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 *, const struct inode *,
  15                         struct qstr *);
  16         int (*d_compare)(const struct dentry *, const struct inode *,
  17                         const struct dentry *, const struct inode *,
  18                         unsigned int, const char *, const struct qstr *);
  19         int (*d_delete)(struct dentry *);
  20         void (*d_release)(struct dentry *);
  21         void (*d_iput)(struct dentry *, struct inode *);
  22         char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
  23         struct vfsmount *(*d_automount)(struct path *path);
  24         int (*d_manage)(struct dentry *, bool);
  25
  26 locking rules:
  27                 rename_lock     ->d_lock        may block       rcu-walk
  28 d_revalidate:   no              no              yes (ref-walk)  maybe
  29 d_weak_revalidate:no            no              yes             no
  30 d_hash          no              no              no              maybe
  31 d_compare:      yes             no              no              maybe
  32 d_delete:       no              yes             no              no
  33 d_release:      no              no              yes             no
  34 d_prune:        no              yes             no              no
  35 d_iput:         no              no              yes             no
  36 d_dname:        no              no              no              no
  37 d_automount:    no              no              yes             no
  38 d_manage:       no              no              yes (ref-walk)  maybe
  39
  40 --------------------------- inode_operations ---------------------------
  41 prototypes:
  42         int (*create) (struct inode *,struct dentry *,umode_t, bool);
  43         struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
  44         int (*link) (struct dentry *,struct inode *,struct dentry *);
  45         int (*unlink) (struct inode *,struct dentry *);
  46         int (*symlink) (struct inode *,struct dentry *,const char *);
  47         int (*mkdir) (struct inode *,struct dentry *,umode_t);
  48         int (*rmdir) (struct inode *,struct dentry *);
  49         int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
  50         int (*rename) (struct inode *, struct dentry *,
  51                         struct inode *, struct dentry *);
  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
  70 locking rules:
  71         all may block
  72                 i_mutex(inode)
  73 lookup:         yes
  74 create:         yes
  75 link:           yes (both)
  76 mknod:          yes
  77 symlink:        yes
  78 mkdir:          yes
  79 unlink:         yes (both)
  80 rmdir:          yes (both)      (see below)
  81 rename:         yes (all)       (see below)
  82 readlink:       no
  83 follow_link:    no
  84 put_link:       no
  85 setattr:        yes
  86 permission:     no (may not block if called in rcu-walk mode)
  87 get_acl:        no
  88 getattr:        no
  89 setxattr:       yes
  90 getxattr:       no
  91 listxattr:      no
  92 removexattr:    yes
  93 fiemap:         no
  94 update_time:    no
  95 atomic_open:    yes
  96
  97         Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
  98 victim.
  99         cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
 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         int (*sync_fs)(struct super_block *sb, int wait);
 114         int (*freeze_fs) (struct super_block *);
 115         int (*unfreeze_fs) (struct super_block *);
 116         int (*statfs) (struct dentry *, struct kstatfs *);
 117         int (*remount_fs) (struct super_block *, int *, char *);
 118         void (*umount_begin) (struct super_block *);
 119         int (*show_options)(struct seq_file *, struct dentry *);
 120         ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
 121         ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
 122         int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
 123
 124 locking rules:
 125         All may block [not true, see below]
 126                         s_umount
 127 alloc_inode:
 128 destroy_inode:
 129 dirty_inode:
 130 write_inode:
 131 drop_inode:                             !!!inode->i_lock!!!
 132 evict_inode:
 133 put_super:              write
 134 sync_fs:                read
 135 freeze_fs:              write
 136 unfreeze_fs:            write
 137 statfs:                 maybe(read)     (see below)
 138 remount_fs:             write
 139 umount_begin:           no
 140 show_options:           no              (namespace_sem)
 141 quota_read:             no              (see below)
 142 quota_write:            no              (see below)
 143 bdev_try_to_free_page:  no              (see below)
 144
 145 ->statfs() has s_umount (shared) when called by ustat(2) (native or
 146 compat), but that's an accident of bad API; s_umount is used to pin
 147 the superblock down when we only have dev_t given us by userland to
 148 identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
 149 doesn't hold it when calling ->statfs() - superblock is pinned down
 150 by resolving the pathname passed to syscall.
