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 dentry *, unsigned int); 21 int (*d_weak_revalidate)(struct dentry *, unsigned int); 22 int (*d_hash)(const struct dentry *, struct qstr *); 23 int (*d_compare)(const struct dentry *, 24 unsigned int, const char *, const struct qstr *); 25 int (*d_delete)(struct dentry *); 26 int (*d_init)(struct dentry *); 27 void (*d_release)(struct dentry *); 28 void (*d_iput)(struct dentry *, struct inode *); 29 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen); 30 struct vfsmount *(*d_automount)(struct path *path); 31 int (*d_manage)(const struct path *, bool); 32 struct dentry *(*d_real)(struct dentry *, const struct inode *); 33 34locking rules: 35 36================== =========== ======== ============== ======== 37ops rename_lock ->d_lock may block rcu-walk 38================== =========== ======== ============== ======== 39d_revalidate: no no yes (ref-walk) maybe 40d_weak_revalidate: no no yes no 41d_hash no no no maybe 42d_compare: yes no no maybe 43d_delete: no yes no no 44d_init: no no yes no 45d_release: no no yes no 46d_prune: no yes no no 47d_iput: no no yes no 48d_dname: no no no no 49d_automount: no no yes no 50d_manage: no no yes (ref-walk) maybe 51d_real no no yes no 52================== =========== ======== ============== ======== 53 54inode_operations 55================ 56 57prototypes:: 58 59 int (*create) (struct inode *,struct dentry *,umode_t, bool); 60 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); 61 int (*link) (struct dentry *,struct inode *,struct dentry *); 62 int (*unlink) (struct inode *,struct dentry *); 63 int (*symlink) (struct inode *,struct dentry *,const char *); 64 int (*mkdir) (struct inode *,struct dentry *,umode_t); 65 int (*rmdir) (struct inode *,struct dentry *); 66 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t); 67 int (*rename) (struct inode *, struct dentry *, 68 struct inode *, struct dentry *, unsigned int); 69 int (*readlink) (struct dentry *, char __user *,int); 70 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *); 71 void (*truncate) (struct inode *); 72 int (*permission) (struct inode *, int, unsigned int); 73 struct posix_acl * (*get_acl)(struct inode *, int, bool); 74 int (*setattr) (struct dentry *, struct iattr *); 75 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int); 76 ssize_t (*listxattr) (struct dentry *, char *, size_t); 77 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len); 78 void (*update_time)(struct inode *, struct timespec *, int); 79 int (*atomic_open)(struct inode *, struct dentry *, 80 struct file *, unsigned open_flag, 81 umode_t create_mode); 82 int (*tmpfile) (struct inode *, struct dentry *, umode_t); 83 int (*fileattr_set)(struct user_namespace *mnt_userns, 84 struct dentry *dentry, struct fileattr *fa); 85 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa); 86 87locking rules: 88 all may block 89 90============= ============================================= 91ops i_rwsem(inode) 92============= ============================================= 93lookup: shared 94create: exclusive 95link: exclusive (both) 96mknod: exclusive 97symlink: exclusive 98mkdir: exclusive 99unlink: exclusive (both) 100rmdir: exclusive (both)(see below) 101rename: exclusive (all) (see below) 102readlink: no 103get_link: no 104setattr: exclusive 105permission: no (may not block if called in rcu-walk mode) 106get_acl: no 107getattr: no 108listxattr: no 109fiemap: no 110update_time: no 111atomic_open: shared (exclusive if O_CREAT is set in open flags) 112tmpfile: no 113fileattr_get: no or exclusive 114fileattr_set: exclusive 115============= ============================================= 116 117 118 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem 119 exclusive on victim. 120 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem. 121 122See Documentation/filesystems/directory-locking.rst for more detailed discussion 123of the locking scheme for directory operations. 