1 ======= 1 =======
2 Locking 2 Locking
3 ======= 3 =======
4 4
5 The text below describes the locking rules for 5 The text below describes the locking rules for VFS-related methods.
6 It is (believed to be) up-to-date. *Please*, i 6 It is (believed to be) up-to-date. *Please*, if you change anything in
7 prototypes or locking protocols - update this 7 prototypes or locking protocols - update this file. And update the relevant
8 instances in the tree, don't leave that to mai 8 instances in the tree, don't leave that to maintainers of filesystems/devices/
9 etc. At the very least, put the list of dubiou 9 etc. At the very least, put the list of dubious cases in the end of this file.
10 Don't turn it into log - maintainers of out-of 10 Don't turn it into log - maintainers of out-of-the-tree code are supposed to
11 be able to use diff(1). 11 be able to use diff(1).
12 12
13 Thing currently missing here: socket operation 13 Thing currently missing here: socket operations. Alexey?
14 14
15 dentry_operations 15 dentry_operations
16 ================= 16 =================
17 17
18 prototypes:: 18 prototypes::
19 19
20 int (*d_revalidate)(struct dentry *, u 20 int (*d_revalidate)(struct dentry *, unsigned int);
21 int (*d_weak_revalidate)(struct dentry 21 int (*d_weak_revalidate)(struct dentry *, unsigned int);
22 int (*d_hash)(const struct dentry *, s 22 int (*d_hash)(const struct dentry *, struct qstr *);
23 int (*d_compare)(const struct dentry * 23 int (*d_compare)(const struct dentry *,
24 unsigned int, const ch 24 unsigned int, const char *, const struct qstr *);
25 int (*d_delete)(struct dentry *); 25 int (*d_delete)(struct dentry *);
26 int (*d_init)(struct dentry *); 26 int (*d_init)(struct dentry *);
27 void (*d_release)(struct dentry *); 27 void (*d_release)(struct dentry *);
28 void (*d_iput)(struct dentry *, struct 28 void (*d_iput)(struct dentry *, struct inode *);
29 char *(*d_dname)((struct dentry *dentr 29 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
30 struct vfsmount *(*d_automount)(struct 30 struct vfsmount *(*d_automount)(struct path *path);
31 int (*d_manage)(const struct path *, b 31 int (*d_manage)(const struct path *, bool);
32 struct dentry *(*d_real)(struct dentry !! 32 struct dentry *(*d_real)(struct dentry *, const struct inode *);
33 33
34 locking rules: 34 locking rules:
35 35
36 ================== =========== ======== 36 ================== =========== ======== ============== ========
37 ops rename_lock ->d_lock 37 ops rename_lock ->d_lock may block rcu-walk
38 ================== =========== ======== 38 ================== =========== ======== ============== ========
39 d_revalidate: no no 39 d_revalidate: no no yes (ref-walk) maybe
40 d_weak_revalidate: no no 40 d_weak_revalidate: no no yes no
41 d_hash no no 41 d_hash no no no maybe
42 d_compare: yes no 42 d_compare: yes no no maybe
43 d_delete: no yes 43 d_delete: no yes no no
44 d_init: no no 44 d_init: no no yes no
45 d_release: no no 45 d_release: no no yes no
46 d_prune: no yes 46 d_prune: no yes no no
47 d_iput: no no 47 d_iput: no no yes no
48 d_dname: no no 48 d_dname: no no no no
49 d_automount: no no 49 d_automount: no no yes no
50 d_manage: no no 50 d_manage: no no yes (ref-walk) maybe
51 d_real no no 51 d_real no no yes no
52 ================== =========== ======== 52 ================== =========== ======== ============== ========
53 53
54 inode_operations 54 inode_operations
55 ================ 55 ================
56 56
57 prototypes:: 57 prototypes::
58 58
59 int (*create) (struct mnt_idmap *, str !! 59 int (*create) (struct inode *,struct dentry *,umode_t, bool);
60 struct dentry * (*lookup) (struct inod 60 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
61 int (*link) (struct dentry *,struct in 61 int (*link) (struct dentry *,struct inode *,struct dentry *);
62 int (*unlink) (struct inode *,struct d 62 int (*unlink) (struct inode *,struct dentry *);
63 int (*symlink) (struct mnt_idmap *, st !! 63 int (*symlink) (struct inode *,struct dentry *,const char *);
64 int (*mkdir) (struct mnt_idmap *, stru !! 64 int (*mkdir) (struct inode *,struct dentry *,umode_t);
