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 int (*get_acl)(struct inode *, int); 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 int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t); 173 174locking rules: 175 All may block [not true, see below] 176 177====================== ============ ======================== 178ops s_umount note 179====================== ============ ======================== 180alloc_inode: 181free_inode: called from RCU callback 182destroy_inode: 183dirty_inode: 184write_inode: 185drop_inode: !!!inode->i_lock!!! 186evict_inode: 187put_super: write 188sync_fs: read 189freeze_fs: write 190unfreeze_fs: write 191statfs: maybe(read) (see below) 192remount_fs: write 193umount_begin: no 194show_options: no (namespace_sem) 195quota_read: no (see below) 196quota_write: no (see below) 197bdev_try_to_free_page: no (see below) 198====================== ============ ======================== 199 200->statfs() has s_umount (shared) when called by ustat(2) (native or 201compat), but that's an accident of bad API; s_umount is used to pin 202the superblock down when we only have dev_t given us by userland to 203identify the superblock. Everything else (statfs(), fstatfs(), etc.) 204doesn't hold it when calling ->statfs() - superblock is pinned down 205by resolving the pathname passed to syscall. 206 207->quota_read() and ->quota_write() functions are both guaranteed to 208be the only ones operating on the quota file by the quota code (via 209dqio_sem) (unless an admin really wants to screw up something and 210writes to quota files with quotas on). For other details about locking 211see also dquot_operations section. 212 213->bdev_try_to_free_page is called from the ->releasepage handler of 214the block device inode. See there for more details. 215 216file_system_type 217================ 218 219prototypes:: 220 221 struct dentry *(*mount) (struct file_system_type *, int, 222 const char *, void *); 223 void (*kill_sb) (struct super_block *); 224 225locking rules: 226 227======= ========= 228ops may block 229======= ========= 230mount yes 231kill_sb yes 232======= ========= 233 234->mount() returns ERR_PTR or the root dentry; its superblock should be locked 235on return. 236 237->kill_sb() takes a write-locked superblock, does all shutdown work on it, 238unlocks and drops the reference. 239 240address_space_operations 241======================== 242prototypes:: 243 244 int (*writepage)(struct page *page, struct writeback_control *wbc); 245 int (*readpage)(struct file *, struct page *); 246 int (*writepages)(struct address_space *, struct writeback_control *); 247 int (*set_page_dirty)(struct page *page); 248 void (*readahead)(struct readahead_control *); 249 int (*readpages)(struct file *filp, struct address_space *mapping, 250 struct list_head *pages, unsigned nr_pages); 251 int (*write_begin)(struct file *, struct address_space *mapping, 252 loff_t pos, unsigned len, unsigned flags, 253 struct page **pagep, void **fsdata); 254 int (*write_end)(struct file *, struct address_space *mapping, 255 loff_t pos, unsigned len, unsigned copied, 256 struct page *page, void *fsdata); 257 sector_t (*bmap)(struct address_space *, sector_t); 258 void (*invalidatepage) (struct page *, unsigned int, unsigned int); 259 int (*releasepage) (struct page *, int); 260 void (*freepage)(struct page *); 261 int (*direct_IO)(struct kiocb *, struct iov_iter *iter); 262 bool (*isolate_page) (struct page *, isolate_mode_t); 263 int (*migratepage)(struct address_space *, struct page *, struct page *); 264 void (*putback_page) (struct page *); 265 int (*launder_page)(struct page *); 266 int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long); 267 int (*error_remove_page)(struct address_space *, struct page *); 268 int (*swap_activate)(struct file *); 269 int (*swap_deactivate)(struct file *); 270 271locking rules: 272 All except set_page_dirty and freepage may block 273 274====================== ======================== ========= 275ops PageLocked(page) i_rwsem 276====================== ======================== ========= 277writepage: yes, unlocks (see below) 278readpage: yes, unlocks 279writepages: 280set_page_dirty no 281readahead: yes, unlocks 282readpages: no 283write_begin: locks the page exclusive 284write_end: yes, unlocks exclusive 285bmap: 286invalidatepage: yes 287releasepage: yes 288freepage: yes 289direct_IO: 290isolate_page: yes 291migratepage: yes (both) 292putback_page: yes 293launder_page: yes 294is_partially_uptodate: yes 295error_remove_page: yes 296swap_activate: no 297swap_deactivate: no 298====================== ======================== ========= 299 300->write_begin(), ->write_end() and ->readpage() may be called from 301the request handler (/dev/loop). 302 303->readpage() unlocks the page, either synchronously or via I/O 304completion. 305 306->readahead() unlocks the pages that I/O is attempted on like ->readpage(). 