1======= 2Locking 3======= 4 5The text below describes the locking rules for VFS-related methods. 6It is (believed to be) up-to-date. *Please*, if you change anything in 7prototypes or locking protocols - update this file. And update the relevant 8instances in the tree, don't leave that to maintainers of filesystems/devices/ 9etc. At the very least, put the list of dubious cases in the end of this file. 10Don't turn it into log - maintainers of out-of-the-tree code are supposed to 11be able to use diff(1). 12 13Thing currently missing here: socket operations. Alexey? 14 15dentry_operations 16================= 17 18prototypes:: 19 20 int (*d_revalidate)(struct inode *, const struct qstr *, 21 struct dentry *, unsigned int); 22 int (*d_weak_revalidate)(struct dentry *, unsigned int); 23 int (*d_hash)(const struct dentry *, struct qstr *); 24 int (*d_compare)(const struct dentry *, 25 unsigned int, const char *, const struct qstr *); 26 int (*d_delete)(struct dentry *); 27 int (*d_init)(struct dentry *); 28 void (*d_release)(struct dentry *); 29 void (*d_iput)(struct dentry *, struct inode *); 30 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen); 31 struct vfsmount *(*d_automount)(struct path *path); 32 int (*d_manage)(const struct path *, bool); 33 struct dentry *(*d_real)(struct dentry *, enum d_real_type type); 34 bool (*d_unalias_trylock)(const struct dentry *); 35 void (*d_unalias_unlock)(const struct dentry *); 36 37locking rules: 38 39================== =========== ======== ============== ======== 40ops rename_lock ->d_lock may block rcu-walk 41================== =========== ======== ============== ======== 42d_revalidate: no no yes (ref-walk) maybe 43d_weak_revalidate: no no yes no 44d_hash no no no maybe 45d_compare: yes no no maybe 46d_delete: no yes no no 47d_init: no no yes no 48d_release: no no yes no 49d_prune: no yes no no 50d_iput: no no yes no 51d_dname: no no no no 52d_automount: no no yes no 53d_manage: no no yes (ref-walk) maybe 54d_real no no yes no 55d_unalias_trylock yes no no no 56d_unalias_unlock yes no no no 57================== =========== ======== ============== ======== 58 59inode_operations 60================ 61 62prototypes:: 63 64 int (*create) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t, bool); 65 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); 66 int (*link) (struct dentry *,struct inode *,struct dentry *); 67 int (*unlink) (struct inode *,struct dentry *); 68 int (*symlink) (struct mnt_idmap *, struct inode *,struct dentry *,const char *); 69 struct dentry *(*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t); 70 int (*rmdir) (struct inode *,struct dentry *); 71 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t); 72 int (*rename) (struct mnt_idmap *, struct inode *, struct dentry *, 73 struct inode *, struct dentry *, unsigned int); 74 int (*readlink) (struct dentry *, char __user *,int); 75 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *); 76 void (*truncate) (struct inode *); 77 int (*permission) (struct mnt_idmap *, struct inode *, int, unsigned int); 78 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool); 79 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *); 80 int (*getattr) (struct mnt_idmap *, const struct path *, struct kstat *, u32, unsigned int); 81 ssize_t (*listxattr) (struct dentry *, char *, size_t); 82 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len); 83 void (*update_time)(struct inode *, struct timespec *, int); 84 int (*atomic_open)(struct inode *, struct dentry *, 85 struct file *, unsigned open_flag, 86 umode_t create_mode); 87 int (*tmpfile) (struct mnt_idmap *, struct inode *, 88 struct file *, umode_t); 89 int (*fileattr_set)(struct mnt_idmap *idmap, 90 struct dentry *dentry, struct file_kattr *fa); 91 int (*fileattr_get)(struct dentry *dentry, struct file_kattr *fa); 92 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int); 93 struct offset_ctx *(*get_offset_ctx)(struct inode *inode); 94 95locking rules: 96 all may block 97 98============== ================================================== 99ops i_rwsem(inode) 100============== ================================================== 101lookup: shared 102create: exclusive 103link: exclusive (both) 104mknod: exclusive 105symlink: exclusive 106mkdir: exclusive 107unlink: exclusive (both) 108rmdir: exclusive (both)(see below) 109rename: exclusive (both parents, some children) (see below) 110readlink: no 111get_link: no 112setattr: exclusive 113permission: no (may not block if called in rcu-walk mode) 114get_inode_acl: no 115get_acl: no 116getattr: no 117listxattr: no 118fiemap: no 119update_time: no 120atomic_open: shared (exclusive if O_CREAT is set in open flags) 121tmpfile: no 122fileattr_get: no or exclusive 123fileattr_set: exclusive 124get_offset_ctx no 125============== ================================================== 126 127 128 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem 129 exclusive on victim. 