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1=======2Locking3=======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 in7prototypes or locking protocols - update this file. And update the relevant8instances 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 to11be able to use diff(1).12 13Thing currently missing here: socket operations. Alexey?14 15dentry_operations16=================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 *, enum d_real_type type);33 34locking rules:35 36================== =========== ======== ============== ========37ops rename_lock ->d_lock may block rcu-walk38================== =========== ======== ============== ========39d_revalidate: no no yes (ref-walk) maybe40d_weak_revalidate: no no yes no41d_hash no no no maybe42d_compare: yes no no maybe43d_delete: no yes no no44d_init: no no yes no45d_release: no no yes no46d_prune: no yes no no47d_iput: no no yes no48d_dname: no no no no49d_automount: no no yes no50d_manage: no no yes (ref-walk) maybe51d_real no no yes no52================== =========== ======== ============== ========53 54inode_operations55================56 57prototypes::58 59 int (*create) (struct mnt_idmap *, 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 mnt_idmap *, struct inode *,struct dentry *,const char *);64 int (*mkdir) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t);65 int (*rmdir) (struct inode *,struct dentry *);66 int (*mknod) (struct mnt_idmap *, struct inode *,struct dentry *,umode_t,dev_t);67 int (*rename) (struct mnt_idmap *, 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 mnt_idmap *, struct inode *, int, unsigned int);73 struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);74 int (*setattr) (struct mnt_idmap *, struct dentry *, struct iattr *);75 int (*getattr) (struct mnt_idmap *, 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 mnt_idmap *, struct inode *,83 struct file *, umode_t);84 int (*fileattr_set)(struct mnt_idmap *idmap,85 struct dentry *dentry, struct fileattr *fa);86 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);87 struct posix_acl * (*get_acl)(struct mnt_idmap *, struct dentry *, int);88 struct offset_ctx *(*get_offset_ctx)(struct inode *inode);89 90locking rules:91 all may block92 93============== ==================================================94ops i_rwsem(inode)95============== ==================================================96lookup: shared97create: exclusive98link: exclusive (both)99mknod: exclusive100symlink: exclusive101mkdir: exclusive102unlink: exclusive (both)103rmdir: exclusive (both)(see below)104rename: exclusive (both parents, some children) (see below)105readlink: no106get_link: no107setattr: exclusive108permission: no (may not block if called in rcu-walk mode)109get_inode_acl: no110get_acl: no111getattr: no112listxattr: no113fiemap: no114update_time: no115atomic_open: shared (exclusive if O_CREAT is set in open flags)116tmpfile: no117fileattr_get: no or exclusive118fileattr_set: exclusive119get_offset_ctx no120============== ==================================================121 122 123 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem124 exclusive on victim.125 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.126 ->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories127 involved.128 ->rename() has ->i_rwsem exclusive on any subdirectory that changes parent.129 130See Documentation/filesystems/directory-locking.rst for more detailed discussion131of the locking scheme for directory operations.132 133xattr_handler operations134========================135 136prototypes::137 138 bool (*list)(struct dentry *dentry);139 int (*get)(const struct xattr_handler *handler, struct dentry *dentry,140 struct inode *inode, const char *name, void *buffer,141 size_t size);142 int (*set)(const struct xattr_handler *handler,143 struct mnt_idmap *idmap,144 struct dentry *dentry, struct inode *inode, const char *name,145 const void *buffer, size_t size, int flags);146 147locking rules:148 all may block149 150===== ==============151ops i_rwsem(inode)152===== ==============153list: no154get: no155set: exclusive156===== ==============157 158super_operations159================160 161prototypes::162 163 struct inode *(*alloc_inode)(struct super_block *sb);164 void (*free_inode)(struct inode *);165 void (*destroy_inode)(struct inode *);166 void (*dirty_inode) (struct inode *, int flags);167 int (*write_inode) (struct inode *, struct writeback_control *wbc);168 int (*drop_inode) (struct inode *);169 void (*evict_inode) (struct inode *);170 void (*put_super) (struct super_block *);171 int (*sync_fs)(struct super_block *sb, int wait);172 int (*freeze_fs) (struct super_block *);173 int (*unfreeze_fs) (struct super_block *);174 int (*statfs) (struct dentry *, struct kstatfs *);175 int (*remount_fs) (struct super_block *, int *, char *);176 void (*umount_begin) (struct super_block *);177 int (*show_options)(struct seq_file *, struct dentry *);178 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);179 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);180 181locking rules:182 All may block [not true, see below]183 184====================== ============ ========================185ops s_umount note186====================== ============ ========================187alloc_inode:188free_inode: called from RCU callback189destroy_inode:190dirty_inode:191write_inode:192drop_inode: !!!inode->i_lock!!!193evict_inode:194put_super: write195sync_fs: read196freeze_fs: write197unfreeze_fs: write198statfs: maybe(read) (see