307 308->readpages() populates the pagecache with the passed pages and starts 309I/O against them. They come unlocked upon I/O completion. 310 311->writepage() is used for two purposes: for "memory cleansing" and for 312"sync". These are quite different operations and the behaviour may differ 313depending upon the mode. 314 315If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then 316it *must* start I/O against the page, even if that would involve 317blocking on in-progress I/O. 318 319If writepage is called for memory cleansing (sync_mode == 320WBC_SYNC_NONE) then its role is to get as much writeout underway as 321possible. So writepage should try to avoid blocking against 322currently-in-progress I/O. 323 324If the filesystem is not called for "sync" and it determines that it 325would need to block against in-progress I/O to be able to start new I/O 326against the page the filesystem should redirty the page with 327redirty_page_for_writepage(), then unlock the page and return zero. 328This may also be done to avoid internal deadlocks, but rarely. 329 330If the filesystem is called for sync then it must wait on any 331in-progress I/O and then start new I/O. 332 333The filesystem should unlock the page synchronously, before returning to the 334caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE 335value. WRITEPAGE_ACTIVATE means that page cannot really be written out 336currently, and VM should stop calling ->writepage() on this page for some 337time. VM does this by moving page to the head of the active list, hence the 338name. 339 340Unless the filesystem is going to redirty_page_for_writepage(), unlock the page 341and return zero, writepage *must* run set_page_writeback() against the page, 342followed by unlocking it. Once set_page_writeback() has been run against the 343page, write I/O can be submitted and the write I/O completion handler must run 344end_page_writeback() once the I/O is complete. If no I/O is submitted, the 345filesystem must run end_page_writeback() against the page before returning from 346writepage. 347 348That is: after 2.5.12, pages which are under writeout are *not* locked. Note, 349if the filesystem needs the page to be locked during writeout, that is ok, too, 350the page is allowed to be unlocked at any point in time between the calls to 351set_page_writeback() and end_page_writeback(). 352 353Note, failure to run either redirty_page_for_writepage() or the combination of 354set_page_writeback()/end_page_writeback() on a page submitted to writepage 355will leave the page itself marked clean but it will be tagged as dirty in the 356radix tree. This incoherency can lead to all sorts of hard-to-debug problems 357in the filesystem like having dirty inodes at umount and losing written data. 358 359->writepages() is used for periodic writeback and for syscall-initiated 360sync operations. The address_space should start I/O against at least 361``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page 362which is written. The address_space implementation may write more (or less) 363pages than ``*nr_to_write`` asks for, but it should try to be reasonably close. 364If nr_to_write is NULL, all dirty pages must be written. 365 366writepages should _only_ write pages which are present on 367mapping->io_pages. 368 369->set_page_dirty() is called from various places in the kernel 370when the target page is marked as needing writeback. It may be called 371under spinlock (it cannot block) and is sometimes called with the page 372not locked. 373 374->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some 375filesystems and by the swapper. The latter will eventually go away. Please, 376keep it that way and don't breed new callers. 377 378->invalidatepage() is called when the filesystem must attempt to drop 379some or all of the buffers from the page when it is being truncated. It 380returns zero on success. If ->invalidatepage is zero, the kernel uses 381block_invalidatepage() instead. 382 383->releasepage() is called when the kernel is about to try to drop the 384buffers from the page in preparation for freeing it. It returns zero to 385indicate that the buffers are (or may be) freeable. If ->releasepage is zero, 386the kernel assumes that the fs has no private interest in the buffers. 387 388->freepage() is called when the kernel is done dropping the page 389from the page cache. 390 391->launder_page() may be called prior to releasing a page if 392it is still found to be dirty. It returns zero if the page was successfully 393cleaned, or an error value if not. Note that in order to prevent the page 394getting mapped back in and redirtied, it needs to be kept locked 395across the entire operation. 396 397->swap_activate will be called with a non-zero argument on 398files backing (non block device backed) swapfiles. A return value 399of zero indicates success, in which case this file can be used for 400backing swapspace. The swapspace operations will be proxied to the 401address space operations. 