locking.rst (27513B)
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 (*read_folio)(struct file *, struct folio *); 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, 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 bool (*release_folio)(struct folio *, gfp_t); 253 void (*free_folio)(struct folio *); 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 swap_info_struct *sis, struct file *f, sector_t *span) 262 int (*swap_deactivate)(struct file *); 263 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter); 264 265locking rules: 266 All except dirty_folio and free_folio may block 267 268====================== ======================== ========= =============== 269ops folio locked i_rwsem invalidate_lock 270====================== ======================== ========= =============== 271writepage: yes, unlocks (see below) 272read_folio: yes, unlocks shared 273writepages: 274dirty_folio: maybe 275readahead: yes, unlocks shared 276write_begin: locks the page exclusive 277write_end: yes, unlocks exclusive 278bmap: 279invalidate_folio: yes exclusive 280release_folio: yes 281free_folio: yes 282direct_IO: 283isolate_page: yes 284migratepage: yes (both) 285putback_page: yes 286launder_folio: yes 287is_partially_uptodate: yes 288error_remove_page: yes 289swap_activate: no 290swap_deactivate: no 291swap_rw: yes, unlocks 292====================== ======================== ========= =============== 293 294->write_begin(), ->write_end() and ->read_folio() may be called from 295the request handler (/dev/loop). 296 297->read_folio() unlocks the folio, either synchronously or via I/O 298completion. 299 300->readahead() unlocks the folios that I/O is attempted on like ->read_folio(). 301 302->writepage() is used for two purposes: for "memory cleansing" and for 303"sync". These are quite different operations and the behaviour may differ 304depending upon the mode. 305 306If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then 307it *must* start I/O against the page, even if that would involve 308blocking on in-progress I/O. 309 310If writepage is called for memory cleansing (sync_mode == 311WBC_SYNC_NONE) then its role is to get as much writeout underway as 312possible. So writepage should try to avoid blocking against 313currently-in-progress I/O. 314 315If the filesystem is not called for "sync" and it determines that it 316would need to block against in-progress I/O to be able to start new I/O 317against the page the filesystem should redirty the page with 318redirty_page_for_writepage(), then unlock the page and return zero. 319This may also be done to avoid internal deadlocks, but rarely. 320 321If the filesystem is called for sync then it must wait on any 322in-progress I/O and then start new I/O. 323 324The filesystem should unlock the page synchronously, before returning to the 325caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE 326value. WRITEPAGE_ACTIVATE means that page cannot really be written out 327currently, and VM should stop calling ->writepage() on this page for some 328time. VM does this by moving page to the head of the active list, hence the 329name. 330 331Unless the filesystem is going to redirty_page_for_writepage(), unlock the page 332and return zero, writepage *must* run set_page_writeback() against the page, 333followed by unlocking it. Once set_page_writeback() has been run against the 334page, write I/O can be submitted and the write I/O completion handler must run 335end_page_writeback() once the I/O is complete. If no I/O is submitted, the 336filesystem must run end_page_writeback() against the page before returning from 337writepage. 338 339That is: after 2.5.12, pages which are under writeout are *not* locked. Note, 340if the filesystem needs the page to be locked during writeout, that is ok, too, 341the page is allowed to be unlocked at any point in time between the calls to 342set_page_writeback() and end_page_writeback(). 343 344Note, failure to run either redirty_page_for_writepage() or the combination of 345set_page_writeback()/end_page_writeback() on a page submitted to writepage 346will leave the page itself marked clean but it will be tagged as dirty in the 347radix tree. This incoherency can lead to all sorts of hard-to-debug problems 348in the filesystem like having dirty inodes at umount and losing written data. 349 350->writepages() is used for periodic writeback and for syscall-initiated 351sync operations. The address_space should start I/O against at least 352``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page 353which is written. The address_space implementation may write more (or less) 354pages than ``*nr_to_write`` asks for, but it should try to be reasonably close. 355If nr_to_write is NULL, all dirty pages must be written. 356 357writepages should _only_ write pages which are present on 358mapping->io_pages. 