below)199remount_fs: write200umount_begin: no201show_options: no (namespace_sem)202quota_read: no (see below)203quota_write: no (see below)204====================== ============ ========================205 206->statfs() has s_umount (shared) when called by ustat(2) (native or207compat), but that's an accident of bad API; s_umount is used to pin208the superblock down when we only have dev_t given us by userland to209identify the superblock. Everything else (statfs(), fstatfs(), etc.)210doesn't hold it when calling ->statfs() - superblock is pinned down211by resolving the pathname passed to syscall.212 213->quota_read() and ->quota_write() functions are both guaranteed to214be the only ones operating on the quota file by the quota code (via215dqio_sem) (unless an admin really wants to screw up something and216writes to quota files with quotas on). For other details about locking217see also dquot_operations section.218 219file_system_type220================221 222prototypes::223 224 struct dentry *(*mount) (struct file_system_type *, int,225 const char *, void *);226 void (*kill_sb) (struct super_block *);227 228locking rules:229 230======= =========231ops may block232======= =========233mount yes234kill_sb yes235======= =========236 237->mount() returns ERR_PTR or the root dentry; its superblock should be locked238on return.239 240->kill_sb() takes a write-locked superblock, does all shutdown work on it,241unlocks and drops the reference.242 243address_space_operations244========================245prototypes::246 247 int (*writepage)(struct page *page, struct writeback_control *wbc);248 int (*read_folio)(struct file *, struct folio *);249 int (*writepages)(struct address_space *, struct writeback_control *);250 bool (*dirty_folio)(struct address_space *, struct folio *folio);251 void (*readahead)(struct readahead_control *);252 int (*write_begin)(struct file *, struct address_space *mapping,253 loff_t pos, unsigned len,254 struct folio **foliop, void **fsdata);255 int (*write_end)(struct file *, struct address_space *mapping,256 loff_t pos, unsigned len, unsigned copied,257 struct folio *folio, void *fsdata);258 sector_t (*bmap)(struct address_space *, sector_t);259 void (*invalidate_folio) (struct folio *, size_t start, size_t len);260 bool (*release_folio)(struct folio *, gfp_t);261 void (*free_folio)(struct folio *);262 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);263 int (*migrate_folio)(struct address_space *, struct folio *dst,264 struct folio *src, enum migrate_mode);265 int (*launder_folio)(struct folio *);266 bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count);267 int (*error_remove_folio)(struct address_space *, struct folio *);268 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)269 int (*swap_deactivate)(struct file *);270 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);271 272locking rules:273 All except dirty_folio and free_folio may block274 275====================== ======================== ========= ===============276ops folio locked i_rwsem invalidate_lock277====================== ======================== ========= ===============278writepage: yes, unlocks (see below)279read_folio: yes, unlocks shared280writepages:281dirty_folio: maybe282readahead: yes, unlocks shared283write_begin: locks the folio exclusive284write_end: yes, unlocks exclusive285bmap:286invalidate_folio: yes exclusive287release_folio: yes288free_folio: yes289direct_IO:290migrate_folio: yes (both)291launder_folio: yes292is_partially_uptodate: yes293error_remove_folio: yes294swap_activate: no295swap_deactivate: no296swap_rw: yes, unlocks297====================== ======================== ========= ===============298 299->write_begin(), ->write_end() and ->read_folio() may be called from300the request handler (/dev/loop).301 302->read_folio() unlocks the folio, either synchronously or via I/O303completion.304 305->readahead() unlocks the folios that I/O is attempted on like ->read_folio().306 307->writepage() is used for two purposes: for "memory cleansing" and for308"sync". These are quite different operations and the behaviour may differ309depending upon the mode.310 311If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then312it *must* start I/O against the page, even if that would involve313blocking on in-progress I/O.314 315If writepage is called for memory cleansing (sync_mode ==316WBC_SYNC_NONE) then its role is to get as much writeout underway as317possible. So writepage should try to avoid blocking against318currently-in-progress I/O.319 320If the filesystem is not called for "sync" and it determines that it321would need to block against in-progress I/O to be able to start new I/O322against the page the filesystem should redirty the page with323redirty_page_for_writepage(), then unlock the page and return zero.324This may also be done to avoid internal deadlocks, but rarely.325 326If the filesystem is called for sync then it must wait on any327in-progress I/O and then start new I/O.328 329The filesystem should unlock the page synchronously, before returning to the330caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE331value. WRITEPAGE_ACTIVATE means that page cannot really be written out332currently, and VM should stop calling ->writepage() on this page for some333time. VM does this by moving page to the head of the active list, hence the334name.335 336Unless the filesystem is going to redirty_page_for_writepage(), unlock the page337and return zero, writepage *must* run set_page_writeback() against the page,338followed by unlocking it. Once