evict_inode) (struct inode *);
127	void (*put_super) (struct super_block *);
128	int (*sync_fs)(struct super_block *sb, int wait);
129	int (*freeze_fs) (struct super_block *);
130	int (*unfreeze_fs) (struct super_block *);
131	int (*statfs) (struct dentry *, struct kstatfs *);
132	int (*remount_fs) (struct super_block *, int *, char *);
133	void (*umount_begin) (struct super_block *);
134	int (*show_options)(struct seq_file *, struct dentry *);
135	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
136	ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
137	int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
138
139locking rules:
140	All may block [not true, see below]
141			s_umount
142alloc_inode:
143destroy_inode:
144dirty_inode:
145write_inode:
146drop_inode:				!!!inode->i_lock!!!
147evict_inode:
148put_super:		write
149sync_fs:		read
150freeze_fs:		write
151unfreeze_fs:		write
152statfs:			maybe(read)	(see below)
153remount_fs:		write
154umount_begin:		no
155show_options:		no		(namespace_sem)
156quota_read:		no		(see below)
157quota_write:		no		(see below)
158bdev_try_to_free_page:	no		(see below)
159
160->statfs() has s_umount (shared) when called by ustat(2) (native or
161compat), but that's an accident of bad API; s_umount is used to pin
162the superblock down when we only have dev_t given us by userland to
163identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
164doesn't hold it when calling ->statfs() - superblock is pinned down
165by resolving the pathname passed to syscall.
166->quota_read() and ->quota_write() functions are both guaranteed to
167be the only ones operating on the quota file by the quota code (via
168dqio_sem) (unless an admin really wants to screw up something and
169writes to quota files with quotas on). For other details about locking
170see also dquot_operations section.
171->bdev_try_to_free_page is called from the ->releasepage handler of
172the block device inode.  See there for more details.
173
174--------------------------- file_system_type ---------------------------
175prototypes:
176	struct dentry *(*mount) (struct file_system_type *, int,
177		       const char *, void *);
178	void (*kill_sb) (struct super_block *);
179locking rules:
180		may block
181mount		yes
182kill_sb		yes
183
184->mount() returns ERR_PTR or the root dentry; its superblock should be locked
185on return.
186->kill_sb() takes a write-locked superblock, does all shutdown work on it,
187unlocks and drops the reference.
188
189--------------------------- address_space_operations --------------------------
190prototypes:
191	int (*writepage)(struct page *page, struct writeback_control *wbc);
192	int (*readpage)(struct file *, struct page *);
193	int (*writepages)(struct address_space *, struct writeback_control *);
194	int (*set_page_dirty)(struct page *page);
195	int (*readpages)(struct file *filp, struct address_space *mapping,
196			struct list_head *pages, unsigned nr_pages);
197	int (*write_begin)(struct file *, struct address_space *mapping,
198				loff_t pos, unsigned len, unsigned flags,
199				struct page **pagep, void **fsdata);
200	int (*write_end)(struct file *, struct address_space *mapping,
201				loff_t pos, unsigned len, unsigned copied,
202				struct page *page, void *fsdata);
203	sector_t (*bmap)(struct address_space *, sector_t);
204	void (*invalidatepage) (struct page *, unsigned int, unsigned int);
205	int (*releasepage) (struct page *, int);
206	void (*freepage)(struct page *);
207	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
208	bool (*isolate_page) (struct page *, isolate_mode_t);
209	int (*migratepage)(struct address_space *, struct page *, struct page *);
210	void (*putback_page) (struct page *);
211	int (*launder_page)(struct page *);
212	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
213	int (*error_remove_page)(struct address_space *, struct page *);
214	int (*swap_activate)(struct file *);
215	int (*swap_deactivate)(struct file *);
216
217locking rules:
218	All except set_page_dirty and freepage may block
219
220			PageLocked(page)	i_mutex
221writepage:		yes, unlocks (see below)
222readpage:		yes, unlocks
223writepages:
224set_page_dirty		no
225readpages:
226write_begin:		locks the page		yes
227write_end:		yes, unlocks		yes
228bmap:
229invalidatepage:		yes
230releasepage:		yes
231freepage:		yes
232direct_IO:
233isolate_page:		yes
234migratepage:		yes (both)
235putback_page:		yes
236launder_page:		yes
237is_partially_uptodate:	yes
238error_remove_page:	yes
239swap_activate:		no
240swap_deactivate:	no
241
242	->write_begin(), ->write_end() and ->readpage() may be called from
243the request handler (/dev/loop).