402 403->swap_deactivate() will be called in the sys_swapoff() 404path after ->swap_activate() returned success. 405 406file_lock_operations 407==================== 408 409prototypes:: 410 411 void (*fl_copy_lock)(struct file_lock *, struct file_lock *); 412 void (*fl_release_private)(struct file_lock *); 413 414 415locking rules: 416 417=================== ============= ========= 418ops inode->i_lock may block 419=================== ============= ========= 420fl_copy_lock: yes no 421fl_release_private: maybe maybe[1]_ 422=================== ============= ========= 423 424.. [1]: 425 ->fl_release_private for flock or POSIX locks is currently allowed 426 to block. Leases however can still be freed while the i_lock is held and 427 so fl_release_private called on a lease should not block. 428 429lock_manager_operations 430======================= 431 432prototypes:: 433 434 void (*lm_notify)(struct file_lock *); /* unblock callback */ 435 int (*lm_grant)(struct file_lock *, struct file_lock *, int); 436 void (*lm_break)(struct file_lock *); /* break_lease callback */ 437 int (*lm_change)(struct file_lock **, int); 438 bool (*lm_breaker_owns_lease)(struct file_lock *); 439 440locking rules: 441 442====================== ============= ================= ========= 443ops inode->i_lock blocked_lock_lock may block 444====================== ============= ================= ========= 445lm_notify: yes yes no 446lm_grant: no no no 447lm_break: yes no no 448lm_change yes no no 449lm_breaker_owns_lease: no no no 450====================== ============= ================= ========= 451 452buffer_head 453=========== 454 455prototypes:: 456 457 void (*b_end_io)(struct buffer_head *bh, int uptodate); 458 459locking rules: 460 461called from interrupts. In other words, extreme care is needed here. 462bh is locked, but that's all warranties we have here. Currently only RAID1, 463highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices 464call this method upon the IO completion. 465 466block_device_operations 467======================= 468prototypes:: 469 470 int (*open) (struct block_device *, fmode_t); 471 int (*release) (struct gendisk *, fmode_t); 472 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 473 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long); 474 int (*direct_access) (struct block_device *, sector_t, void **, 475 unsigned long *); 476 void (*unlock_native_capacity) (struct gendisk *); 477 int (*getgeo)(struct block_device *, struct hd_geometry *); 478 void (*swap_slot_free_notify) (struct block_device *, unsigned long); 479 480locking rules: 481 482======================= =================== 483ops bd_mutex 484======================= =================== 485open: yes 486release: yes 487ioctl: no 488compat_ioctl: no 489direct_access: no 490unlock_native_capacity: no 491getgeo: no 492swap_slot_free_notify: no (see below) 493======================= =================== 494 495swap_slot_free_notify is called with swap_lock and sometimes the page lock 496held. 497 498 499file_operations 500=============== 501 502prototypes:: 503 504 loff_t (*llseek) (struct file *, loff_t, int); 505 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); 506 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); 507 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); 508 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); 509 int (*iterate) (struct file *, struct dir_context *); 510 int (*iterate_shared) (struct file *, struct dir_context *); 511 __poll_t (*poll) (struct file *, struct poll_table_struct *); 512 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); 513 long (*compat_ioctl) (struct file *, unsigned int, unsigned long); 514 int (*mmap) (struct file *, struct vm_area_struct *); 515 int (*open) (struct inode *, struct file *); 516 int (*flush) (struct file *); 517 int (*release) (struct inode *, struct file *); 518 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync); 519 int (*fasync) (int, struct file *, int); 520 int (*lock) (struct file *, int, struct file_lock *); 521 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, 522 loff_t *); 523 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, 524 loff_t *); 525 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, 526 void __user *); 527 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, 528 loff_t *, int); 529 unsigned long (*get_unmapped_area)(struct file *, unsigned long, 530 unsigned long, unsigned long, unsigned long); 531 int (*check_flags)(int); 532 int (*flock) (struct file *, int, struct file_lock *); 533 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, 534 size_t, unsigned int); 535 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, 536 size_t, unsigned int); 537 int (*setlease)(struct file *, long, struct file_lock **, void **); 538 long (*fallocate)(struct file *, int, loff_t, loff_t); 539 540locking rules: 541 All may block. 