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