65 int (*rmdir) (struct inode *,struct de 65 int (*rmdir) (struct inode *,struct dentry *);
66 int (*mknod) (struct mnt_idmap *, stru !! 66 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
67 int (*rename) (struct mnt_idmap *, str !! 67 int (*rename) (struct inode *, struct dentry *,
68 struct inode *, struct 68 struct inode *, struct dentry *, unsigned int);
69 int (*readlink) (struct dentry *, char 69 int (*readlink) (struct dentry *, char __user *,int);
70 const char *(*get_link) (struct dentry 70 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
71 void (*truncate) (struct inode *); 71 void (*truncate) (struct inode *);
72 int (*permission) (struct mnt_idmap *, !! 72 int (*permission) (struct inode *, int, unsigned int);
73 struct posix_acl * (*get_inode_acl)(st !! 73 int (*get_acl)(struct inode *, int);
74 int (*setattr) (struct mnt_idmap *, st !! 74 int (*setattr) (struct dentry *, struct iattr *);
75 int (*getattr) (struct mnt_idmap *, co !! 75 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
76 ssize_t (*listxattr) (struct dentry *, 76 ssize_t (*listxattr) (struct dentry *, char *, size_t);
77 int (*fiemap)(struct inode *, struct f 77 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
78 void (*update_time)(struct inode *, st 78 void (*update_time)(struct inode *, struct timespec *, int);
79 int (*atomic_open)(struct inode *, str 79 int (*atomic_open)(struct inode *, struct dentry *,
80 struct file *, 80 struct file *, unsigned open_flag,
81 umode_t create 81 umode_t create_mode);
82 int (*tmpfile) (struct mnt_idmap *, st !! 82 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
83 struct file *, umode_t <<
84 int (*fileattr_set)(struct mnt_idmap * <<
85 struct dentry *den <<
86 int (*fileattr_get)(struct dentry *den <<
87 struct posix_acl * (*get_acl)(struct m <<
88 struct offset_ctx *(*get_offset_ctx)(s <<
89 83
90 locking rules: 84 locking rules:
91 all may block 85 all may block
92 86
93 ============== ============================== !! 87 ============ =============================================
94 ops i_rwsem(inode) 88 ops i_rwsem(inode)
95 ============== ============================== !! 89 ============ =============================================
96 lookup: shared 90 lookup: shared
97 create: exclusive 91 create: exclusive
98 link: exclusive (both) 92 link: exclusive (both)
99 mknod: exclusive 93 mknod: exclusive
100 symlink: exclusive 94 symlink: exclusive
101 mkdir: exclusive 95 mkdir: exclusive
102 unlink: exclusive (both) 96 unlink: exclusive (both)
103 rmdir: exclusive (both)(see below) 97 rmdir: exclusive (both)(see below)
104 rename: exclusive (both parents, some !! 98 rename: exclusive (all) (see below)
105 readlink: no 99 readlink: no
106 get_link: no 100 get_link: no
107 setattr: exclusive 101 setattr: exclusive
108 permission: no (may not block if called in 102 permission: no (may not block if called in rcu-walk mode)
109 get_inode_acl: no <<
110 get_acl: no 103 get_acl: no
111 getattr: no 104 getattr: no
112 listxattr: no 105 listxattr: no
113 fiemap: no 106 fiemap: no
114 update_time: no 107 update_time: no
115 atomic_open: shared (exclusive if O_CREAT i 108 atomic_open: shared (exclusive if O_CREAT is set in open flags)
116 tmpfile: no 109 tmpfile: no
117 fileattr_get: no or exclusive !! 110 ============ =============================================
118 fileattr_set: exclusive <<
119 get_offset_ctx no <<
120 ============== ============================== <<
121 111
122 112
123 Additionally, ->rmdir(), ->unlink() an 113 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
124 exclusive on victim. 114 exclusive on victim.
125 cross-directory ->rename() has (per-su 115 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
126 ->unlink() and ->rename() have ->i_rws <<
127 involved. <<
128 ->rename() has ->i_rwsem exclusive on <<
129 116
130 See Documentation/filesystems/directory-lockin 117 See Documentation/filesystems/directory-locking.rst for more detailed discussion
131 of the locking scheme for directory operations 118 of the locking scheme for directory operations.
