| blob | 0a926e2ba3ab68ffd541f5a619cd673e59826821 |
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?
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);
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
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 *);
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
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.
107 See Documentation/filesystems/directory-locking for more detailed discussion
108 of the locking scheme for directory operations.
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);
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)
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.
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
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.
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 *);
207 locking rules:
208 All except set_page_dirty and freepage may block
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
231 ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
232 may be called from the request handler (/dev/loop).
234 ->readpage() unlocks the page, either synchronously or via I/O
235 completion.
237 ->readpages() populates the pagecache with the passed pages and starts
238 I/O against them. They come unlocked upon I/O completion.
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.
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.
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.
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.
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.
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.
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.
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().
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.
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...
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.
300 writepages should _only_ write pages which are present on
301 mapping->io_pages.
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.
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.
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.
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.
322 ->freepage() is called when the kernel is done dropping the page
323 from the page cache.
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.
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.
337 ->swap_deactivate() will be called in the sys_swapoff()
338 path after ->swap_activate() returned success.
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 *);
346 locking rules:
347 inode->i_lock may block
348 fl_copy_lock: yes no
349 fl_release_private: maybe maybe[1]
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.
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);
364 locking rules:
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
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.
384 --------------------------- buffer_head -----------------------------------
385 prototypes:
386 void (*b_end_io)(struct buffer_head *bh, int uptodate);
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.
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);
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)
420 media_changed, unlock_native_capacity and revalidate_disk are called only from
421 check_disk_change().
423 swap_slot_free_notify is called with swap_lock and sometimes the page lock
424 held.
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 };
466 locking rules:
467 All may block.
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.
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.
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...
488 ->read on directories probably must go away - we should just enforce -EISDIR
489 in sys_read() and friends.
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
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);
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.
506 What filesystem should expect from the generic quota functions:
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
515 FS recursion means calling ->quota_read() and ->quota_write() from superblock
516 operations.
518 More details about quota locking can be found in fs/dquot.c.
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);
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
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.
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".
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.
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.
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.
574 ================================================================================
575 Dubious stuff
577 (if you break something or notice that it is broken and do not fix it yourself
578 - at least put it here)