124 125xattr_handler operations 126======================== 127 128prototypes:: 129 130 bool (*list)(struct dentry *dentry); 131 int (*get)(const struct xattr_handler *handler, struct dentry *dentry, 132 struct inode *inode, const char *name, void *buffer, 133 size_t size); 134 int (*set)(const struct xattr_handler *handler, 135 struct user_namespace *mnt_userns, 136 struct dentry *dentry, struct inode *inode, const char *name, 137 const void *buffer, size_t size, int flags); 138 139locking rules: 140 all may block 141 142===== ============== 143ops i_rwsem(inode) 144===== ============== 145list: no 146get: no 147set: exclusive 148===== ============== 149 150super_operations 151================ 152 153prototypes:: 154 155 struct inode *(*alloc_inode)(struct super_block *sb); 156 void (*free_inode)(struct inode *); 157 void (*destroy_inode)(struct inode *); 158 void (*dirty_inode) (struct inode *, int flags); 159 int (*write_inode) (struct inode *, struct writeback_control *wbc); 160 int (*drop_inode) (struct inode *); 161 void (*evict_inode) (struct inode *); 162 void (*put_super) (struct super_block *); 163 int (*sync_fs)(struct super_block *sb, int wait); 164 int (*freeze_fs) (struct super_block *); 165 int (*unfreeze_fs) (struct super_block *); 166 int (*statfs) (struct dentry *, struct kstatfs *); 167 int (*remount_fs) (struct super_block *, int *, char *); 168 void (*umount_begin) (struct super_block *); 169 int (*show_options)(struct seq_file *, struct dentry *); 170 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); 171 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); 172 173locking rules: 174 All may block [not true, see below] 175 176====================== ============ ======================== 177ops s_umount note 178====================== ============ ======================== 179alloc_inode: 180free_inode: called from RCU callback 181destroy_inode: 182dirty_inode: 183write_inode: 184drop_inode: !!!inode->i_lock!!! 185evict_inode: 186put_super: write 187sync_fs: read 188freeze_fs: write 189unfreeze_fs: write 190statfs: maybe(read) (see below) 191remount_fs: write 192umount_begin: no 193show_options: no (namespace_sem) 194quota_read: no (see below) 195quota_write: no (see below) 196====================== ============ ======================== 197 198->statfs() has s_umount (shared) when called by ustat(2) (native or 199compat), but that's an accident of bad API; s_umount is used to pin 200the superblock down when we only have dev_t given us by userland to 201identify the superblock. Everything else (statfs(), fstatfs(), etc.) 202doesn't hold it when calling ->statfs() - superblock is pinned down 203by resolving the pathname passed to syscall. 204 205->quota_read() and ->quota_write() functions are both guaranteed to 206be the only ones operating on the quota file by the quota code (via 207dqio_sem) (unless an admin really wants to screw up something and 208writes to quota files with quotas on). For other details about locking 209see also dquot_operations section. 210 211file_system_type 212================ 213 214prototypes:: 215 216 struct dentry *(*mount) (struct file_system_type *, int, 217 const char *, void *); 218 void (*kill_sb) (struct super_block *); 219 220locking rules: 221 222======= ========= 223ops may block 224======= ========= 225mount yes 226kill_sb yes 227======= ========= 228 229->mount() returns ERR_PTR or the root dentry; its superblock should be locked 230on return. 231 232->kill_sb() takes a write-locked superblock, does all shutdown work on it, 233unlocks and drops the reference. 234 235address_space_operations 236======================== 237prototypes:: 238 239 int (*writepage)(struct page *page, struct writeback_control *wbc); 240 int (*readpage)(struct file *, struct page *); 241 int (*writepages)(struct address_space *, struct writeback_control *); 242 bool (*dirty_folio)(struct address_space *, struct folio *folio); 243 void (*readahead)(struct readahead_control *); 244 int (*write_begin)(struct file *, struct address_space *mapping, 245 loff_t pos, unsigned len, unsigned flags, 246 struct page **pagep, void **fsdata); 247 int (*write_end)(struct file *, struct address_space *mapping, 248 loff_t pos, unsigned len, unsigned copied, 249 struct page *page, void *fsdata); 250 sector_t (*bmap)(struct address_space *, sector_t); 251 void (*invalidate_folio) (struct folio *, size_t start, size_t len); 252 int (*releasepage) (struct page *, int); 253 void (*freepage)(struct page *); 254 int (*direct_IO)(struct kiocb *, struct iov_iter *iter); 255 bool (*isolate_page) (struct page *, isolate_mode_t); 256 int (*migratepage)(struct address_space *, struct page *, struct page *); 257 void (*putback_page) (struct page *); 258 int (*launder_folio)(struct folio *); 259 bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count); 260 int (*error_remove_page)(struct address_space *, struct page *); 261 int (*swap_activate)(struct file *); 262 int (*swap_deactivate)(struct file *); 263 264locking rules: 265 All except dirty_folio and freepage may block 266 267====================== ======================== ========= =============== 