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