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