Documentation/filesystems/Locking

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