359 360->dirty_folio() is called from various places in the kernel when 361the target folio is marked as needing writeback. The folio cannot be 362truncated because either the caller holds the folio lock, or the caller 363has found the folio while holding the page table lock which will block 364truncation. 365 366->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some 367filesystems and by the swapper. The latter will eventually go away. Please, 368keep it that way and don't breed new callers. 369 370->invalidate_folio() is called when the filesystem must attempt to drop 371some or all of the buffers from the page when it is being truncated. It 372returns zero on success. The filesystem must exclusively acquire 373invalidate_lock before invalidating page cache in truncate / hole punch 374path (and thus calling into ->invalidate_folio) to block races between page 375cache invalidation and page cache filling functions (fault, read, ...). 376 377->release_folio() is called when the kernel is about to try to drop the 378buffers from the folio in preparation for freeing it. It returns false to 379indicate that the buffers are (or may be) freeable. If ->release_folio is 380NULL, the kernel assumes that the fs has no private interest in the buffers. 381 382->free_folio() is called when the kernel has dropped the folio 383from the page cache. 384 385->launder_folio() may be called prior to releasing a folio if 386it is still found to be dirty. It returns zero if the folio was successfully 387cleaned, or an error value if not. Note that in order to prevent the folio 388getting mapped back in and redirtied, it needs to be kept locked 389across the entire operation. 390 391->swap_activate() will be called to prepare the given file for swap. It 392should perform any validation and preparation necessary to ensure that 393writes can be performed with minimal memory allocation. It should call 394add_swap_extent(), or the helper iomap_swapfile_activate(), and return 395the number of extents added. If IO should be submitted through 396->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted 397directly to the block device ``sis->bdev``. 398 399->swap_deactivate() will be called in the sys_swapoff() 400path after ->swap_activate() returned success. 401 402->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate(). 403 404file_lock_operations 405==================== 406 407prototypes:: 408 409 void (*fl_copy_lock)(struct file_lock *, struct file_lock *); 410 void (*fl_release_private)(struct file_lock *); 411 412 413locking rules: 414 415=================== ============= ========= 416ops inode->i_lock may block 417=================== ============= ========= 418fl_copy_lock: yes no 419fl_release_private: maybe maybe[1]_ 420=================== ============= ========= 421 422.. [1]: 423 ->fl_release_private for flock or POSIX locks is currently allowed 424 to block. Leases however can still be freed while the i_lock is held and 425 so fl_release_private called on a lease should not block. 426 427lock_manager_operations 428======================= 429 430prototypes:: 431 432 void (*lm_notify)(struct file_lock *); /* unblock callback */ 433 int (*lm_grant)(struct file_lock *, struct file_lock *, int); 434 void (*lm_break)(struct file_lock *); /* break_lease callback */ 435 int (*lm_change)(struct file_lock **, int); 436 bool (*lm_breaker_owns_lease)(struct file_lock *); 437 bool (*lm_lock_expirable)(struct file_lock *); 438 void (*lm_expire_lock)(void); 439 440locking rules: 441 442====================== ============= ================= ========= 443ops flc_lock blocked_lock_lock may block 444====================== ============= ================= ========= 445lm_notify: no yes no 446lm_grant: no no no 447lm_break: yes no no 448lm_change yes no no 449lm_breaker_owns_lease: yes no no 450lm_lock_expirable yes no no 451lm_expire_lock no no yes 452====================== ============= ================= ========= 453 454buffer_head 455=========== 456 457prototypes:: 458 459 void (*b_end_io)(struct buffer_head *bh, int uptodate); 460 461locking rules: 462 463called from interrupts. In other words, extreme care is needed here. 464bh is locked, but that's all warranties we have here. Currently only RAID1, 465highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices 466call this method upon the IO completion. 