 151 ->quota_read() and ->quota_write() functions are both guaranteed to
 152 be the only ones operating on the quota file by the quota code (via
 153 dqio_sem) (unless an admin really wants to screw up something and
 154 writes to quota files with quotas on). For other details about locking
 155 see also dquot_operations section.
 156 ->bdev_try_to_free_page is called from the ->releasepage handler of
 157 the block device inode.  See there for more details.
 158
 159 --------------------------- file_system_type ---------------------------
 160 prototypes:
 161         int (*get_sb) (struct file_system_type *, int,
 162                        const char *, void *, struct vfsmount *);
 163         struct dentry *(*mount) (struct file_system_type *, int,
 164                        const char *, void *);
 165         void (*kill_sb) (struct super_block *);
 166 locking rules:
 167                 may block
 168 mount           yes
 169 kill_sb         yes
 170
 171 ->mount() returns ERR_PTR or the root dentry; its superblock should be locked
 172 on return.
 173 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
 174 unlocks and drops the reference.
 175
 176 --------------------------- address_space_operations --------------------------
 177 prototypes:
 178         int (*writepage)(struct page *page, struct writeback_control *wbc);
 179         int (*readpage)(struct file *, struct page *);
 180         int (*sync_page)(struct page *);
 181         int (*writepages)(struct address_space *, struct writeback_control *);
 182         int (*set_page_dirty)(struct page *page);
 183         int (*readpages)(struct file *filp, struct address_space *mapping,
 184                         struct list_head *pages, unsigned nr_pages);
 185         int (*write_begin)(struct file *, struct address_space *mapping,
 186                                 loff_t pos, unsigned len, unsigned flags,
 187                                 struct page **pagep, void **fsdata);
 188         int (*write_end)(struct file *, struct address_space *mapping,
 189                                 loff_t pos, unsigned len, unsigned copied,
 190                                 struct page *page, void *fsdata);
 191         sector_t (*bmap)(struct address_space *, sector_t);
 192         int (*invalidatepage) (struct page *, unsigned long);
 193         int (*releasepage) (struct page *, int);
 194         void (*freepage)(struct page *);
 195         int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
 196                         loff_t offset, unsigned long nr_segs);
 197         int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,
 198                                 unsigned long *);
 199         int (*migratepage)(struct address_space *, struct page *, struct page *);
 200         int (*launder_page)(struct page *);
 201         int (*is_partially_uptodate)(struct page *, read_descriptor_t *, unsigned long);
 202         int (*error_remove_page)(struct address_space *, struct page *);
 203         int (*swap_activate)(struct file *);
 204         int (*swap_deactivate)(struct file *);
 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 swap_activate:          no
 229 swap_deactivate:        no
 230
 231         ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
 232 may be called from the request handler (/dev/loop).
 233
 234         ->readpage() unlocks the page, either synchronously or via I/O
 235 completion.
 236
 237         ->readpages() populates the pagecache with the passed pages and starts
 238 I/O against them.  They come unlocked upon I/O completion.
 239
 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.
 243
 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.
 247
 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.
 252
 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.
 258
 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.
 261
 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.
 268
 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.
 276
 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().
 281
 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.
 287
 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...
 292
 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.
 299
 300 writepages should _only_ write pages which are present on
 301 mapping->io_pages.
 302
 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.
 307
 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.
 311
 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.
 316
 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.
 321
 322         ->freepage() is called when the kernel is done dropping the page
 323 from the page cache.
 324
 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.
 330
 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.
 336
 337         ->swap_deactivate() will be called in the sys_swapoff()
 338 path after ->swap_activate() returned success.
