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