268ops PageLocked(page) i_rwsem invalidate_lock 269====================== ======================== ========= =============== 270writepage: yes, unlocks (see below) 271readpage: yes, unlocks shared 272writepages: 273dirty_folio maybe 274readahead: yes, unlocks shared 275write_begin: locks the page exclusive 276write_end: yes, unlocks exclusive 277bmap: 278invalidate_folio: yes exclusive 279releasepage: yes 280freepage: yes 281direct_IO: 282isolate_page: yes 283migratepage: yes (both) 284putback_page: yes 285launder_folio: yes 286is_partially_uptodate: yes 287error_remove_page: yes 288swap_activate: no 289swap_deactivate: no 290====================== ======================== ========= =============== 291 292->write_begin(), ->write_end() and ->readpage() may be called from 293the request handler (/dev/loop). 294 295->readpage() unlocks the page, either synchronously or via I/O 296completion. 297 298->readahead() unlocks the pages that I/O is attempted on like ->readpage(). 299 300->writepage() is used for two purposes: for "memory cleansing" and for 301"sync". These are quite different operations and the behaviour may differ 302depending upon the mode. 303 304If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then 305it *must* start I/O against the page, even if that would involve 306blocking on in-progress I/O. 307 308If writepage is called for memory cleansing (sync_mode == 309WBC_SYNC_NONE) then its role is to get as much writeout underway as 310possible. So writepage should try to avoid blocking against 311currently-in-progress I/O. 312 313If the filesystem is not called for "sync" and it determines that it 314would need to block against in-progress I/O to be able to start new I/O 315against the page the filesystem should redirty the page with 316redirty_page_for_writepage(), then unlock the page and return zero. 317This may also be done to avoid internal deadlocks, but rarely. 318 319If the filesystem is called for sync then it must wait on any 320in-progress I/O and then start new I/O. 321 322The filesystem should unlock the page synchronously, before returning to the 323caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE 324value. WRITEPAGE_ACTIVATE means that page cannot really be written out 325currently, and VM should stop calling ->writepage() on this page for some 326time. VM does this by moving page to the head of the active list, hence the 327name. 328 329Unless the filesystem is going to redirty_page_for_writepage(), unlock the page 330and return zero, writepage *must* run set_page_writeback() against the page, 331followed by unlocking it. Once set_page_writeback() has been run against the 332page, write I/O can be submitted and the write I/O completion handler must run 333end_page_writeback() once the I/O is complete. If no I/O is submitted, the 334filesystem must run end_page_writeback() against the page before returning from 335writepage. 336 337That is: after 2.5.12, pages which are under writeout are *not* locked. Note, 338if the filesystem needs the page to be locked during writeout, that is ok, too, 339the page is allowed to be unlocked at any point in time between the calls to 340set_page_writeback() and end_page_writeback(). 341 342Note, failure to run either redirty_page_for_writepage() or the combination of 343set_page_writeback()/end_page_writeback() on a page submitted to writepage 344will leave the page itself marked clean but it will be tagged as dirty in the 345radix tree. This incoherency can lead to all sorts of hard-to-debug problems 346in the filesystem like having dirty inodes at umount and losing written data. 347 348->writepages() is used for periodic writeback and for syscall-initiated 349sync operations. The address_space should start I/O against at least 350``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page 351which is written. The address_space implementation may write more (or less) 352pages than ``*nr_to_write`` asks for, but it should try to be reasonably close. 353If nr_to_write is NULL, all dirty pages must be written. 354 355writepages should _only_ write pages which are present on 356mapping->io_pages. 357 358->dirty_folio() is called from various places in the kernel when 359the target folio is marked as needing writeback. The folio cannot be 360truncated because either the caller holds the folio lock, or the caller 361has found the folio while holding the page table lock which will block 362truncation. 363 364->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some 365filesystems and by the swapper. The latter will eventually go away. Please, 366keep it that way and don't breed new callers. 