467 468block_device_operations 469======================= 470prototypes:: 471 472 int (*open) (struct block_device *, fmode_t); 473 int (*release) (struct gendisk *, fmode_t); 474 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 475 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 476 int (*direct_access) (struct block_device *, sector_t, void **, 477 unsigned long *); 478 void (*unlock_native_capacity) (struct gendisk *); 479 int (*getgeo)(struct block_device *, struct hd_geometry *); 480 void (*swap_slot_free_notify) (struct block_device *, unsigned long); 481 482locking rules: 483 484======================= =================== 485ops open_mutex 486======================= =================== 487open: yes 488release: yes 489ioctl: no 490compat_ioctl: no 491direct_access: no 492unlock_native_capacity: no 493getgeo: no 494swap_slot_free_notify: no (see below) 495======================= =================== 496 497swap_slot_free_notify is called with swap_lock and sometimes the page lock 498held. 499 500 501file_operations 502=============== 503 504prototypes:: 505 506 loff_t (*llseek) (struct file *, loff_t, int); 507 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 508 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 509 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 510 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 511 int (*iopoll) (struct kiocb *kiocb, bool spin); 512 int (*iterate) (struct file *, struct dir_context *); 513 int (*iterate_shared) (struct file *, struct dir_context *); 514 __poll_t (*poll) (struct file *, struct poll_table_struct *); 515 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 516 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 517 int (*mmap) (struct file *, struct vm_area_struct *); 518 int (*open) (struct inode *, struct file *); 519 int (*flush) (struct file *); 520 int (*release) (struct inode *, struct file *); 521 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync); 522 int (*fasync) (int, struct file *, int); 523 int (*lock) (struct file *, int, struct file_lock *); 524 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, 525 loff_t *, int); 526 unsigned long (*get_unmapped_area)(struct file *, unsigned long, 527 unsigned long, unsigned long, unsigned long); 528 int (*check_flags)(int); 529 int (*flock) (struct file *, int, struct file_lock *); 530 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, 531 size_t, unsigned int); 532 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, 533 size_t, unsigned int); 534 int (*setlease)(struct file *, long, struct file_lock **, void **); 535 long (*fallocate)(struct file *, int, loff_t, loff_t); 536 void (*show_fdinfo)(struct seq_file *m, struct file *f); 537 unsigned (*mmap_capabilities)(struct file *); 538 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, 539 loff_t, size_t, unsigned int); 540 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, 541 struct file *file_out, loff_t pos_out, 542 loff_t len, unsigned int remap_flags); 543 int (*fadvise)(struct file *, loff_t, loff_t, int); 544 545locking rules: 546 All may block. 547 548->llseek() locking has moved from llseek to the individual llseek 549implementations. If your fs is not using generic_file_llseek, you 550need to acquire and release the appropriate locks in your ->llseek(). 551For many filesystems, it is probably safe to acquire the inode 552mutex or just to use i_size_read() instead. 553Note: this does not protect the file->f_pos against concurrent modifications 554since this is something the userspace has to take care about. 555 556->iterate() is called with i_rwsem exclusive. 557 558->iterate_shared() is called with i_rwsem at least shared. 559 560->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags. 561Most instances call fasync_helper(), which does that maintenance, so it's 562not normally something one needs to worry about. Return values > 0 will be 563mapped to zero in the VFS layer. 564 565->readdir() and ->ioctl() on directories must be changed. Ideally we would 566move ->readdir() to inode_operations and use a separate method for directory 567->ioctl() or kill the latter completely. One of the problems is that for 568anything that resembles union-mount we won't have a struct file for all 569components. And there are other reasons why the current interface is a mess... 570 571->read on directories probably must go away - we should just enforce -EISDIR 572in sys_read() and friends. 573 574->setlease operations should call generic_setlease() before or after setting 575the lease within the individual filesystem to record the result of the 576operation 577 578->fallocate implementation must be really careful to maintain page cache 579consistency when punching holes or performing other operations that invalidate 580page cache contents. Usually the filesystem needs to call 581truncate_inode_pages_range() to invalidate relevant range of the page cache. 582However the filesystem usually also needs to update its internal (and on disk) 583view of file offset -> disk block mapping. Until this update is finished, the 584filesystem needs to block page faults and reads from reloading now-stale page 585cache contents from the disk. Since VFS acquires mapping->invalidate_lock in 586shared mode when loading pages from disk (filemap_fault(), filemap_read(), 587readahead paths), the fallocate implementation must take the invalidate_lock to 588prevent reloading. 