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_iput:         no              no              yes             no
  33 d_dname:        no              no              no              no
  34 d_automount:    no              no              yes             no
  35 d_manage:       no              no              yes (ref-walk)  maybe
  36
  37 --------------------------- inode_operations ---------------------------
  38 prototypes:
  39         int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
  40         struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
  41 ata *);
  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 *,int);
  46         int (*rmdir) (struct inode *,struct dentry *);
  47         int (*mknod) (struct inode *,struct dentry *,int,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 (*check_acl)(struct inode *, int, unsigned 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         void (*truncate_range)(struct inode *, loff_t, loff_t);
  63         long (*fallocate)(struct inode *inode, int mode, loff_t offset, loff_t len);
  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 check_acl:      no
  85 getattr:        no
  86 setxattr:       yes
  87 getxattr:       no
  88 listxattr:      no
  89 removexattr:    yes
  90 truncate_range: yes
  91 fallocate:      no
  92 fiemap:         no
  93         Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
  94 victim.
  95         cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
  96         ->truncate() is never called directly - it's a callback, not a
  97 method. It's called by vmtruncate() - deprecated library function used by
  98 ->setattr(). Locking information above applies to that call (i.e. is
  99 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
 100 passed).
 101
 102 See Documentation/filesystems/directory-locking for more detailed discussion
 103 of the locking scheme for directory operations.
 104
 105 --------------------------- super_operations ---------------------------
 106 prototypes:
 107         struct inode *(*alloc_inode)(struct super_block *sb);
 108         void (*destroy_inode)(struct inode *);
 109         void (*dirty_inode) (struct inode *);
 110         int (*write_inode) (struct inode *, struct writeback_control *wbc);
 111         int (*drop_inode) (struct inode *);
 112         void (*evict_inode) (struct inode *);
 113         void (*put_super) (struct super_block *);
 114         void (*write_super) (struct super_block *);
 115         int (*sync_fs)(struct super_block *sb, int wait);
 116         int (*freeze_fs) (struct super_block *);
 117         int (*unfreeze_fs) (struct super_block *);
 118         int (*statfs) (struct dentry *, struct kstatfs *);
 119         int (*remount_fs) (struct super_block *, int *, char *);
 120         void (*umount_begin) (struct super_block *);
 121         int (*show_options)(struct seq_file *, struct vfsmount *);
 122         ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
 123         ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
 124         int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
 125
 126 locking rules:
 127         All may block [not true, see below]
 128                         s_umount
 129 alloc_inode:
 130 destroy_inode:
 131 dirty_inode:                            (must not sleep)
 132 write_inode:
 133 drop_inode:                             !!!inode_lock!!!
 134 evict_inode:
 135 put_super:              write
 136 write_super:            read
 137 sync_fs:                read
 138 freeze_fs:              read
 139 unfreeze_fs:            read
 140 statfs:                 maybe(read)     (see below)
 141 remount_fs:             write
 142 umount_begin:           no
 143 show_options:           no              (namespace_sem)
 144 quota_read:             no              (see below)
 145 quota_write:            no              (see below)
 146 bdev_try_to_free_page:  no              (see below)
 147
 148 ->statfs() has s_umount (shared) when called by ustat(2) (native or
 149 compat), but that's an accident of bad API; s_umount is used to pin
 150 the superblock down when we only have dev_t given us by userland to
 151 identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
 152 doesn't hold it when calling ->statfs() - superblock is pinned down
 153 by resolving the pathname passed to syscall.
 154 ->quota_read() and ->quota_write() functions are both guaranteed to
 155 be the only ones operating on the quota file by the quota code (via
 156 dqio_sem) (unless an admin really wants to screw up something and
 157 writes to quota files with quotas on). For other details about locking
 158 see also dquot_operations section.
 159 ->bdev_try_to_free_page is called from the ->releasepage handler of
 160 the block device inode.  See there for more details.
 161
 162 --------------------------- file_system_type ---------------------------
 163 prototypes:
 164         int (*get_sb) (struct file_system_type *, int,
 165                        const char *, void *, struct vfsmount *);
 166         struct dentry *(*mount) (struct file_system_type *, int,
 167                        const char *, void *);
 168         void (*kill_sb) (struct super_block *);
 169 locking rules:
 170                 may block
 171 get_sb          yes
 172 mount           yes
 173 kill_sb         yes
 174
 175 ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
 176 (exclusive on ->s_umount).
 177 ->mount() returns ERR_PTR or the root dentry.