130 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem. 131 ->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories 132 involved. 133 ->rename() has ->i_rwsem exclusive on any subdirectory that changes parent. 134 135See Documentation/filesystems/directory-locking.rst for more detailed discussion 136of the locking scheme for directory operations. 137 138xattr_handler operations 139======================== 140 141prototypes:: 142 143 bool (*list)(struct dentry *dentry); 144 int (*get)(const struct xattr_handler *handler, struct dentry *dentry, 145 struct inode *inode, const char *name, void *buffer, 146 size_t size); 147 int (*set)(const struct xattr_handler *handler, 148 struct mnt_idmap *idmap, 149 struct dentry *dentry, struct inode *inode, const char *name, 150 const void *buffer, size_t size, int flags); 151 152locking rules: 153 all may block 154 155===== ============== 156ops i_rwsem(inode) 157===== ============== 158list: no 159get: no 160set: exclusive 161===== ============== 162 163super_operations 164================ 165 166prototypes:: 167 168 struct inode *(*alloc_inode)(struct super_block *sb); 169 void (*free_inode)(struct inode *); 170 void (*destroy_inode)(struct inode *); 171 void (*dirty_inode) (struct inode *, int flags); 172 int (*write_inode) (struct inode *, struct writeback_control *wbc); 173 int (*drop_inode) (struct inode *); 174 void (*evict_inode) (struct inode *); 175 void (*put_super) (struct super_block *); 176 int (*sync_fs)(struct super_block *sb, int wait); 177 int (*freeze_fs) (struct super_block *); 178 int (*unfreeze_fs) (struct super_block *); 179 int (*statfs) (struct dentry *, struct kstatfs *); 180 int (*remount_fs) (struct super_block *, int *, char *); 181 void (*umount_begin) (struct super_block *); 182 int (*show_options)(struct seq_file *, struct dentry *); 183 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); 184 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); 185 186locking rules: 187 All may block [not true, see below] 188 189====================== ============ ======================== 190ops s_umount note 191====================== ============ ======================== 192alloc_inode: 193free_inode: called from RCU callback 194destroy_inode: 195dirty_inode: 196write_inode: 197drop_inode: !!!inode->i_lock!!! 198evict_inode: 199put_super: write 200sync_fs: read 201freeze_fs: write 202unfreeze_fs: write 203statfs: maybe(read) (see below) 204remount_fs: write 205umount_begin: no 206show_options: no (namespace_sem) 207quota_read: no (see below) 208quota_write: no (see below) 209====================== ============ ======================== 210 211->statfs() has s_umount (shared) when called by ustat(2) (native or 212compat), but that's an accident of bad API; s_umount is used to pin 213the superblock down when we only have dev_t given us by userland to 214identify the superblock. Everything else (statfs(), fstatfs(), etc.) 215doesn't hold it when calling ->statfs() - superblock is pinned down 216by resolving the pathname passed to syscall. 217 218->quota_read() and ->quota_write() functions are both guaranteed to 219be the only ones operating on the quota file by the quota code (via 220dqio_sem) (unless an admin really wants to screw up something and 221writes to quota files with quotas on). For other details about locking 222see also dquot_operations section. 223 224file_system_type 225================ 226 227prototypes:: 228 229 struct dentry *(*mount) (struct file_system_type *, int, 230 const char *, void *); 231 void (*kill_sb) (struct super_block *); 232 233locking rules: 234 235======= ========= 236ops may block 237======= ========= 238mount yes 239kill_sb yes 240======= ========= 241 242->mount() returns ERR_PTR or the root dentry; its superblock should be locked 243on return. 244 245->kill_sb() takes a write-locked superblock, does all shutdown work on it, 246unlocks and drops the reference. 