set_page_writeback() has been run against the339page, write I/O can be submitted and the write I/O completion handler must run340end_page_writeback() once the I/O is complete. If no I/O is submitted, the341filesystem must run end_page_writeback() against the page before returning from342writepage.343 344That is: after 2.5.12, pages which are under writeout are *not* locked. Note,345if the filesystem needs the page to be locked during writeout, that is ok, too,346the page is allowed to be unlocked at any point in time between the calls to347set_page_writeback() and end_page_writeback().348 349Note, failure to run either redirty_page_for_writepage() or the combination of350set_page_writeback()/end_page_writeback() on a page submitted to writepage351will leave the page itself marked clean but it will be tagged as dirty in the352radix tree. This incoherency can lead to all sorts of hard-to-debug problems353in the filesystem like having dirty inodes at umount and losing written data.354 355->writepages() is used for periodic writeback and for syscall-initiated356sync operations. The address_space should start I/O against at least357``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page358which is written. The address_space implementation may write more (or less)359pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.360If nr_to_write is NULL, all dirty pages must be written.361 362writepages should _only_ write pages which are present on363mapping->io_pages.364 365->dirty_folio() is called from various places in the kernel when366the target folio is marked as needing writeback. The folio cannot be367truncated because either the caller holds the folio lock, or the caller368has found the folio while holding the page table lock which will block369truncation.370 371->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some372filesystems and by the swapper. The latter will eventually go away. Please,373keep it that way and don't breed new callers.374 375->invalidate_folio() is called when the filesystem must attempt to drop376some or all of the buffers from the page when it is being truncated. It377returns zero on success. The filesystem must exclusively acquire378invalidate_lock before invalidating page cache in truncate / hole punch379path (and thus calling into ->invalidate_folio) to block races between page380cache invalidation and page cache filling functions (fault, read, ...).381 382->release_folio() is called when the MM wants to make a change to the383folio that would invalidate the filesystem's private data. For example,384it may be about to be removed from the address_space or split. The folio385is locked and not under writeback. It may be dirty. The gfp parameter386is not usually used for allocation, but rather to indicate what the387filesystem may do to attempt to free the private data. The filesystem may388return false to indicate that the folio's private data cannot be freed.389If it returns true, it should have already removed the private data from390the folio. If a filesystem does not provide a ->release_folio method,391the pagecache will assume that private data is buffer_heads and call392try_to_free_buffers().393 394->free_folio() is called when the kernel has dropped the folio395from the page cache.396 397->launder_folio() may be called prior to releasing a folio if398it is still found to be dirty. It returns zero if the folio was successfully399cleaned, or an error value if not. Note that in order to prevent the folio400getting mapped back in and redirtied, it needs to be kept locked401across the entire operation.402 403->swap_activate() will be called to prepare the given file for swap. It404should perform any validation and preparation necessary to ensure that405writes can be performed with minimal memory allocation. It should call406add_swap_extent(), or the helper iomap_swapfile_activate(), and return407the number of extents added. If IO should be submitted through408->swap_rw(), it should set SWP_FS_OPS, otherwise IO will be submitted409directly to the block device ``sis->bdev``.410 411->swap_deactivate() will be called in the sys_swapoff()412path after ->swap_activate() returned success.413 414->swap_rw will be called for swap IO if SWP_FS_OPS was set by ->swap_activate().415 416file_lock_operations417====================418 419prototypes::420 421 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);422 void (*fl_release_private)(struct file_lock *);423 424 425locking rules:426 427=================== ============= =========428ops inode->i_lock may block429=================== ============= =========430fl_copy_lock: yes no431fl_release_private: maybe maybe[1]_432=================== ============= =========433 434.. [1]:435 ->fl_release_private for flock or POSIX locks is currently allowed436 to block. Leases however can still be freed while the i_lock is held and437 so fl_release_private called on a lease should not block.438 439lock_manager_operations440=======================441 442prototypes::443 444 void (*lm_notify)(struct file_lock *); /* unblock callback */445 int (*lm_grant)(struct file_lock *, struct file_lock *, int);446 void (*lm_break)(struct file_lock *); /* break_lease callback */447 int (*lm_change)(struct file_lock **, int);448 bool (*lm_breaker_owns_lease)(struct file_lock *);449 bool (*lm_lock_expirable)(struct file_lock *);450 void (*lm_expire_lock)(void);451 452locking rules:453 454====================== ============= ================= =========455ops flc_lock blocked_lock_lock may