132 119
133 xattr_handler operations 120 xattr_handler operations
134 ======================== 121 ========================
135 122
136 prototypes:: 123 prototypes::
137 124
138 bool (*list)(struct dentry *dentry); 125 bool (*list)(struct dentry *dentry);
139 int (*get)(const struct xattr_handler 126 int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
140 struct inode *inode, const 127 struct inode *inode, const char *name, void *buffer,
141 size_t size); 128 size_t size);
142 int (*set)(const struct xattr_handler !! 129 int (*set)(const struct xattr_handler *handler, struct dentry *dentry,
143 struct mnt_idmap *idmap, !! 130 struct inode *inode, const char *name, const void *buffer,
144 struct dentry *dentry, stru !! 131 size_t size, int flags);
145 const void *buffer, size_t <<
146 132
147 locking rules: 133 locking rules:
148 all may block 134 all may block
149 135
150 ===== ============== 136 ===== ==============
151 ops i_rwsem(inode) 137 ops i_rwsem(inode)
152 ===== ============== 138 ===== ==============
153 list: no 139 list: no
154 get: no 140 get: no
155 set: exclusive 141 set: exclusive
156 ===== ============== 142 ===== ==============
157 143
158 super_operations 144 super_operations
159 ================ 145 ================
160 146
161 prototypes:: 147 prototypes::
162 148
163 struct inode *(*alloc_inode)(struct su 149 struct inode *(*alloc_inode)(struct super_block *sb);
164 void (*free_inode)(struct inode *); 150 void (*free_inode)(struct inode *);
165 void (*destroy_inode)(struct inode *); 151 void (*destroy_inode)(struct inode *);
166 void (*dirty_inode) (struct inode *, i 152 void (*dirty_inode) (struct inode *, int flags);
167 int (*write_inode) (struct inode *, st 153 int (*write_inode) (struct inode *, struct writeback_control *wbc);
168 int (*drop_inode) (struct inode *); 154 int (*drop_inode) (struct inode *);
169 void (*evict_inode) (struct inode *); 155 void (*evict_inode) (struct inode *);
170 void (*put_super) (struct super_block 156 void (*put_super) (struct super_block *);
171 int (*sync_fs)(struct super_block *sb, 157 int (*sync_fs)(struct super_block *sb, int wait);
172 int (*freeze_fs) (struct super_block * 158 int (*freeze_fs) (struct super_block *);
173 int (*unfreeze_fs) (struct super_block 159 int (*unfreeze_fs) (struct super_block *);
174 int (*statfs) (struct dentry *, struct 160 int (*statfs) (struct dentry *, struct kstatfs *);
175 int (*remount_fs) (struct super_block 161 int (*remount_fs) (struct super_block *, int *, char *);
176 void (*umount_begin) (struct super_blo 162 void (*umount_begin) (struct super_block *);
177 int (*show_options)(struct seq_file *, 163 int (*show_options)(struct seq_file *, struct dentry *);
178 ssize_t (*quota_read)(struct super_blo 164 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
179 ssize_t (*quota_write)(struct super_bl 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);
180 167
181 locking rules: 168 locking rules:
182 All may block [not true, see below] 169 All may block [not true, see below]
183 170
184 ====================== ============ ====== 171 ====================== ============ ========================
185 ops s_umount note 172 ops s_umount note
186 ====================== ============ ====== 173 ====================== ============ ========================
187 alloc_inode: 174 alloc_inode:
188 free_inode: called 175 free_inode: called from RCU callback
189 destroy_inode: 176 destroy_inode:
190 dirty_inode: 177 dirty_inode:
191 write_inode: 178 write_inode:
192 drop_inode: !!!ino 179 drop_inode: !!!inode->i_lock!!!
193 evict_inode: 180 evict_inode:
194 put_super: write 181 put_super: write
195 sync_fs: read 182 sync_fs: read
196 freeze_fs: write 183 freeze_fs: write
197 unfreeze_fs: write 184 unfreeze_fs: write
198 statfs: maybe(read) (see b 185 statfs: maybe(read) (see below)
199 remount_fs: write 186 remount_fs: write
200 umount_begin: no 187 umount_begin: no
201 show_options: no (names 188 show_options: no (namespace_sem)
202 quota_read: no (see b 189 quota_read: no (see below)
203 quota_write: no (see b 190 quota_write: no (see below)
>> 191 bdev_try_to_free_page: no (see below)
204 ====================== ============ ====== 192 ====================== ============ ========================
205 193
206 ->statfs() has s_umount (shared) when called b 194 ->statfs() has s_umount (shared) when called by ustat(2) (native or
207 compat), but that's an accident of bad API; s_ 195 compat), but that's an accident of bad API; s_umount is used to pin
208 the superblock down when we only have dev_t gi 196 the superblock down when we only have dev_t given us by userland to
209 identify the superblock. Everything else (sta 197 identify the superblock. Everything else (statfs(), fstatfs(), etc.)
210 doesn't hold it when calling ->statfs() - supe 198 doesn't hold it when calling ->statfs() - superblock is pinned down
211 by resolving the pathname passed to syscall. 199 by resolving the pathname passed to syscall.