244
245	->readpage() unlocks the page, either synchronously or via I/O
246completion.
247
248	->readpages() populates the pagecache with the passed pages and starts
249I/O against them.  They come unlocked upon I/O completion.
250
251	->writepage() is used for two purposes: for "memory cleansing" and for
252"sync".  These are quite different operations and the behaviour may differ
253depending upon the mode.
254
255If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
256it *must* start I/O against the page, even if that would involve
257blocking on in-progress I/O.
258
259If writepage is called for memory cleansing (sync_mode ==
260WBC_SYNC_NONE) then its role is to get as much writeout underway as
261possible.  So writepage should try to avoid blocking against
262currently-in-progress I/O.
263
264If the filesystem is not called for "sync" and it determines that it
265would need to block against in-progress I/O to be able to start new I/O
266against the page the filesystem should redirty the page with
267redirty_page_for_writepage(), then unlock the page and return zero.
268This may also be done to avoid internal deadlocks, but rarely.
269
270If the filesystem is called for sync then it must wait on any
271in-progress I/O and then start new I/O.
272
273The filesystem should unlock the page synchronously, before returning to the
274caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
275value. WRITEPAGE_ACTIVATE means that page cannot really be written out
276currently, and VM should stop calling ->writepage() on this page for some
277time. VM does this by moving page to the head of the active list, hence the
278name.
279
280Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
281and return zero, writepage *must* run set_page_writeback() against the page,
282followed by unlocking it.  Once set_page_writeback() has been run against the
283page, write I/O can be submitted and the write I/O completion handler must run
284end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
285filesystem must run end_page_writeback() against the page before returning from
286writepage.
287
288That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
289if the filesystem needs the page to be locked during writeout, that is ok, too,
290the page is allowed to be unlocked at any point in time between the calls to
291set_page_writeback() and end_page_writeback().
292
293Note, failure to run either redirty_page_for_writepage() or the combination of
294set_page_writeback()/end_page_writeback() on a page submitted to writepage
295will leave the page itself marked clean but it will be tagged as dirty in the
296radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
297in the filesystem like having dirty inodes at umount and losing written data.
298
299	->writepages() is used for periodic writeback and for syscall-initiated
300sync operations.  The address_space should start I/O against at least
301*nr_to_write pages.  *nr_to_write must be decremented for each page which is
302written.  The address_space implementation may write more (or less) pages
303than *nr_to_write asks for, but it should try to be reasonably close.  If
304nr_to_write is NULL, all dirty pages must be written.
305
306writepages should _only_ write pages which are present on
307mapping->io_pages.
308
309	->set_page_dirty() is called from various places in the kernel
310when the target page is marked as needing writeback.  It may be called
311under spinlock (it cannot block) and is sometimes called with the page
312not locked.
313
314	->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
315filesystems and by the swapper. The latter will eventually go away.  Please,
316keep it that way and don't breed new callers.
317
318	->invalidatepage() is called when the filesystem must attempt to drop
319some or all of the buffers from the page when it is being truncated. It
320returns zero on success. If ->invalidatepage is zero, the kernel uses
321block_invalidatepage() instead.
322
323	->releasepage() is called when the kernel is about to try to drop the
324buffers from the page in preparation for freeing it.  It returns zero to
325indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
326the kernel assumes that the fs has no private interest in the buffers.
327
328	->freepage() is called when the kernel is done dropping the page
329from the page cache.
330
331	->launder_page() may be called prior to releasing a page if
332it is still found to be dirty. It returns zero if the page was successfully
333cleaned, or an error value if not. Note that in order to prevent the page
334getting mapped back in and redirtied, it needs to be kept locked
335across the entire operation.
336
337	->swap_activate will be called with a non-zero argument on
338files backing (non block device backed) swapfiles. A return value
339of zero indicates success, in which case this file can be used for
340backing swapspace. The swapspace operations will be proxied to the
341address space operations.
342
343	->swap_deactivate() will be called in the sys_swapoff()
344path after ->swap_activate() returned success.
