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