212 200
213 ->quota_read() and ->quota_write() functions a 201 ->quota_read() and ->quota_write() functions are both guaranteed to
214 be the only ones operating on the quota file b 202 be the only ones operating on the quota file by the quota code (via
215 dqio_sem) (unless an admin really wants to scr 203 dqio_sem) (unless an admin really wants to screw up something and
216 writes to quota files with quotas on). For oth 204 writes to quota files with quotas on). For other details about locking
217 see also dquot_operations section. 205 see also dquot_operations section.
218 206
>> 207 ->bdev_try_to_free_page is called from the ->releasepage handler of
>> 208 the block device inode. See there for more details.
>> 209
219 file_system_type 210 file_system_type
220 ================ 211 ================
221 212
222 prototypes:: 213 prototypes::
223 214
224 struct dentry *(*mount) (struct file_s 215 struct dentry *(*mount) (struct file_system_type *, int,
225 const char *, void *); 216 const char *, void *);
226 void (*kill_sb) (struct super_block *) 217 void (*kill_sb) (struct super_block *);
227 218
228 locking rules: 219 locking rules:
229 220
230 ======= ========= 221 ======= =========
231 ops may block 222 ops may block
232 ======= ========= 223 ======= =========
233 mount yes 224 mount yes
234 kill_sb yes 225 kill_sb yes
235 ======= ========= 226 ======= =========
236 227
237 ->mount() returns ERR_PTR or the root dentry; 228 ->mount() returns ERR_PTR or the root dentry; its superblock should be locked
238 on return. 229 on return.
239 230
240 ->kill_sb() takes a write-locked superblock, d 231 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
241 unlocks and drops the reference. 232 unlocks and drops the reference.
242 233
243 address_space_operations 234 address_space_operations
244 ======================== 235 ========================
245 prototypes:: 236 prototypes::
246 237
247 int (*writepage)(struct page *page, st 238 int (*writepage)(struct page *page, struct writeback_control *wbc);
248 int (*read_folio)(struct file *, struc !! 239 int (*readpage)(struct file *, struct page *);
249 int (*writepages)(struct address_space 240 int (*writepages)(struct address_space *, struct writeback_control *);
250 bool (*dirty_folio)(struct address_spa !! 241 int (*set_page_dirty)(struct page *page);
251 void (*readahead)(struct readahead_con 242 void (*readahead)(struct readahead_control *);
>> 243 int (*readpages)(struct file *filp, struct address_space *mapping,
>> 244 struct list_head *pages, unsigned nr_pages);
252 int (*write_begin)(struct file *, stru 245 int (*write_begin)(struct file *, struct address_space *mapping,
253 loff_t pos, un !! 246 loff_t pos, unsigned len, unsigned flags,
254 struct folio * !! 247 struct page **pagep, void **fsdata);
255 int (*write_end)(struct file *, struct 248 int (*write_end)(struct file *, struct address_space *mapping,
256 loff_t pos, un 249 loff_t pos, unsigned len, unsigned copied,
257 struct folio * !! 250 struct page *page, void *fsdata);
258 sector_t (*bmap)(struct address_space 251 sector_t (*bmap)(struct address_space *, sector_t);
259 void (*invalidate_folio) (struct folio !! 252 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
260 bool (*release_folio)(struct folio *, !! 253 int (*releasepage) (struct page *, int);
261 void (*free_folio)(struct folio *); !! 254 void (*freepage)(struct page *);
262 int (*direct_IO)(struct kiocb *, struc 255 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
263 int (*migrate_folio)(struct address_sp !! 256 bool (*isolate_page) (struct page *, isolate_mode_t);
264 struct folio *src, enu !! 257 int (*migratepage)(struct address_space *, struct page *, struct page *);
265 int (*launder_folio)(struct folio *); !! 258 void (*putback_page) (struct page *);
266 bool (*is_partially_uptodate)(struct f !! 259 int (*launder_page)(struct page *);
267 int (*error_remove_folio)(struct addre !! 260 int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
268 int (*swap_activate)(struct swap_info_ !! 261 int (*error_remove_page)(struct address_space *, struct page *);
>> 262 int (*swap_activate)(struct file *);
269 int (*swap_deactivate)(struct file *); 263 int (*swap_deactivate)(struct file *);
270 int (*swap_rw)(struct kiocb *iocb, str <<
271 264
272 locking rules: 265 locking rules:
273 All except dirty_folio and free_folio !! 266 All except set_page_dirty and freepage may block
274 267
275 ====================== ====================== !! 268 ====================== ======================== =========
276 ops folio locked !! 269 ops PageLocked(page) i_rwsem
277 ====================== ====================== !! 270 ====================== ======================== =========
278 writepage: yes, unlocks (see belo 271 writepage: yes, unlocks (see below)
279 read_folio: yes, unlocks !! 272 readpage: yes, unlocks