 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 (*sync_page)(struct page *);
 186         int (*writepages)(struct address_space *, struct writeback_control *);
 187         int (*set_page_dirty)(struct page *page);
 188         int (*readpages)(struct file *filp, struct address_space *mapping,
 189                         struct list_head *pages, unsigned nr_pages);
 190         int (*write_begin)(struct file *, struct address_space *mapping,
 191                                 loff_t pos, unsigned len, unsigned flags,
 192                                 struct page **pagep, void **fsdata);
 193         int (*write_end)(struct file *, struct address_space *mapping,
 194                                 loff_t pos, unsigned len, unsigned copied,
 195                                 struct page *page, void *fsdata);
 196         sector_t (*bmap)(struct address_space *, sector_t);
 197         int (*invalidatepage) (struct page *, unsigned long);
 198         int (*releasepage) (struct page *, int);
 199         void (*freepage)(struct page *);
 200         int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
 201                         loff_t offset, unsigned long nr_segs);
 202         int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,
 203                                 unsigned long *);
 204         int (*migratepage)(struct address_space *, struct page *, struct page *);
 205         int (*launder_page)(struct page *);
 206         int (*is_partially_uptodate)(struct page *, read_descriptor_t *, unsigned long);
 207         int (*error_remove_page)(struct address_space *, struct page *);
 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 sync_page:              maybe
 216 writepages:
 217 set_page_dirty          no
 218 readpages:
 219 write_begin:            locks the page          yes
 220 write_end:              yes, unlocks            yes
 221 bmap:
 222 invalidatepage:         yes
 223 releasepage:            yes
 224 freepage:               yes
 225 direct_IO:
 226 get_xip_mem:                                    maybe
 227 migratepage:            yes (both)
 228 launder_page:           yes
 229 is_partially_uptodate:  yes
 230 error_remove_page:      yes
 231
 232         ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
 233 may be called from the request handler (/dev/loop).
 234
 235         ->readpage() unlocks the page, either synchronously or via I/O
 236 completion.
 237
 238         ->readpages() populates the pagecache with the passed pages and starts
 239 I/O against them.  They come unlocked upon I/O completion.
 240
 241         ->writepage() is used for two purposes: for "memory cleansing" and for
 242 "sync".  These are quite different operations and the behaviour may differ
 243 depending upon the mode.
 244
 245 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
 246 it *must* start I/O against the page, even if that would involve
 247 blocking on in-progress I/O.
 248
 249 If writepage is called for memory cleansing (sync_mode ==
 250 WBC_SYNC_NONE) then its role is to get as much writeout underway as
 251 possible.  So writepage should try to avoid blocking against
 252 currently-in-progress I/O.
 253
 254 If the filesystem is not called for "sync" and it determines that it
 255 would need to block against in-progress I/O to be able to start new I/O
 256 against the page the filesystem should redirty the page with
 257 redirty_page_for_writepage(), then unlock the page and return zero.
 258 This may also be done to avoid internal deadlocks, but rarely.
 259
 260 If the filesystem is called for sync then it must wait on any
 261 in-progress I/O and then start new I/O.
 262
 263 The filesystem should unlock the page synchronously, before returning to the
 264 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
 265 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
 266 currently, and VM should stop calling ->writepage() on this page for some
 267 time. VM does this by moving page to the head of the active list, hence the
 268 name.
 269
 270 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
 271 and return zero, writepage *must* run set_page_writeback() against the page,
 272 followed by unlocking it.  Once set_page_writeback() has been run against the
 273 page, write I/O can be submitted and the write I/O completion handler must run
 274 end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
 275 filesystem must run end_page_writeback() against the page before returning from
 276 writepage.
 277
 278 That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
 279 if the filesystem needs the page to be locked during writeout, that is ok, too,
 280 the page is allowed to be unlocked at any point in time between the calls to
 281 set_page_writeback() and end_page_writeback().
 282
 283 Note, failure to run either redirty_page_for_writepage() or the combination of
 284 set_page_writeback()/end_page_writeback() on a page submitted to writepage
 285 will leave the page itself marked clean but it will be tagged as dirty in the
 286 radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
 287 in the filesystem like having dirty inodes at umount and losing written data.
 288
 289         ->sync_page() locking rules are not well-defined - usually it is called
 290 with lock on page, but that is not guaranteed. Considering the currently
 291 existing instances of this method ->sync_page() itself doesn't look
 292 well-defined...
 293
 294         ->writepages() is used for periodic writeback and for syscall-initiated
 295 sync operations.  The address_space should start I/O against at least
 296 *nr_to_write pages.  *nr_to_write must be decremented for each page which is
 297 written.  The address_space implementation may write more (or less) pages
 298 than *nr_to_write asks for, but it should try to be reasonably close.  If
 299 nr_to_write is NULL, all dirty pages must be written.
 300
 301 writepages should _only_ write pages which are present on
 302 mapping->io_pages.