367 368->invalidate_folio() is called when the filesystem must attempt to drop 369some or all of the buffers from the page when it is being truncated. It 370returns zero on success. The filesystem must exclusively acquire 371invalidate_lock before invalidating page cache in truncate / hole punch 372path (and thus calling into ->invalidate_folio) to block races between page 373cache invalidation and page cache filling functions (fault, read, ...). 374 375->releasepage() is called when the kernel is about to try to drop the 376buffers from the page in preparation for freeing it. It returns zero to 377indicate that the buffers are (or may be) freeable. If ->releasepage is zero, 378the kernel assumes that the fs has no private interest in the buffers. 379 380->freepage() is called when the kernel is done dropping the page 381from the page cache. 382 383->launder_folio() may be called prior to releasing a folio if 384it is still found to be dirty. It returns zero if the folio was successfully 385cleaned, or an error value if not. Note that in order to prevent the folio 386getting mapped back in and redirtied, it needs to be kept locked 387across the entire operation. 388 389->swap_activate will be called with a non-zero argument on 390files backing (non block device backed) swapfiles. A return value 391of zero indicates success, in which case this file can be used for 392backing swapspace. The swapspace operations will be proxied to the 393address space operations. 394 395->swap_deactivate() will be called in the sys_swapoff() 396path after ->swap_activate() returned success. 397 398file_lock_operations 399==================== 400 401prototypes:: 402 403 void (*fl_copy_lock)(struct file_lock *, struct file_lock *); 404 void (*fl_release_private)(struct file_lock *); 405 406 407locking rules: 408 409=================== ============= ========= 410ops inode->i_lock may block 411=================== ============= ========= 412fl_copy_lock: yes no 413fl_release_private: maybe maybe[1]_ 414=================== ============= ========= 415 416.. [1]: 417 ->fl_release_private for flock or POSIX locks is currently allowed 418 to block. Leases however can still be freed while the i_lock is held and 419 so fl_release_private called on a lease should not block. 420 421lock_manager_operations 422======================= 423 424prototypes:: 425 426 void (*lm_notify)(struct file_lock *); /* unblock callback */ 427 int (*lm_grant)(struct file_lock *, struct file_lock *, int); 428 void (*lm_break)(struct file_lock *); /* break_lease callback */ 429 int (*lm_change)(struct file_lock **, int); 430 bool (*lm_breaker_owns_lease)(struct file_lock *); 431 432locking rules: 433 434====================== ============= ================= ========= 435ops flc_lock blocked_lock_lock may block 436====================== ============= ================= ========= 437lm_notify: no yes no 438lm_grant: no no no 439lm_break: yes no no 440lm_change yes no no 441lm_breaker_owns_lease: yes no no 442====================== ============= ================= ========= 443 444buffer_head 445=========== 446 447prototypes:: 448 449 void (*b_end_io)(struct buffer_head *bh, int uptodate); 450 451locking rules: 452 453called from interrupts. In other words, extreme care is needed here. 454bh is locked, but that's all warranties we have here. Currently only RAID1, 455highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices 456call this method upon the IO completion. 457 458block_device_operations 459======================= 460prototypes:: 461 462 int (*open) (struct block_device *, fmode_t); 463 int (*release) (struct gendisk *, fmode_t); 464 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 465 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 466 int (*direct_access) (struct block_device *, sector_t, void **, 467 unsigned long *); 468 void (*unlock_native_capacity) (struct gendisk *); 469 int (*getgeo)(struct block_device *, struct hd_geometry *); 470 void (*swap_slot_free_notify) (struct block_device *, unsigned long); 471 472locking rules: 473 474======================= =================== 475ops open_mutex 476======================= =================== 477open: yes 478release: yes 479ioctl: no 480compat_ioctl: no 481direct_access: no 482unlock_native_capacity: no 483getgeo: no 484swap_slot_free_notify: no (see below) 485======================= =================== 486 487swap_slot_free_notify is called with swap_lock and sometimes the page