280 writepages: 273 writepages:
281 dirty_folio: maybe !! 274 set_page_dirty no
282 readahead: yes, unlocks !! 275 readahead: yes, unlocks
283 write_begin: locks the folio !! 276 readpages: no
>> 277 write_begin: locks the page exclusive
284 write_end: yes, unlocks 278 write_end: yes, unlocks exclusive
285 bmap: 279 bmap:
286 invalidate_folio: yes !! 280 invalidatepage: yes
287 release_folio: yes !! 281 releasepage: yes
288 free_folio: yes !! 282 freepage: yes
289 direct_IO: 283 direct_IO:
290 migrate_folio: yes (both) !! 284 isolate_page: yes
291 launder_folio: yes !! 285 migratepage: yes (both)
>> 286 putback_page: yes
>> 287 launder_page: yes
292 is_partially_uptodate: yes 288 is_partially_uptodate: yes
293 error_remove_folio: yes !! 289 error_remove_page: yes
294 swap_activate: no 290 swap_activate: no
295 swap_deactivate: no 291 swap_deactivate: no
296 swap_rw: yes, unlocks !! 292 ====================== ======================== =========
297 ====================== ====================== <<
298 293
299 ->write_begin(), ->write_end() and ->read_foli !! 294 ->write_begin(), ->write_end() and ->readpage() may be called from
300 the request handler (/dev/loop). 295 the request handler (/dev/loop).
301 296
302 ->read_folio() unlocks the folio, either synch !! 297 ->readpage() unlocks the page, either synchronously or via I/O
303 completion. 298 completion.
304 299
305 ->readahead() unlocks the folios that I/O is a !! 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
>> 303 I/O against them. They come unlocked upon I/O completion.
306 304
307 ->writepage() is used for two purposes: for "m 305 ->writepage() is used for two purposes: for "memory cleansing" and for
308 "sync". These are quite different operations 306 "sync". These are quite different operations and the behaviour may differ
309 depending upon the mode. 307 depending upon the mode.
310 308
311 If writepage is called for sync (wbc->sync_mod 309 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
312 it *must* start I/O against the page, even if 310 it *must* start I/O against the page, even if that would involve
313 blocking on in-progress I/O. 311 blocking on in-progress I/O.
314 312
315 If writepage is called for memory cleansing (s 313 If writepage is called for memory cleansing (sync_mode ==
316 WBC_SYNC_NONE) then its role is to get as much 314 WBC_SYNC_NONE) then its role is to get as much writeout underway as
317 possible. So writepage should try to avoid bl 315 possible. So writepage should try to avoid blocking against
318 currently-in-progress I/O. 316 currently-in-progress I/O.
319 317
320 If the filesystem is not called for "sync" and 318 If the filesystem is not called for "sync" and it determines that it
321 would need to block against in-progress I/O to 319 would need to block against in-progress I/O to be able to start new I/O
322 against the page the filesystem should redirty 320 against the page the filesystem should redirty the page with
323 redirty_page_for_writepage(), then unlock the 321 redirty_page_for_writepage(), then unlock the page and return zero.
324 This may also be done to avoid internal deadlo 322 This may also be done to avoid internal deadlocks, but rarely.
325 323
326 If the filesystem is called for sync then it m 324 If the filesystem is called for sync then it must wait on any
327 in-progress I/O and then start new I/O. 325 in-progress I/O and then start new I/O.
328 326
329 The filesystem should unlock the page synchron 327 The filesystem should unlock the page synchronously, before returning to the
330 caller, unless ->writepage() returns special W 328 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
331 value. WRITEPAGE_ACTIVATE means that page cann 329 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
332 currently, and VM should stop calling ->writep 330 currently, and VM should stop calling ->writepage() on this page for some
333 time. VM does this by moving page to the head 331 time. VM does this by moving page to the head of the active list, hence the
334 name. 332 name.
335 333
336 Unless the filesystem is going to redirty_page 334 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
337 and return zero, writepage *must* run set_page 335 and return zero, writepage *must* run set_page_writeback() against the page,
338 followed by unlocking it. Once set_page_write 336 followed by unlocking it. Once set_page_writeback() has been run against the
339 page, write I/O can be submitted and the write 337 page, write I/O can be submitted and the write I/O completion handler must run
340 end_page_writeback() once the I/O is complete. 338 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
341 filesystem must run end_page_writeback() again 339 filesystem must run end_page_writeback() against the page before returning from
342 writepage. 340 writepage.