345
346----------------------- file_lock_operations ------------------------------
347prototypes:
348	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
349	void (*fl_release_private)(struct file_lock *);
350
351
352locking rules:
353			inode->i_lock	may block
354fl_copy_lock:		yes		no
355fl_release_private:	maybe		maybe[1]
356
357[1]:	->fl_release_private for flock or POSIX locks is currently allowed
358to block. Leases however can still be freed while the i_lock is held and
359so fl_release_private called on a lease should not block.
360
361----------------------- lock_manager_operations ---------------------------
362prototypes:
363	int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
364	unsigned long (*lm_owner_key)(struct file_lock *);
365	void (*lm_notify)(struct file_lock *);  /* unblock callback */
366	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
367	void (*lm_break)(struct file_lock *); /* break_lease callback */
368	int (*lm_change)(struct file_lock **, int);
369
370locking rules:
371
372			inode->i_lock	blocked_lock_lock	may block
373lm_compare_owner:	yes[1]		maybe			no
374lm_owner_key		yes[1]		yes			no
375lm_notify:		yes		yes			no
376lm_grant:		no		no			no
377lm_break:		yes		no			no
378lm_change		yes		no			no
379
380[1]:	->lm_compare_owner and ->lm_owner_key are generally called with
381*an* inode->i_lock held. It may not be the i_lock of the inode
382associated with either file_lock argument! This is the case with deadlock
383detection, since the code has to chase down the owners of locks that may
384be entirely unrelated to the one on which the lock is being acquired.
385For deadlock detection however, the blocked_lock_lock is also held. The
386fact that these locks are held ensures that the file_locks do not
387disappear out from under you while doing the comparison or generating an
388owner key.
389
390--------------------------- buffer_head -----------------------------------
391prototypes:
392	void (*b_end_io)(struct buffer_head *bh, int uptodate);
393
394locking rules:
395	called from interrupts. In other words, extreme care is needed here.
396bh is locked, but that's all warranties we have here. Currently only RAID1,
397highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
398call this method upon the IO completion.
399
400--------------------------- block_device_operations -----------------------
401prototypes:
402	int (*open) (struct block_device *, fmode_t);
403	int (*release) (struct gendisk *, fmode_t);
404	int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
405	int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
406	int (*direct_access) (struct block_device *, sector_t, void **,
407				unsigned long *);
408	int (*media_changed) (struct gendisk *);
409	void (*unlock_native_capacity) (struct gendisk *);
410	int (*revalidate_disk) (struct gendisk *);
411	int (*getgeo)(struct block_device *, struct hd_geometry *);
412	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
413
414locking rules:
415			bd_mutex
416open:			yes
417release:		yes
418ioctl:			no
419compat_ioctl:		no
420direct_access:		no
421media_changed:		no
422unlock_native_capacity:	no
423revalidate_disk:	no
424getgeo:			no
425swap_slot_free_notify:	no	(see below)
426
427media_changed, unlock_native_capacity and revalidate_disk are called only from
428check_disk_change().
429
430swap_slot_free_notify is called with swap_lock and sometimes the page lock
431held.
432
433
434--------------------------- file_operations -------------------------------
435prototypes:
436	loff_t (*llseek) (struct file *, loff_t, int);
437	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
438	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
439	ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
440	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
441	int (*iterate) (struct file *, struct dir_context *);
442	unsigned int (*poll) (struct file *, struct poll_table_struct *);
443	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
444	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
445	int (*mmap) (struct file *, struct vm_area_struct *);
446	int (*open) (struct inode *, struct file *);
447	int (*flush) (struct file *);
448	int (*release) (struct inode *, struct file *);
449	int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
450	int (*fasync) (int, struct file *, int);
451	int (*lock) (struct file *, int, struct file_lock *);
452	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
453			loff_t *);
454	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
455			loff_t *);
456	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
457			void __user *);
458	ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
459			loff_t *, int);
460	unsigned long (*get_unmapped_area)(struct file *, unsigned long,
461			unsigned long, unsigned long, unsigned long);
462	int (*check_flags)(int);
463	int (*flock) (struct file *, int, struct file_lock *);
464	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
465			size_t, unsigned int);
466	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
467			size_t, unsigned int);
468	int (*setlease)(struct file *, long, struct file_lock **, void **);
469	long (*fallocate)(struct file *, int, loff_t, loff_t);
470};
471
472locking rules:
473	All may block.
474
475->llseek() locking has moved from llseek to the individual llseek
476implementations.  If your fs is not using generic_file_llseek, you
477need to acquire and release the appropriate locks in your ->llseek().