247 248address_space_operations 249======================== 250prototypes:: 251 252 int (*read_folio)(struct file *, struct folio *); 253 int (*writepages)(struct address_space *, struct writeback_control *); 254 bool (*dirty_folio)(struct address_space *, struct folio *folio); 255 void (*readahead)(struct readahead_control *); 256 int (*write_begin)(const struct kiocb *, struct address_space *mapping, 257 loff_t pos, unsigned len, 258 struct folio **foliop, void **fsdata); 259 int (*write_end)(const struct kiocb *, struct address_space *mapping, 260 loff_t pos, unsigned len, unsigned copied, 261 struct folio *folio, void *fsdata); 262 sector_t (*bmap)(struct address_space *, sector_t); 263 void (*invalidate_folio) (struct folio *, size_t start, size_t len); 264 bool (*release_folio)(struct folio *, gfp_t); 265 void (*free_folio)(struct folio *); 266 int (*direct_IO)(struct kiocb *, struct iov_iter *iter); 267 int (*migrate_folio)(struct address_space *, struct folio *dst, 268 struct folio *src, enum migrate_mode); 269 int (*launder_folio)(struct folio *); 270 bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count); 271 int (*error_remove_folio)(struct address_space *, struct folio *); 272 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span) 273 int (*swap_deactivate)(struct file *); 274 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter); 275 276locking rules: 277 All except dirty_folio and free_folio may block 278 279====================== ======================== ========= =============== 280ops folio locked i_rwsem invalidate_lock 281====================== ======================== ========= =============== 282read_folio: yes, unlocks shared 283writepages: 284dirty_folio: maybe 285readahead: yes, unlocks shared 286write_begin: locks the folio exclusive 287write_end: yes, unlocks exclusive 288bmap: 289invalidate_folio: yes exclusive 290release_folio: yes 291free_folio: yes 292direct_IO: 293migrate_folio: yes (both) 294launder_folio: yes 295is_partially_uptodate: yes 296error_remove_folio: yes 297swap_activate: no 298swap_deactivate: no 299swap_rw: yes, unlocks 300====================== ======================== ========= =============== 301 302->write_begin(), ->write_end() and ->read_folio() may be called from 303the request handler (/dev/loop). 304 305->read_folio() unlocks the folio, either synchronously or via I/O 306completion. 307 308->readahead() unlocks the folios that I/O is attempted on like ->read_folio(). 309 310->writepages() is used for periodic writeback and for syscall-initiated 311sync operations. The address_space should start I/O against at least 312``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page 313which is written. The address_space implementation may write more (or less) 314pages than ``*nr_to_write`` asks for, but it should try to be reasonably close. 315If nr_to_write is NULL, all dirty pages must be written. 316 317writepages should _only_ write pages which are present in 318mapping->i_pages. 319 320->dirty_folio() is called from various places in the kernel when 321the target folio is marked as needing writeback. The folio cannot be 322truncated because either the caller holds the folio lock, or the caller 323has found the folio while holding the page table lock which will block 324truncation. 325 326->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some 327filesystems and by the swapper. The latter will eventually go away. Please, 328keep it that way and don't breed new callers. 329 330->invalidate_folio() is called when the filesystem must attempt to drop 331some or all of the buffers from the page when it is being truncated. It 332returns zero on success. The filesystem must exclusively acquire 333invalidate_lock before invalidating page cache in truncate / hole punch 334path (and thus calling into ->invalidate_folio) to block races between page 335cache invalidation and page cache filling functions (fault, read, ...). 336 337->release_folio() is called when the MM wants to make a change to the 338folio that would invalidate the filesystem's private data. For example, 339it may be about to be removed from the address_space or split. The folio 340is locked and not under writeback. It may be dirty. The gfp parameter 341is not usually used for allocation, but rather to indicate what the 342filesystem may do to attempt to free the private data. The filesystem may 343return false to indicate that the folio's private data cannot be freed. 344If it returns true, it should have already removed the private data from 345the folio. If a filesystem does not provide a ->release_folio method, 346the pagecache will assume that private data is buffer_heads and call 347try_to_free_buffers(). 