343 341
344 That is: after 2.5.12, pages which are under w 342 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
345 if the filesystem needs the page to be locked 343 if the filesystem needs the page to be locked during writeout, that is ok, too,
346 the page is allowed to be unlocked at any poin 344 the page is allowed to be unlocked at any point in time between the calls to
347 set_page_writeback() and end_page_writeback(). 345 set_page_writeback() and end_page_writeback().
348 346
349 Note, failure to run either redirty_page_for_w 347 Note, failure to run either redirty_page_for_writepage() or the combination of
350 set_page_writeback()/end_page_writeback() on a 348 set_page_writeback()/end_page_writeback() on a page submitted to writepage
351 will leave the page itself marked clean but it 349 will leave the page itself marked clean but it will be tagged as dirty in the
352 radix tree. This incoherency can lead to all 350 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
353 in the filesystem like having dirty inodes at 351 in the filesystem like having dirty inodes at umount and losing written data.
354 352
355 ->writepages() is used for periodic writeback 353 ->writepages() is used for periodic writeback and for syscall-initiated
356 sync operations. The address_space should sta 354 sync operations. The address_space should start I/O against at least
357 ``*nr_to_write`` pages. ``*nr_to_write`` must 355 ``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page
358 which is written. The address_space implement 356 which is written. The address_space implementation may write more (or less)
359 pages than ``*nr_to_write`` asks for, but it s 357 pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
360 If nr_to_write is NULL, all dirty pages must b 358 If nr_to_write is NULL, all dirty pages must be written.
361 359
362 writepages should _only_ write pages which are 360 writepages should _only_ write pages which are present on
363 mapping->io_pages. 361 mapping->io_pages.
364 362
365 ->dirty_folio() is called from various places !! 363 ->set_page_dirty() is called from various places in the kernel
366 the target folio is marked as needing writebac !! 364 when the target page is marked as needing writeback. It may be called
367 truncated because either the caller holds the !! 365 under spinlock (it cannot block) and is sometimes called with the page
368 has found the folio while holding the page tab !! 366 not locked.
369 truncation. <<
370 367
371 ->bmap() is currently used by legacy ioctl() ( 368 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
372 filesystems and by the swapper. The latter wil 369 filesystems and by the swapper. The latter will eventually go away. Please,
373 keep it that way and don't breed new callers. 370 keep it that way and don't breed new callers.
374 371
375 ->invalidate_folio() is called when the filesy !! 372 ->invalidatepage() is called when the filesystem must attempt to drop
376 some or all of the buffers from the page when 373 some or all of the buffers from the page when it is being truncated. It
377 returns zero on success. The filesystem must !! 374 returns zero on success. If ->invalidatepage is zero, the kernel uses
378 invalidate_lock before invalidating page cache !! 375 block_invalidatepage() instead.
379 path (and thus calling into ->invalidate_folio <<
380 cache invalidation and page cache filling func <<
381 <<
382 ->release_folio() is called when the MM wants <<
383 folio that would invalidate the filesystem's p <<
384 it may be about to be removed from the address <<
385 is locked and not under writeback. It may be <<
386 is not usually used for allocation, but rather <<
387 filesystem may do to attempt to free the priva <<
388 return false to indicate that the folio's priv <<
389 If it returns true, it should have already rem <<
390 the folio. If a filesystem does not provide a <<
391 the pagecache will assume that private data is <<
392 try_to_free_buffers(). <<
393 376
394 ->free_folio() is called when the kernel has d !! 377 ->releasepage() is called when the kernel is about to try to drop the
>> 378 buffers from the page in preparation for freeing it. It returns zero to
>> 379 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
>> 380 the 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
395 from the page cache. 383 from the page cache.
396 384
397 ->launder_folio() may be called prior to relea !! 385 ->launder_page() may be called prior to releasing a page if
398 it is still found to be dirty. It returns zero !! 386 it is still found to be dirty. It returns zero if the page was successfully
399 cleaned, or an error value if not. Note that i !! 387 cleaned, or an error value if not. Note that in order to prevent the page
400 getting mapped back in and redirtied, it needs 388 getting mapped back in and redirtied, it needs to be kept locked
401 across the entire operation. 389 across the entire operation.
402 390
403 ->swap_activate() will be called to prepare th !! 391 ->swap_activate will be called with a non-zero argument on
404 should perform any validation and preparation !! 392 files backing (non block device backed) swapfiles. A return value
405 writes can be performed with minimal memory al !! 393 of zero indicates success, in which case this file can be used for
406 add_swap_extent(), or the helper iomap_swapfil !! 394 backing swapspace. The swapspace operations will be proxied to the
407 the number of extents added. If IO should be !! 395 address space operations.