478For many filesystems, it is probably safe to acquire the inode
479mutex or just to use i_size_read() instead.
480Note: this does not protect the file->f_pos against concurrent modifications
481since this is something the userspace has to take care about.
482
483->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
484Most instances call fasync_helper(), which does that maintenance, so it's
485not normally something one needs to worry about.  Return values > 0 will be
486mapped to zero in the VFS layer.
487
488->readdir() and ->ioctl() on directories must be changed. Ideally we would
489move ->readdir() to inode_operations and use a separate method for directory
490->ioctl() or kill the latter completely. One of the problems is that for
491anything that resembles union-mount we won't have a struct file for all
492components. And there are other reasons why the current interface is a mess...
493
494->read on directories probably must go away - we should just enforce -EISDIR
495in sys_read() and friends.
496
497->setlease operations should call generic_setlease() before or after setting
498the lease within the individual filesystem to record the result of the
499operation
500
501--------------------------- dquot_operations -------------------------------
502prototypes:
503	int (*write_dquot) (struct dquot *);
504	int (*acquire_dquot) (struct dquot *);
505	int (*release_dquot) (struct dquot *);
506	int (*mark_dirty) (struct dquot *);
507	int (*write_info) (struct super_block *, int);
508
509These operations are intended to be more or less wrapping functions that ensure
510a proper locking wrt the filesystem and call the generic quota operations.
511
512What filesystem should expect from the generic quota functions:
513
514		FS recursion	Held locks when called
515write_dquot:	yes		dqonoff_sem or dqptr_sem
516acquire_dquot:	yes		dqonoff_sem or dqptr_sem
517release_dquot:	yes		dqonoff_sem or dqptr_sem
518mark_dirty:	no		-
519write_info:	yes		dqonoff_sem
520
521FS recursion means calling ->quota_read() and ->quota_write() from superblock
522operations.
523
524More details about quota locking can be found in fs/dquot.c.
525
526--------------------------- vm_operations_struct -----------------------------
527prototypes:
528	void (*open)(struct vm_area_struct*);
529	void (*close)(struct vm_area_struct*);
530	int (*fault)(struct vm_area_struct*, struct vm_fault *);
531	int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
532	int (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
533	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
534
535locking rules:
536		mmap_sem	PageLocked(page)
537open:		yes
538close:		yes
539fault:		yes		can return with page locked
540map_pages:	yes
541page_mkwrite:	yes		can return with page locked
542pfn_mkwrite:	yes
543access:		yes
544
545	->fault() is called when a previously not present pte is about
546to be faulted in. The filesystem must find and return the page associated
547with the passed in "pgoff" in the vm_fault structure. If it is possible that
548the page may be truncated and/or invalidated, then the filesystem must lock
549the page, then ensure it is not already truncated (the page lock will block
550subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
551locked. The VM will unlock the page.
552
553	->map_pages() is called when VM asks to map easy accessible pages.
554Filesystem should find and map pages associated with offsets from "start_pgoff"
555till "end_pgoff". ->map_pages() is called with page table locked and must
556not block.  If it's not possible to reach a page without blocking,
557filesystem should skip it. Filesystem should use do_set_pte() to setup
558page table entry. Pointer to entry associated with the page is passed in
559"pte" field in vm_fault structure. Pointers to entries for other offsets
560should be calculated relative to "pte".
561
562	->page_mkwrite() is called when a previously read-only pte is
563about to become writeable. The filesystem again must ensure that there are
564no truncate/invalidate races, and then return with the page locked. If
565the page has been truncated, the filesystem should not look up a new page
566like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
567will cause the VM to retry the fault.
568
569	->pfn_mkwrite() is the same as page_mkwrite but when the pte is
570VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
571VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
572after this call is to make the pte read-write, unless pfn_mkwrite returns
573an error.
574
575	->access() is called when get_user_pages() fails in
576access_process_vm(), typically used to debug a process through
577/proc/pid/mem or ptrace.  This function is needed only for
578VM_IO | VM_PFNMAP VMAs.
579
580================================================================================
581			Dubious stuff
582
583(if you break something or notice that it is broken and do not fix it yourself
584- at least put it here)