 339
 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 *);
 344
 345
 346 locking rules:
 347                         file_lock_lock  may block
 348 fl_copy_lock:           yes             no
 349 fl_release_private:     maybe           no
 350
 351 ----------------------- lock_manager_operations ---------------------------
 352 prototypes:
 353         int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
 354         void (*lm_notify)(struct file_lock *);  /* unblock callback */
 355         int (*lm_grant)(struct file_lock *, struct file_lock *, int);
 356         void (*lm_break)(struct file_lock *); /* break_lease callback */
 357         int (*lm_change)(struct file_lock **, int);
 358
 359 locking rules:
 360                         file_lock_lock  may block
 361 lm_compare_owner:       yes             no
 362 lm_notify:              yes             no
 363 lm_grant:               no              no
 364 lm_break:               yes             no
 365 lm_change               yes             no
 366
 367 --------------------------- buffer_head -----------------------------------
 368 prototypes:
 369         void (*b_end_io)(struct buffer_head *bh, int uptodate);
 370
 371 locking rules:
 372         called from interrupts. In other words, extreme care is needed here.
 373 bh is locked, but that's all warranties we have here. Currently only RAID1,
 374 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
 375 call this method upon the IO completion.
 376
 377 --------------------------- block_device_operations -----------------------
 378 prototypes:
 379         int (*open) (struct block_device *, fmode_t);
 380         int (*release) (struct gendisk *, fmode_t);
 381         int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
 382         int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
 383         int (*direct_access) (struct block_device *, sector_t, void **, unsigned long *);
 384         int (*media_changed) (struct gendisk *);
 385         void (*unlock_native_capacity) (struct gendisk *);
 386         int (*revalidate_disk) (struct gendisk *);
 387         int (*getgeo)(struct block_device *, struct hd_geometry *);
 388         void (*swap_slot_free_notify) (struct block_device *, unsigned long);
 389
 390 locking rules:
 391                         bd_mutex
 392 open:                   yes
 393 release:                yes
 394 ioctl:                  no
 395 compat_ioctl:           no
 396 direct_access:          no
 397 media_changed:          no
 398 unlock_native_capacity: no
 399 revalidate_disk:        no
 400 getgeo:                 no
 401 swap_slot_free_notify:  no      (see below)
 402
 403 media_changed, unlock_native_capacity and revalidate_disk are called only from
 404 check_disk_change().
 405
 406 swap_slot_free_notify is called with swap_lock and sometimes the page lock
 407 held.
 408
 409
 410 --------------------------- file_operations -------------------------------
 411 prototypes:
 412         loff_t (*llseek) (struct file *, loff_t, int);
 413         ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 414         ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
 415         ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 416         ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 417         int (*readdir) (struct file *, void *, filldir_t);
 418         unsigned int (*poll) (struct file *, struct poll_table_struct *);
 419         long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 420         long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
 421         int (*mmap) (struct file *, struct vm_area_struct *);
 422         int (*open) (struct inode *, struct file *);
 423         int (*flush) (struct file *);
 424         int (*release) (struct inode *, struct file *);
 425         int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
 426         int (*aio_fsync) (struct kiocb *, int datasync);
 427         int (*fasync) (int, struct file *, int);
 428         int (*lock) (struct file *, int, struct file_lock *);
 429         ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
 430                         loff_t *);
 431         ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
 432                         loff_t *);
 433         ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
 434                         void __user *);
 435         ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
 436                         loff_t *, int);
 437         unsigned long (*get_unmapped_area)(struct file *, unsigned long,
 438                         unsigned long, unsigned long, unsigned long);
 439         int (*check_flags)(int);
 440         int (*flock) (struct file *, int, struct file_lock *);
 441         ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
 442                         size_t, unsigned int);
 443         ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
 444                         size_t, unsigned int);
 445         int (*setlease)(struct file *, long, struct file_lock **);
 446         long (*fallocate)(struct file *, int, loff_t, loff_t);
 447 };
 448
 449 locking rules:
 450         All may block except for ->setlease.
 451         No VFS locks held on entry except for ->setlease.
 452
 453 ->setlease has the file_list_lock held and must not sleep.