408 ->swap_rw(), it should set SWP_FS_OPS, otherwi <<
409 directly to the block device ``sis->bdev``. <<
410 396
411 ->swap_deactivate() will be called in the sys_ 397 ->swap_deactivate() will be called in the sys_swapoff()
412 path after ->swap_activate() returned success. 398 path after ->swap_activate() returned success.
413 399
414 ->swap_rw will be called for swap IO if SWP_FS <<
415 <<
416 file_lock_operations 400 file_lock_operations
417 ==================== 401 ====================
418 402
419 prototypes:: 403 prototypes::
420 404
421 void (*fl_copy_lock)(struct file_lock 405 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
422 void (*fl_release_private)(struct file 406 void (*fl_release_private)(struct file_lock *);
423 407
424 408
425 locking rules: 409 locking rules:
426 410
427 =================== ============= ====== 411 =================== ============= =========
428 ops inode->i_lock may bl 412 ops inode->i_lock may block
429 =================== ============= ====== 413 =================== ============= =========
430 fl_copy_lock: yes no 414 fl_copy_lock: yes no
431 fl_release_private: maybe maybe[ 415 fl_release_private: maybe maybe[1]_
432 =================== ============= ====== 416 =================== ============= =========
433 417
434 .. [1]: 418 .. [1]:
435 ->fl_release_private for flock or POSIX loc 419 ->fl_release_private for flock or POSIX locks is currently allowed
436 to block. Leases however can still be freed 420 to block. Leases however can still be freed while the i_lock is held and
437 so fl_release_private called on a lease sho 421 so fl_release_private called on a lease should not block.
438 422
439 lock_manager_operations 423 lock_manager_operations
440 ======================= 424 =======================
441 425
442 prototypes:: 426 prototypes::
443 427
444 void (*lm_notify)(struct file_lock *); 428 void (*lm_notify)(struct file_lock *); /* unblock callback */
445 int (*lm_grant)(struct file_lock *, st 429 int (*lm_grant)(struct file_lock *, struct file_lock *, int);
446 void (*lm_break)(struct file_lock *); 430 void (*lm_break)(struct file_lock *); /* break_lease callback */
447 int (*lm_change)(struct file_lock **, 431 int (*lm_change)(struct file_lock **, int);
448 bool (*lm_breaker_owns_lease)(struct f 432 bool (*lm_breaker_owns_lease)(struct file_lock *);
449 bool (*lm_lock_expirable)(struct file_ <<
450 void (*lm_expire_lock)(void); <<
451 433
452 locking rules: 434 locking rules:
453 435
454 ====================== ============= ====== 436 ====================== ============= ================= =========
455 ops flc_lock blocke !! 437 ops inode->i_lock blocked_lock_lock may block
456 ====================== ============= ====== 438 ====================== ============= ================= =========
457 lm_notify: no yes !! 439 lm_notify: yes yes no
458 lm_grant: no no 440 lm_grant: no no no
459 lm_break: yes no 441 lm_break: yes no no
460 lm_change yes no 442 lm_change yes no no
461 lm_breaker_owns_lease: yes no !! 443 lm_breaker_owns_lease: no no no
462 lm_lock_expirable yes no <<
463 lm_expire_lock no no <<
464 ====================== ============= ====== 444 ====================== ============= ================= =========
465 445
466 buffer_head 446 buffer_head
467 =========== 447 ===========
468 448
469 prototypes:: 449 prototypes::
470 450
471 void (*b_end_io)(struct buffer_head *b 451 void (*b_end_io)(struct buffer_head *bh, int uptodate);
472 452
473 locking rules: 453 locking rules:
474 454
475 called from interrupts. In other words, extrem 455 called from interrupts. In other words, extreme care is needed here.
476 bh is locked, but that's all warranties we hav 456 bh is locked, but that's all warranties we have here. Currently only RAID1,
477 highmem, fs/buffer.c, and fs/ntfs/aops.c are p 457 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
478 call this method upon the IO completion. 458 call this method upon the IO completion.