542 543->llseek() locking has moved from llseek to the individual llseek 544implementations. If your fs is not using generic_file_llseek, you 545need to acquire and release the appropriate locks in your ->llseek(). 546For many filesystems, it is probably safe to acquire the inode 547mutex or just to use i_size_read() instead. 548Note: this does not protect the file->f_pos against concurrent modifications 549since this is something the userspace has to take care about. 550 551->iterate() is called with i_rwsem exclusive. 552 553->iterate_shared() is called with i_rwsem at least shared. 554 555->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags. 556Most instances call fasync_helper(), which does that maintenance, so it's 557not normally something one needs to worry about. Return values > 0 will be 558mapped to zero in the VFS layer. 559 560->readdir() and ->ioctl() on directories must be changed. Ideally we would 561move ->readdir() to inode_operations and use a separate method for directory 562->ioctl() or kill the latter completely. One of the problems is that for 563anything that resembles union-mount we won't have a struct file for all 564components. And there are other reasons why the current interface is a mess... 565 566->read on directories probably must go away - we should just enforce -EISDIR 567in sys_read() and friends. 568 569->setlease operations should call generic_setlease() before or after setting 570the lease within the individual filesystem to record the result of the 571operation 572 573dquot_operations 574================ 575 576prototypes:: 577 578 int (*write_dquot) (struct dquot *); 579 int (*acquire_dquot) (struct dquot *); 580 int (*release_dquot) (struct dquot *); 581 int (*mark_dirty) (struct dquot *); 582 int (*write_info) (struct super_block *, int); 583 584These operations are intended to be more or less wrapping functions that ensure 585a proper locking wrt the filesystem and call the generic quota operations. 586 587What filesystem should expect from the generic quota functions: 588 589============== ============ ========================= 590ops FS recursion Held locks when called 591============== ============ ========================= 592write_dquot: yes dqonoff_sem or dqptr_sem 593acquire_dquot: yes dqonoff_sem or dqptr_sem 594release_dquot: yes dqonoff_sem or dqptr_sem 595mark_dirty: no - 596write_info: yes dqonoff_sem 597============== ============ ========================= 598 599FS recursion means calling ->quota_read() and ->quota_write() from superblock 600operations. 601 602More details about quota locking can be found in fs/dquot.c. 603 604vm_operations_struct 605==================== 606 607prototypes:: 608 609 void (*open)(struct vm_area_struct*); 610 void (*close)(struct vm_area_struct*); 611 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *); 612 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *); 613 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *); 614 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); 615 616locking rules: 617 618============= ========= =========================== 619ops mmap_lock PageLocked(page) 620============= ========= =========================== 621open: yes 622close: yes 623fault: yes can return with page locked 624map_pages: yes 625page_mkwrite: yes can return with page locked 626pfn_mkwrite: yes 627access: yes 628============= ========= =========================== 629 630->fault() is called when a previously not present pte is about 631to be faulted in. The filesystem must find and return the page associated 632with the passed in "pgoff" in the vm_fault structure. If it is possible that 633the page may be truncated and/or invalidated, then the filesystem must lock 634the page, then ensure it is not already truncated (the page lock will block 635subsequent truncate), and then return with VM_FAULT_LOCKED, and the page 636locked. The VM will unlock the page. 637 638->map_pages() is called when VM asks to map easy accessible pages. 639Filesystem should find and map pages associated with offsets from "start_pgoff" 640till "end_pgoff". ->map_pages() is called with page table locked and must 641not block. If it's not possible to reach a page without blocking, 642filesystem should skip it. Filesystem should use do_set_pte() to setup 643page table entry. Pointer to entry associated with the page is passed in 644"pte" field in vm_fault structure. Pointers to entries for other offsets 645should be calculated relative to "pte". 646 647->page_mkwrite() is called when a previously read-only pte is 648about to become writeable. The filesystem again must ensure that there are 649no truncate/invalidate races, and then return with the page locked. If 650the page has been truncated, the filesystem should not look up a new page 651like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which 652will cause the VM to retry 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