589 590->copy_file_range and ->remap_file_range implementations need to serialize 591against modifications of file data while the operation is running. For 592blocking changes through write(2) and similar operations inode->i_rwsem can be 593used. To block changes to file contents via a memory mapping during the 594operation, the filesystem must take mapping->invalidate_lock to coordinate 595with ->page_mkwrite. 596 597dquot_operations 598================ 599 600prototypes:: 601 602 int (*write_dquot) (struct dquot *); 603 int (*acquire_dquot) (struct dquot *); 604 int (*release_dquot) (struct dquot *); 605 int (*mark_dirty) (struct dquot *); 606 int (*write_info) (struct super_block *, int); 607 608These operations are intended to be more or less wrapping functions that ensure 609a proper locking wrt the filesystem and call the generic quota operations. 610 611What filesystem should expect from the generic quota functions: 612 613============== ============ ========================= 614ops FS recursion Held locks when called 615============== ============ ========================= 616write_dquot: yes dqonoff_sem or dqptr_sem 617acquire_dquot: yes dqonoff_sem or dqptr_sem 618release_dquot: yes dqonoff_sem or dqptr_sem 619mark_dirty: no - 620write_info: yes dqonoff_sem 621============== ============ ========================= 622 623FS recursion means calling ->quota_read() and ->quota_write() from superblock 624operations. 625 626More details about quota locking can be found in fs/dquot.c. 627 628vm_operations_struct 629==================== 630 631prototypes:: 632 633 void (*open)(struct vm_area_struct*); 634 void (*close)(struct vm_area_struct*); 635 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *); 636 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *); 637 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *); 638 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); 639 640locking rules: 641 642============= ========= =========================== 643ops mmap_lock PageLocked(page) 644============= ========= =========================== 645open: yes 646close: yes 647fault: yes can return with page locked 648map_pages: yes 649page_mkwrite: yes can return with page locked 650pfn_mkwrite: yes 651access: yes 652============= ========= =========================== 653 654->fault() is called when a previously not present pte is about to be faulted 655in. The filesystem must find and return the page associated with the passed in 656"pgoff" in the vm_fault structure. If it is possible that the page may be 657truncated and/or invalidated, then the filesystem must lock invalidate_lock, 658then ensure the page is not already truncated (invalidate_lock will block 659subsequent truncate), and then return with VM_FAULT_LOCKED, and the page 660locked. The VM will unlock the page. 661 662->map_pages() is called when VM asks to map easy accessible pages. 663Filesystem should find and map pages associated with offsets from "start_pgoff" 664till "end_pgoff". ->map_pages() is called with page table locked and must 665not block. If it's not possible to reach a page without blocking, 666filesystem should skip it. Filesystem should use do_set_pte() to setup 667page table entry. Pointer to entry associated with the page is passed in 668"pte" field in vm_fault structure. Pointers to entries for other offsets 669should be calculated relative to "pte". 670 671->page_mkwrite() is called when a previously read-only pte is about to become 672writeable. The filesystem again must ensure that there are no 673truncate/invalidate races or races with operations such as ->remap_file_range 674or ->copy_file_range, and then return with the page locked. Usually 675mapping->invalidate_lock is suitable for proper serialization. If the page has 676been truncated, the filesystem should not look up a new page like the ->fault() 677handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to 678retry the fault. 679 680->pfn_mkwrite() is the same as page_mkwrite but when the pte is 681VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is 682VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior 683after this call is to make the pte read-write, unless pfn_mkwrite returns 684an error. 685 686->access() is called when get_user_pages() fails in 687access_process_vm(), typically used to debug a process through 688/proc/pid/mem or ptrace. This function is needed only for 689VM_IO | VM_PFNMAP VMAs. 690 691-------------------------------------------------------------------------------- 692 693 Dubious stuff 694 695(if you break something or notice that it is broken and do not fix it yourself 696- at least put it here)