 303
 304         ->set_page_dirty() is called from various places in the kernel
 305 when the target page is marked as needing writeback.  It may be called
 306 under spinlock (it cannot block) and is sometimes called with the page
 307 not locked.
 308
 309         ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
 310 filesystems and by the swapper. The latter will eventually go away.  Please,
 311 keep it that way and don't breed new callers.
 312
 313         ->invalidatepage() is called when the filesystem must attempt to drop
 314 some or all of the buffers from the page when it is being truncated.  It
 315 returns zero on success.  If ->invalidatepage is zero, the kernel uses
 316 block_invalidatepage() instead.
 317
 318         ->releasepage() is called when the kernel is about to try to drop the
 319 buffers from the page in preparation for freeing it.  It returns zero to
 320 indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
 321 the kernel assumes that the fs has no private interest in the buffers.
 322
 323         ->freepage() is called when the kernel is done dropping the page
 324 from the page cache.
 325
 326         ->launder_page() may be called prior to releasing a page if
 327 it is still found to be dirty. It returns zero if the page was successfully
 328 cleaned, or an error value if not. Note that in order to prevent the page
 329 getting mapped back in and redirtied, it needs to be kept locked
 330 across the entire operation.
 331
 332 ----------------------- file_lock_operations ------------------------------
 333 prototypes:
 334         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 335         void (*fl_release_private)(struct file_lock *);
 336
 337
 338 locking rules:
 339                         file_lock_lock  may block
 340 fl_copy_lock:           yes             no
 341 fl_release_private:     maybe           no
 342
 343 ----------------------- lock_manager_operations ---------------------------
 344 prototypes:
 345         int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
 346         void (*fl_notify)(struct file_lock *);  /* unblock callback */
 347         int (*fl_grant)(struct file_lock *, struct file_lock *, int);
 348         void (*fl_release_private)(struct file_lock *);
 349         void (*fl_break)(struct file_lock *); /* break_lease callback */
 350         int (*fl_change)(struct file_lock **, int);
 351
 352 locking rules:
 353                         file_lock_lock  may block
 354 fl_compare_owner:       yes             no
 355 fl_notify:              yes             no
 356 fl_grant:               no              no
 357 fl_release_private:     maybe           no
 358 fl_break:               yes             no
 359 fl_change               yes             no
 360
 361 --------------------------- buffer_head -----------------------------------
 362 prototypes:
 363         void (*b_end_io)(struct buffer_head *bh, int uptodate);
 364
 365 locking rules:
 366         called from interrupts. In other words, extreme care is needed here.
 367 bh is locked, but that's all warranties we have here. Currently only RAID1,
 368 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
 369 call this method upon the IO completion.
 370
 371 --------------------------- block_device_operations -----------------------
 372 prototypes:
 373         int (*open) (struct block_device *, fmode_t);
 374         int (*release) (struct gendisk *, fmode_t);
 375         int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
 376         int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
 377         int (*direct_access) (struct block_device *, sector_t, void **, unsigned long *);
 378         int (*media_changed) (struct gendisk *);
 379         void (*unlock_native_capacity) (struct gendisk *);
 380         int (*revalidate_disk) (struct gendisk *);
 381         int (*getgeo)(struct block_device *, struct hd_geometry *);
 382         void (*swap_slot_free_notify) (struct block_device *, unsigned long);
 383
 384 locking rules:
 385                         bd_mutex
 386 open:                   yes
 387 release:                yes
 388 ioctl:                  no
 389 compat_ioctl:           no
 390 direct_access:          no
 391 media_changed:          no
 392 unlock_native_capacity: no
 393 revalidate_disk:        no
 394 getgeo:                 no
 395 swap_slot_free_notify:  no      (see below)
 396
 397 media_changed, unlock_native_capacity and revalidate_disk are called only from
 398 check_disk_change().
 399
 400 swap_slot_free_notify is called with swap_lock and sometimes the page lock
 401 held.
 402
 403
 404 --------------------------- file_operations -------------------------------
 405 prototypes:
 406         loff_t (*llseek) (struct file *, loff_t, int);
 407         ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 408         ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
 409         ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 410         ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 411         int (*readdir) (struct file *, void *, filldir_t);
 412         unsigned int (*poll) (struct file *, struct poll_table_struct *);
 413         long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 414         long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
 415         int (*mmap) (struct file *, struct vm_area_struct *);
 416         int (*open) (struct inode *, struct file *);
 417         int (*flush) (struct file *);
 418         int (*release) (struct inode *, struct file *);
 419         int (*fsync) (struct file *, int datasync);
 420         int (*aio_fsync) (struct kiocb *, int datasync);
 421         int (*fasync) (int, struct file *, int);
 422         int (*lock) (struct file *, int, struct file_lock *);
 423         ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
 424                         loff_t *);
 425         ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
 426                         loff_t *);
 427         ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
 428                         void __user *);
 429         ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
 430                         loff_t *, int);
 431         unsigned long (*get_unmapped_area)(struct file *, unsigned long,
 432                         unsigned long, unsigned long, unsigned long);
 433         int (*check_flags)(int);
 434         int (*flock) (struct file *, int, struct file_lock *);
 435         ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
 436                         size_t, unsigned int);
 437         ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
 438                         size_t, unsigned int);
 439         int (*setlease)(struct file *, long, struct file_lock **);
 440 };
 441
 442 locking rules:
 443         All may block except for ->setlease.
 444         No VFS locks held on entry except for ->fsync and ->setlease.
 445
 446 ->fsync() has i_mutex on inode.
 447
 448 ->setlease has the file_list_lock held and must not sleep.
 449
 450 ->llseek() locking has moved from llseek to the individual llseek
 451 implementations.  If your fs is not using generic_file_llseek, you
 452 need to acquire and release the appropriate locks in your ->llseek().
 453 For many filesystems, it is probably safe to acquire the inode
 454 mutex or just to use i_size_read() instead.
 455 Note: this does not protect the file->f_pos against concurrent modifications
 456 since this is something the userspace has to take care about.
 457
 458 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
 459 Most instances call fasync_helper(), which does that maintenance, so it's
 460 not normally something one needs to worry about.  Return values > 0 will be
 461 mapped to zero in the VFS layer.
 462
 463 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
 464 move ->readdir() to inode_operations and use a separate method for directory
 465 ->ioctl() or kill the latter completely. One of the problems is that for
 466 anything that resembles union-mount we won't have a struct file for all
 467 components. And there are other reasons why the current interface is a mess...
 468
 469 ->read on directories probably must go away - we should just enforce -EISDIR
 470 in sys_read() and friends.
 471
 472 --------------------------- dquot_operations -------------------------------
 473 prototypes:
 474         int (*write_dquot) (struct dquot *);
 475         int (*acquire_dquot) (struct dquot *);
 476         int (*release_dquot) (struct dquot *);
 477         int (*mark_dirty) (struct dquot *);
 478         int (*write_info) (struct super_block *, int);
 479
 480 These operations are intended to be more or less wrapping functions that ensure
 481 a proper locking wrt the filesystem and call the generic quota operations.
 482
 483 What filesystem should expect from the generic quota functions:
 484
 485                 FS recursion    Held locks when called
 486 write_dquot:    yes             dqonoff_sem or dqptr_sem
 487 acquire_dquot:  yes             dqonoff_sem or dqptr_sem
 488 release_dquot:  yes             dqonoff_sem or dqptr_sem
 489 mark_dirty:     no              -
 490 write_info:     yes             dqonoff_sem
 491
 492 FS recursion means calling ->quota_read() and ->quota_write() from superblock
 493 operations.
 494
 495 More details about quota locking can be found in fs/dquot.c.
 496
 497 --------------------------- vm_operations_struct -----------------------------
 498 prototypes:
 499         void (*open)(struct vm_area_struct*);
 500         void (*close)(struct vm_area_struct*);
 501         int (*fault)(struct vm_area_struct*, struct vm_fault *);
 502         int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
 503         int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
 504
 505 locking rules:
 506                 mmap_sem        PageLocked(page)
 507 open:           yes
 508 close:          yes
 509 fault:          yes             can return with page locked
 510 page_mkwrite:   yes             can return with page locked
 511 access:         yes
 512
 513         ->fault() is called when a previously not present pte is about
 514 to be faulted in. The filesystem must find and return the page associated
 515 with the passed in "pgoff" in the vm_fault structure. If it is possible that
 516 the page may be truncated and/or invalidated, then the filesystem must lock
 517 the page, then ensure it is not already truncated (the page lock will block
 518 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
 519 locked. The VM will unlock the page.
 520
 521         ->page_mkwrite() is called when a previously read-only pte is
 522 about to become writeable. The filesystem again must ensure that there are
 523 no truncate/invalidate races, and then return with the page locked. If
 524 the page has been truncated, the filesystem should not look up a new page
 525 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
 526 will cause the VM to retry the fault.
 527
 528         ->access() is called when get_user_pages() fails in
 529 acces_process_vm(), typically used to debug a process through
 530 /proc/pid/mem or ptrace.  This function is needed only for
 531 VM_IO | VM_PFNMAP VMAs.
 532
 533 ================================================================================
 534                         Dubious stuff
 535
 536 (if you break something or notice that it is broken and do not fix it yourself
 537 - at least put it here)