block456====================== ============= ================= =========457lm_notify: no yes no458lm_grant: no no no459lm_break: yes no no460lm_change yes no no461lm_breaker_owns_lease: yes no no462lm_lock_expirable yes no no463lm_expire_lock no no yes464====================== ============= ================= =========465 466buffer_head467===========468 469prototypes::470 471 void (*b_end_io)(struct buffer_head *bh, int uptodate);472 473locking rules:474 475called from interrupts. In other words, extreme care is needed here.476bh is locked, but that's all warranties we have here. Currently only RAID1,477highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices478call this method upon the IO completion.479 480block_device_operations481=======================482prototypes::483 484 int (*open) (struct block_device *, fmode_t);485 int (*release) (struct gendisk *, fmode_t);486 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);487 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);488 int (*direct_access) (struct block_device *, sector_t, void **,489 unsigned long *);490 void (*unlock_native_capacity) (struct gendisk *);491 int (*getgeo)(struct block_device *, struct hd_geometry *);492 void (*swap_slot_free_notify) (struct block_device *, unsigned long);493 494locking rules:495 496======================= ===================497ops open_mutex498======================= ===================499open: yes500release: yes501ioctl: no502compat_ioctl: no503direct_access: no504unlock_native_capacity: no505getgeo: no506swap_slot_free_notify: no (see below)507======================= ===================508 509swap_slot_free_notify is called with swap_lock and sometimes the page lock510held.511 512 513file_operations514===============515 516prototypes::517 518 loff_t (*llseek) (struct file *, loff_t, int);519 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);520 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);521 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);522 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);523 int (*iopoll) (struct kiocb *kiocb, bool spin);524 int (*iterate_shared) (struct file *, struct dir_context *);525 __poll_t (*poll) (struct file *, struct poll_table_struct *);526 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);527 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);528 int (*mmap) (struct file *, struct vm_area_struct *);529 int (*open) (struct inode *, struct file *);530 int (*flush) (struct file *);531 int (*release) (struct inode *, struct file *);532 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);533 int (*fasync) (int, struct file *, int);534 int (*lock) (struct file *, int, struct file_lock *);535 unsigned long (*get_unmapped_area)(struct file *, unsigned long,536 unsigned long, unsigned long, unsigned long);537 int (*check_flags)(int);538 int (*flock) (struct file *, int, struct file_lock *);539 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,540 size_t, unsigned int);541 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,542 size_t, unsigned int);543 int (*setlease)(struct file *, long, struct file_lock **, void **);544 long (*fallocate)(struct file *, int, loff_t, loff_t);545 void (*show_fdinfo)(struct seq_file *m, struct file *f);546 unsigned (*mmap_capabilities)(struct file *);547 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,548 loff_t, size_t, unsigned int);549 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,550 struct file *file_out, loff_t pos_out,551 loff_t len, unsigned int remap_flags);552 int (*fadvise)(struct file *, loff_t, loff_t, int);553 554locking rules:555 All may block.556 557->llseek() locking has moved from llseek to the individual llseek558implementations. If your fs is not using generic_file_llseek, you559need to acquire and release the appropriate locks in your ->llseek().560For many filesystems, it is probably safe to acquire the inode561mutex or just to use i_size_read() instead.562Note: this does not protect the file->f_pos against concurrent modifications563since this is something the userspace has to take care about.564 565->iterate_shared() is called with i_rwsem held for reading, and with the566file f_pos_lock held exclusively567 568->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.569Most instances call fasync_helper(), which does that maintenance, so it's570not normally something one needs to worry about. Return values > 0 will be571mapped to zero in the VFS layer.572 573->readdir() and ->ioctl() on directories must be changed. Ideally we would574move ->readdir() to inode_operations and use a separate method for directory575->ioctl() or kill the latter completely. One of the problems is that for576anything that resembles union-mount we won't have a struct file for all577components. And there are other reasons why the current interface is a mess...578 579->read on directories probably must go away - we should just enforce -EISDIR580in sys_read() and friends.581 582->setlease operations should call generic_setlease() before or after setting583the lease within the individual filesystem to record the result of the584operation585 586->fallocate implementation must be really careful to maintain page cache587consistency when punching holes or performing other operations that invalidate588page cache contents. Usually the filesystem needs to call589truncate_inode_pages_range() to invalidate relevant range of the page cache.590However the filesystem usually also needs to update its internal (and on disk)591view of file offset -> disk block mapping. Until this update