lock 488held. 489 490 491file_operations 492=============== 493 494prototypes:: 495 496 loff_t (*llseek) (struct file *, loff_t, int); 497 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 498 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 499 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 500 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 501 int (*iopoll) (struct kiocb *kiocb, bool spin); 502 int (*iterate) (struct file *, struct dir_context *); 503 int (*iterate_shared) (struct file *, struct dir_context *); 504 __poll_t (*poll) (struct file *, struct poll_table_struct *); 505 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 506 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 507 int (*mmap) (struct file *, struct vm_area_struct *); 508 int (*open) (struct inode *, struct file *); 509 int (*flush) (struct file *); 510 int (*release) (struct inode *, struct file *); 511 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync); 512 int (*fasync) (int, struct file *, int); 513 int (*lock) (struct file *, int, struct file_lock *); 514 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, 515 loff_t *, int); 516 unsigned long (*get_unmapped_area)(struct file *, unsigned long, 517 unsigned long, unsigned long, unsigned long); 518 int (*check_flags)(int); 519 int (*flock) (struct file *, int, struct file_lock *); 520 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, 521 size_t, unsigned int); 522 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, 523 size_t, unsigned int); 524 int (*setlease)(struct file *, long, struct file_lock **, void **); 525 long (*fallocate)(struct file *, int, loff_t, loff_t); 526 void (*show_fdinfo)(struct seq_file *m, struct file *f); 527 unsigned (*mmap_capabilities)(struct file *); 528 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, 529 loff_t, size_t, unsigned int); 530 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 531 struct file *file_out, loff_t pos_out, 532 loff_t len, unsigned int remap_flags); 533 int (*fadvise)(struct file *, loff_t, loff_t, int); 534 535locking rules: 536 All may block. 537 538->llseek() locking has moved from llseek to the individual llseek 539implementations. If your fs is not using generic_file_llseek, you 540need to acquire and release the appropriate locks in your ->llseek(). 541For many filesystems, it is probably safe to acquire the inode 542mutex or just to use i_size_read() instead. 543Note: this does not protect the file->f_pos against concurrent modifications 544since this is something the userspace has to take care about. 545 546->iterate() is called with i_rwsem exclusive. 547 548->iterate_shared() is called with i_rwsem at least shared. 549 550->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags. 551Most instances call fasync_helper(), which does that maintenance, so it's 552not normally something one needs to worry about. Return values > 0 will be 553mapped to zero in the VFS layer. 554 555->readdir() and ->ioctl() on directories must be changed. Ideally we would 556move ->readdir() to inode_operations and use a separate method for directory 557->ioctl() or kill the latter completely. One of the problems is that for 558anything that resembles union-mount we won't have a struct file for all 559components. And there are other reasons why the current interface is a mess... 560 561->read on directories probably must go away - we should just enforce -EISDIR 562in sys_read() and friends. 563 564->setlease operations should call generic_setlease() before or after setting 565the lease within the individual filesystem to record the result of the 566operation 567 568->fallocate implementation must be really careful to maintain page cache 569consistency when punching holes or performing other operations that invalidate 570page cache contents. Usually the filesystem needs to call 571truncate_inode_pages_range() to invalidate relevant range of the page cache. 572However the filesystem usually also needs to update its internal (and on disk) 573view of file offset -> disk block mapping. Until this update is finished, the 574filesystem needs to block page faults and reads from reloading now-stale page 575cache contents from the disk. Since VFS acquires mapping->invalidate_lock in 576shared mode when loading pages from disk (filemap_fault(), filemap_read(), 577readahead paths), the fallocate implementation must take the invalidate_lock to 578prevent reloading. 