348 349->free_folio() is called when the kernel has dropped the folio 350from the page cache. 351 352->launder_folio() may be called prior to releasing a folio if 353it is still found to be dirty. It returns zero if the folio was successfully 354cleaned, or an error value if not. Note that in order to prevent the folio 355getting mapped back in and redirtied, it needs to be kept locked 356across the entire operation. 357 358->swap_activate() will be called to prepare the given file for swap. It 359should perform any validation and preparation necessary to ensure that 360writes can be performed with minimal memory allocation. It should call 361add_swap_extent(), or the helper iomap_swapfile_activate(), and return 362the number of extents added. If IO should be submitted through 363->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted 364directly to the block device ``sis->bdev``. 365 366->swap_deactivate() will be called in the sys_swapoff() 367path after ->swap_activate() returned success. 368 369->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate(). 370 371file_lock_operations 372==================== 373 374prototypes:: 375 376 void (*fl_copy_lock)(struct file_lock *, struct file_lock *); 377 void (*fl_release_private)(struct file_lock *); 378 379 380locking rules: 381 382=================== ============= ========= 383ops inode->i_lock may block 384=================== ============= ========= 385fl_copy_lock: yes no 386fl_release_private: maybe maybe[1]_ 387=================== ============= ========= 388 389.. [1]: 390 ->fl_release_private for flock or POSIX locks is currently allowed 391 to block. Leases however can still be freed while the i_lock is held and 392 so fl_release_private called on a lease should not block. 393 394lock_manager_operations 395======================= 396 397prototypes:: 398 399 void (*lm_notify)(struct file_lock *); /* unblock callback */ 400 int (*lm_grant)(struct file_lock *, struct file_lock *, int); 401 void (*lm_break)(struct file_lock *); /* break_lease callback */ 402 int (*lm_change)(struct file_lock **, int); 403 bool (*lm_breaker_owns_lease)(struct file_lock *); 404 bool (*lm_lock_expirable)(struct file_lock *); 405 void (*lm_expire_lock)(void); 406 407locking rules: 408 409====================== ============= ================= ========= 410ops flc_lock blocked_lock_lock may block 411====================== ============= ================= ========= 412lm_notify: no yes no 413lm_grant: no no no 414lm_break: yes no no 415lm_change yes no no 416lm_breaker_owns_lease: yes no no 417lm_lock_expirable yes no no 418lm_expire_lock no no yes 419lm_open_conflict yes no no 420====================== ============= ================= ========= 421 422buffer_head 423=========== 424 425prototypes:: 426 427 void (*b_end_io)(struct buffer_head *bh, int uptodate); 428 429locking rules: 430 431called from interrupts. In other words, extreme care is needed here. 432bh is locked, but that's all warranties we have here. Currently only RAID1, 433highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices 434call this method upon the IO completion. 435 436block_device_operations 437======================= 438prototypes:: 439 440 int (*open) (struct block_device *, fmode_t); 441 int (*release) (struct gendisk *, fmode_t); 442 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 443 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 444 int (*direct_access) (struct block_device *, sector_t, void **, 445 unsigned long *); 446 void (*unlock_native_capacity) (struct gendisk *); 447 int (*getgeo)(struct gendisk *, struct hd_geometry *); 448 void (*swap_slot_free_notify) (struct block_device *, unsigned long); 449 450locking rules: 451 452======================= =================== 453ops open_mutex 454======================= =================== 455open: yes 456release: yes 457ioctl: no 458compat_ioctl: no 459direct_access: no 460unlock_native_capacity: no 461getgeo: no 462swap_slot_free_notify: no (see below) 463======================= =================== 464 465swap_slot_free_notify is called with swap_lock and sometimes the page lock 466held. 467 468 469file_operations 470=============== 471 472prototypes:: 473 474 loff_t (*llseek) (struct file *, loff_t, int); 475 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 476 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 477 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 478 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 479 int (*iopoll) (struct kiocb *kiocb, bool spin); 480 int (*iterate_shared) (struct file *, struct dir_context *); 481 __poll_t (*poll) (struct file *, struct poll_table_struct *); 482 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 483 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 484 int (*mmap) (struct file *, struct vm_area_struct *); 485 int (*open) (struct inode *, struct file *); 486 int (*flush) (struct file *); 487 int (*release) (struct inode *, struct file *); 488 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync); 489 int (*fasync) (int, struct file *, int); 490 int (*lock) (struct file *, int, struct file_lock *); 491 unsigned long (*get_unmapped_area)(struct file *, unsigned long, 492 unsigned long, unsigned long, unsigned long); 493 int (*check_flags)(int); 494 int (*flock) (struct file *, int, struct file_lock *); 495 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, 496 size_t, unsigned int); 497 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, 498 size_t, unsigned int); 499 int (*setlease)(struct file *, long, struct file_lock **, void **); 500 long (*fallocate)(struct file *, int, loff_t, loff_t); 501 void (*show_fdinfo)(struct seq_file *m, struct file *f); 502 unsigned (*mmap_capabilities)(struct file *); 503 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, 504 loff_t, size_t, unsigned int); 505 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 506 struct file *file_out, loff_t pos_out, 507 loff_t len, unsigned int remap_flags); 508 int (*fadvise)(struct file *, loff_t, loff_t, int); 509 510locking rules: 511 All may block. 512 513->llseek() locking has moved from llseek to the individual llseek 514implementations. If your fs is not using generic_file_llseek, you 515need to acquire and release the appropriate locks in your ->llseek(). 516For many filesystems, it is probably safe to acquire the inode 517mutex or just to use i_size_read() instead. 518Note: this does not protect the file->f_pos against concurrent modifications 519since this is something the userspace has to take care about. 520 521->iterate_shared() is called with i_rwsem held for reading, and with the 522file f_pos_lock held exclusively 523 524->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags. 525Most instances call fasync_helper(), which does that maintenance, so it's 526not normally something one needs to worry about. Return values > 0 will be 527mapped to zero in the VFS layer. 528 529->readdir() and ->ioctl() on directories must be changed. Ideally we would 530move ->readdir() to inode_operations and use a separate method for directory 531->ioctl() or kill the latter completely. One of the problems is that for 532anything that resembles union-mount we won't have a struct file for all 533components. And there are other reasons why the current interface is a mess... 534 535->read on directories probably must go away - we should just enforce -EISDIR 536in sys_read() and friends. 537 538->setlease operations should call generic_setlease() before or after setting 539the lease within the individual filesystem to record the result of the 540operation 541 542->fallocate implementation must be really careful to maintain page cache 543consistency when punching holes or performing other operations that invalidate 544page cache contents. Usually the filesystem needs to call 545truncate_inode_pages_range() to invalidate relevant range of the page cache. 546However the filesystem usually also needs to update its internal (and on disk) 547view of file offset -> disk block mapping. Until this update is finished, the 548filesystem needs to block page faults and reads from reloading now-stale page 549cache contents from the disk. Since VFS acquires mapping->invalidate_lock in 550shared mode when loading pages from disk (filemap_fault(), filemap_read(), 551readahead paths), the fallocate implementation must take the invalidate_lock to 552prevent reloading. 553 554->copy_file_range and ->remap_file_range implementations need to serialize 555against modifications of file data while the operation is running. For 556blocking changes through write(2) and similar operations inode->i_rwsem can be 557used. To block changes to file contents via a memory mapping during the 558operation, the filesystem must take mapping->invalidate_lock to coordinate 559with ->page_mkwrite. 560 561dquot_operations 562================ 563 564prototypes:: 565 566 int (*write_dquot) (struct dquot *); 567 int (*acquire_dquot) (struct dquot *); 568 int (*release_dquot) (struct dquot *); 569 int (*mark_dirty) (struct dquot *); 570 int (*write_info) (struct super_block *, int); 571 572These operations are intended to be more or less wrapping functions that ensure 573a proper locking wrt the filesystem and call the generic quota operations. 