is finished, the592filesystem needs to block page faults and reads from reloading now-stale page593cache contents from the disk. Since VFS acquires mapping->invalidate_lock in594shared mode when loading pages from disk (filemap_fault(), filemap_read(),595readahead paths), the fallocate implementation must take the invalidate_lock to596prevent reloading.597 598->copy_file_range and ->remap_file_range implementations need to serialize599against modifications of file data while the operation is running. For600blocking changes through write(2) and similar operations inode->i_rwsem can be601used. To block changes to file contents via a memory mapping during the602operation, the filesystem must take mapping->invalidate_lock to coordinate603with ->page_mkwrite.604 605dquot_operations606================607 608prototypes::609 610 int (*write_dquot) (struct dquot *);611 int (*acquire_dquot) (struct dquot *);612 int (*release_dquot) (struct dquot *);613 int (*mark_dirty) (struct dquot *);614 int (*write_info) (struct super_block *, int);615 616These operations are intended to be more or less wrapping functions that ensure617a proper locking wrt the filesystem and call the generic quota operations.618 619What filesystem should expect from the generic quota functions:620 621============== ============ =========================622ops FS recursion Held locks when called623============== ============ =========================624write_dquot: yes dqonoff_sem or dqptr_sem625acquire_dquot: yes dqonoff_sem or dqptr_sem626release_dquot: yes dqonoff_sem or dqptr_sem627mark_dirty: no -628write_info: yes dqonoff_sem629============== ============ =========================630 631FS recursion means calling ->quota_read() and ->quota_write() from superblock632operations.633 634More details about quota locking can be found in fs/dquot.c.635 636vm_operations_struct637====================638 639prototypes::640 641 void (*open)(struct vm_area_struct *);642 void (*close)(struct vm_area_struct *);643 vm_fault_t (*fault)(struct vm_fault *);644 vm_fault_t (*huge_fault)(struct vm_fault *, unsigned int order);645 vm_fault_t (*map_pages)(struct vm_fault *, pgoff_t start, pgoff_t end);646 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);647 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);648 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);649 650locking rules:651 652============= ========== ===========================653ops mmap_lock PageLocked(page)654============= ========== ===========================655open: write656close: read/write657fault: read can return with page locked658huge_fault: maybe-read659map_pages: maybe-read660page_mkwrite: read can return with page locked661pfn_mkwrite: read662access: read663============= ========== ===========================664 665->fault() is called when a previously not present pte is about to be faulted666in. The filesystem must find and return the page associated with the passed in667"pgoff" in the vm_fault structure. If it is possible that the page may be668truncated and/or invalidated, then the filesystem must lock invalidate_lock,669then ensure the page is not already truncated (invalidate_lock will block670subsequent truncate), and then return with VM_FAULT_LOCKED, and the page671locked. The VM will unlock the page.672 673->huge_fault() is called when there is no PUD or PMD entry present. This674gives the filesystem the opportunity to install a PUD or PMD sized page.675Filesystems can also use the ->fault method to return a PMD sized page,676so implementing this function may not be necessary. In particular,677filesystems should not call filemap_fault() from ->huge_fault().678The mmap_lock may not be held when this method is called.679 680->map_pages() is called when VM asks to map easy accessible pages.681Filesystem should find and map pages associated with offsets from "start_pgoff"682till "end_pgoff". ->map_pages() is called with the RCU lock held and must683not block. If it's not possible to reach a page without blocking,684filesystem should skip it. Filesystem should use set_pte_range() to setup685page table entry. Pointer to entry associated with the page is passed in686"pte" field in vm_fault structure. Pointers to entries for other offsets687should be calculated relative to "pte".688 689->page_mkwrite() is called when a previously read-only pte is about to become690writeable. The filesystem again must ensure that there are no691truncate/invalidate races or races with operations such as ->remap_file_range692or ->copy_file_range, and then return with the page locked. Usually693mapping->invalidate_lock is suitable for proper serialization. If the page has694been truncated, the filesystem should not look up a new page like the ->fault()695handler, but simply return with VM_FAULT_NOPAGE, which will cause the VM to696retry the fault.697 698->pfn_mkwrite() is the same as page_mkwrite but when the pte is699VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is700VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior701after this call is to make the pte read-write, unless pfn_mkwrite returns702an error.703 704->access() is called when get_user_pages() fails in705access_process_vm(), typically used to debug a process through706/proc/pid/mem or ptrace. This function is needed only for707VM_IO | VM_PFNMAP VMAs.708 709--------------------------------------------------------------------------------710 711 Dubious stuff712 713(if you break something or notice that it is broken and do not fix it yourself714- at least put it here)715