 454
 455 ->llseek() locking has moved from llseek to the individual llseek
 456 implementations.  If your fs is not using generic_file_llseek, you
 457 need to acquire and release the appropriate locks in your ->llseek().
 458 For many filesystems, it is probably safe to acquire the inode
 459 mutex or just to use i_size_read() instead.
 460 Note: this does not protect the file->f_pos against concurrent modifications
 461 since this is something the userspace has to take care about.
 462
 463 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
 464 Most instances call fasync_helper(), which does that maintenance, so it's
 465 not normally something one needs to worry about.  Return values > 0 will be
 466 mapped to zero in the VFS layer.
 467
 468 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
 469 move ->readdir() to inode_operations and use a separate method for directory
 470 ->ioctl() or kill the latter completely. One of the problems is that for
 471 anything that resembles union-mount we won't have a struct file for all
 472 components. And there are other reasons why the current interface is a mess...
 473
 474 ->read on directories probably must go away - we should just enforce -EISDIR
 475 in sys_read() and friends.
 476
 477 --------------------------- dquot_operations -------------------------------
 478 prototypes:
 479         int (*write_dquot) (struct dquot *);
 480         int (*acquire_dquot) (struct dquot *);
 481         int (*release_dquot) (struct dquot *);
 482         int (*mark_dirty) (struct dquot *);
 483         int (*write_info) (struct super_block *, int);
 484
 485 These operations are intended to be more or less wrapping functions that ensure
 486 a proper locking wrt the filesystem and call the generic quota operations.
 487
 488 What filesystem should expect from the generic quota functions:
 489
 490                 FS recursion    Held locks when called
 491 write_dquot:    yes             dqonoff_sem or dqptr_sem
 492 acquire_dquot:  yes             dqonoff_sem or dqptr_sem
 493 release_dquot:  yes             dqonoff_sem or dqptr_sem
 494 mark_dirty:     no              -
 495 write_info:     yes             dqonoff_sem
 496
 497 FS recursion means calling ->quota_read() and ->quota_write() from superblock
 498 operations.
 499
 500 More details about quota locking can be found in fs/dquot.c.
 501
 502 --------------------------- vm_operations_struct -----------------------------
 503 prototypes:
 504         void (*open)(struct vm_area_struct*);
 505         void (*close)(struct vm_area_struct*);
 506         int (*fault)(struct vm_area_struct*, struct vm_fault *);
 507         int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
 508         int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
 509
 510 locking rules:
 511                 mmap_sem        PageLocked(page)
 512 open:           yes
 513 close:          yes
 514 fault:          yes             can return with page locked
 515 page_mkwrite:   yes             can return with page locked
 516 access:         yes
 517
 518         ->fault() is called when a previously not present pte is about
 519 to be faulted in. The filesystem must find and return the page associated
 520 with the passed in "pgoff" in the vm_fault structure. If it is possible that
 521 the page may be truncated and/or invalidated, then the filesystem must lock
 522 the page, then ensure it is not already truncated (the page lock will block
 523 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
 524 locked. The VM will unlock the page.
 525
 526         ->page_mkwrite() is called when a previously read-only pte is
 527 about to become writeable. The filesystem again must ensure that there are
 528 no truncate/invalidate races, and then return with the page locked. If
 529 the page has been truncated, the filesystem should not look up a new page
 530 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
 531 will cause the VM to retry the fault.
 532
 533         ->access() is called when get_user_pages() fails in
 534 acces_process_vm(), typically used to debug a process through
 535 /proc/pid/mem or ptrace.  This function is needed only for
 536 VM_IO | VM_PFNMAP VMAs.
 537
 538 ================================================================================
 539                         Dubious stuff
 540
 541 (if you break something or notice that it is broken and do not fix it yourself
 542 - at least put it here)