574 575What filesystem should expect from the generic quota functions: 576 577============== ============ ========================= 578ops FS recursion Held locks when called 579============== ============ ========================= 580write_dquot: yes dqonoff_sem or dqptr_sem 581acquire_dquot: yes dqonoff_sem or dqptr_sem 582release_dquot: yes dqonoff_sem or dqptr_sem 583mark_dirty: no - 584write_info: yes dqonoff_sem 585============== ============ ========================= 586 587FS recursion means calling ->quota_read() and ->quota_write() from superblock 588operations. 589 590More details about quota locking can be found in fs/dquot.c. 591 592vm_operations_struct 593==================== 594 595prototypes:: 596 597 void (*open)(struct vm_area_struct *); 598 void (*close)(struct vm_area_struct *); 599 vm_fault_t (*fault)(struct vm_fault *); 600 vm_fault_t (*huge_fault)(struct vm_fault *, unsigned int order); 601 vm_fault_t (*map_pages)(struct vm_fault *, pgoff_t start, pgoff_t end); 602 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *); 603 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *); 604 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); 605 606locking rules: 607 608============= ========== =========================== 609ops mmap_lock PageLocked(page) 610============= ========== =========================== 611open: write 612close: read/write 613fault: read can return with page locked 614huge_fault: maybe-read 615map_pages: maybe-read 616page_mkwrite: read can return with page locked 617pfn_mkwrite: read 618access: read 619============= ========== =========================== 620 621->fault() is called when a previously not present pte is about to be faulted 622in. The filesystem must find and return the page associated with the passed in 623"pgoff" in the vm_fault structure. If it is possible that the page may be 624truncated and/or invalidated, then the filesystem must lock invalidate_lock, 625then ensure the page is not already truncated (invalidate_lock will block 626subsequent truncate), and then return with VM_FAULT_LOCKED, and the page 627locked. The VM will unlock the page. 628 629->huge_fault() is called when there is no PUD or PMD entry present. This 630gives the filesystem the opportunity to install a PUD or PMD sized page. 631Filesystems can also use the ->fault method to return a PMD sized page, 632so implementing this function may not be necessary. In particular, 633filesystems should not call filemap_fault() from ->huge_fault(). 634The mmap_lock may not be held when this method is called. 635 636->map_pages() is called when VM asks to map easy accessible pages. 637Filesystem should find and map pages associated with offsets from "start_pgoff" 638till "end_pgoff". ->map_pages() is called with the RCU lock held and must 639not block. If it's not possible to reach a page without blocking, 640filesystem should skip it. Filesystem should use set_pte_range() to setup 641page table entry. Pointer to entry associated with the page is passed in 642"pte" field in vm_fault structure. Pointers to entries for other offsets 643should be calculated relative to "pte". 644 645->page_mkwrite() is called when a previously read-only pte is about to become 646writeable. The filesystem again must ensure that there are no 647truncate/invalidate races or races with operations such as ->remap_file_range 648or ->copy_file_range, and then return with the page locked. Usually 649mapping->invalidate_lock is suitable for proper serialization. If the page has 650been truncated, the filesystem should not look up a new page like the ->fault() 651handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to 652retry the fault. 653 654->pfn_mkwrite() is the same as page_mkwrite but when the pte is 655VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is 656VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior 657after this call is to make the pte read-write, unless pfn_mkwrite returns 658an error. 659 660->access() is called when get_user_pages() fails in 661access_process_vm(), typically used to debug a process through 662/proc/pid/mem or ptrace. This function is needed only for 663VM_IO | VM_PFNMAP VMAs. 664 665-------------------------------------------------------------------------------- 666 667 Dubious stuff 668 669(if you break something or notice that it is broken and do not fix it yourself 670- at least put it here) 671