579 580->copy_file_range and ->remap_file_range implementations need to serialize 581against modifications of file data while the operation is running. For 582blocking changes through write(2) and similar operations inode->i_rwsem can be 583used. To block changes to file contents via a memory mapping during the 584operation, the filesystem must take mapping->invalidate_lock to coordinate 585with ->page_mkwrite. 586 587dquot_operations 588================ 589 590prototypes:: 591 592 int (*write_dquot) (struct dquot *); 593 int (*acquire_dquot) (struct dquot *); 594 int (*release_dquot) (struct dquot *); 595 int (*mark_dirty) (struct dquot *); 596 int (*write_info) (struct super_block *, int); 597 598These operations are intended to be more or less wrapping functions that ensure 599a proper locking wrt the filesystem and call the generic quota operations. 600 601What filesystem should expect from the generic quota functions: 602 603============== ============ ========================= 604ops FS recursion Held locks when called 605============== ============ ========================= 606write_dquot: yes dqonoff_sem or dqptr_sem 607acquire_dquot: yes dqonoff_sem or dqptr_sem 608release_dquot: yes dqonoff_sem or dqptr_sem 609mark_dirty: no - 610write_info: yes dqonoff_sem 611============== ============ ========================= 612 613FS recursion means calling ->quota_read() and ->quota_write() from superblock 614operations. 615 616More details about quota locking can be found in fs/dquot.c. 617 618vm_operations_struct 619==================== 620 621prototypes:: 622 623 void (*open)(struct vm_area_struct*); 624 void (*close)(struct vm_area_struct*); 625 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *); 626 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *); 627 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *); 628 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); 629 630locking rules: 631 632============= ========= =========================== 633ops mmap_lock PageLocked(page) 634============= ========= =========================== 635open: yes 636close: yes 637fault: yes can return with page locked 638map_pages: yes 639page_mkwrite: yes can return with page locked 640pfn_mkwrite: yes 641access: yes 642============= ========= =========================== 643 644->fault() is called when a previously not present pte is about to be faulted 645in. The filesystem must find and return the page associated with the passed in 646"pgoff" in the vm_fault structure. If it is possible that the page may be 647truncated and/or invalidated, then the filesystem must lock invalidate_lock, 648then ensure the page is not already truncated (invalidate_lock will block 649subsequent truncate), and then return with VM_FAULT_LOCKED, and the page 650locked. The VM will unlock the page. 651 652->map_pages() is called when VM asks to map easy accessible pages. 653Filesystem should find and map pages associated with offsets from "start_pgoff" 654till "end_pgoff". ->map_pages() is called with page table locked and must 655not block. If it's not possible to reach a page without blocking, 656filesystem should skip it. Filesystem should use do_set_pte() to setup 657page table entry. Pointer to entry associated with the page is passed in 658"pte" field in vm_fault structure. Pointers to entries for other offsets 659should be calculated relative to "pte". 660 661->page_mkwrite() is called when a previously read-only pte is about to become 662writeable. The filesystem again must ensure that there are no 663truncate/invalidate races or races with operations such as ->remap_file_range 664or ->copy_file_range, and then return with the page locked. Usually 665mapping->invalidate_lock is suitable for proper serialization. If the page has 666been truncated, the filesystem should not look up a new page like the ->fault() 667handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to 668retry the fault. 669 670->pfn_mkwrite() is the same as page_mkwrite but when the pte is 671VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is 672VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior 673after this call is to make the pte read-write, unless pfn_mkwrite returns 674an error. 675 676->access() is called when get_user_pages() fails in 677access_process_vm(), typically used to debug a process through 678/proc/pid/mem or ptrace. This function is needed only for 679VM_IO | VM_PFNMAP VMAs. 680 681-------------------------------------------------------------------------------- 682 683 Dubious stuff 684 685(if you break something or notice that it is broken and do not fix it yourself 686- at least put it here) 687