479 459
480 block_device_operations 460 block_device_operations
481 ======================= 461 =======================
482 prototypes:: 462 prototypes::
483 463
484 int (*open) (struct block_device *, fm 464 int (*open) (struct block_device *, fmode_t);
485 int (*release) (struct gendisk *, fmod 465 int (*release) (struct gendisk *, fmode_t);
486 int (*ioctl) (struct block_device *, f 466 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
487 int (*compat_ioctl) (struct block_devi 467 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
488 int (*direct_access) (struct block_dev 468 int (*direct_access) (struct block_device *, sector_t, void **,
489 unsigned long 469 unsigned long *);
490 void (*unlock_native_capacity) (struct 470 void (*unlock_native_capacity) (struct gendisk *);
>> 471 int (*revalidate_disk) (struct gendisk *);
491 int (*getgeo)(struct block_device *, s 472 int (*getgeo)(struct block_device *, struct hd_geometry *);
492 void (*swap_slot_free_notify) (struct 473 void (*swap_slot_free_notify) (struct block_device *, unsigned long);
493 474
494 locking rules: 475 locking rules:
495 476
496 ======================= =================== 477 ======================= ===================
497 ops open_mutex !! 478 ops bd_mutex
498 ======================= =================== 479 ======================= ===================
499 open: yes 480 open: yes
500 release: yes 481 release: yes
501 ioctl: no 482 ioctl: no
502 compat_ioctl: no 483 compat_ioctl: no
503 direct_access: no 484 direct_access: no
504 unlock_native_capacity: no 485 unlock_native_capacity: no
>> 486 revalidate_disk: no
505 getgeo: no 487 getgeo: no
506 swap_slot_free_notify: no (see below) 488 swap_slot_free_notify: no (see below)
507 ======================= =================== 489 ======================= ===================
508 490
509 swap_slot_free_notify is called with swap_lock 491 swap_slot_free_notify is called with swap_lock and sometimes the page lock
510 held. 492 held.
511 493
512 494
513 file_operations 495 file_operations
514 =============== 496 ===============
515 497
516 prototypes:: 498 prototypes::
517 499
518 loff_t (*llseek) (struct file *, loff_ 500 loff_t (*llseek) (struct file *, loff_t, int);
519 ssize_t (*read) (struct file *, char _ 501 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
520 ssize_t (*write) (struct file *, const 502 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
521 ssize_t (*read_iter) (struct kiocb *, 503 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
522 ssize_t (*write_iter) (struct kiocb *, 504 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
523 int (*iopoll) (struct kiocb *kiocb, bo !! 505 int (*iterate) (struct file *, struct dir_context *);
524 int (*iterate_shared) (struct file *, 506 int (*iterate_shared) (struct file *, struct dir_context *);
525 __poll_t (*poll) (struct file *, struc 507 __poll_t (*poll) (struct file *, struct poll_table_struct *);
526 long (*unlocked_ioctl) (struct file *, 508 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
527 long (*compat_ioctl) (struct file *, u 509 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
528 int (*mmap) (struct file *, struct vm_ 510 int (*mmap) (struct file *, struct vm_area_struct *);
529 int (*open) (struct inode *, struct fi 511 int (*open) (struct inode *, struct file *);
530 int (*flush) (struct file *); 512 int (*flush) (struct file *);
531 int (*release) (struct inode *, struct 513 int (*release) (struct inode *, struct file *);
532 int (*fsync) (struct file *, loff_t st 514 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
533 int (*fasync) (int, struct file *, int 515 int (*fasync) (int, struct file *, int);
534 int (*lock) (struct file *, int, struc 516 int (*lock) (struct file *, int, struct file_lock *);
>> 517 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
>> 518 loff_t *);
>> 519 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
>> 520 loff_t *);
>> 521 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
>> 522 void __user *);
>> 523 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
>> 524 loff_t *, int);
535 unsigned long (*get_unmapped_area)(str 525 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
536 unsigned long, unsigne 526 unsigned long, unsigned long, unsigned long);
537 int (*check_flags)(int); 527 int (*check_flags)(int);
538 int (*flock) (struct file *, int, stru 528 int (*flock) (struct file *, int, struct file_lock *);
539 ssize_t (*splice_write)(struct pipe_in 529 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
540 size_t, unsigned int); 530 size_t, unsigned int);
541 ssize_t (*splice_read)(struct file *, 531 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
542 size_t, unsigned int); 532 size_t, unsigned int);
543 int (*setlease)(struct file *, long, s 533 int (*setlease)(struct file *, long, struct file_lock **, void **);
544 long (*fallocate)(struct file *, int, 534 long (*fallocate)(struct file *, int, loff_t, loff_t);
545 void (*show_fdinfo)(struct seq_file *m <<
546 unsigned (*mmap_capabilities)(struct f <<
547 ssize_t (*copy_file_range)(struct file <<
548 loff_t, size_t, unsign <<
549 loff_t (*remap_file_range)(struct file <<
550 struct file *file_out, <<
551 loff_t len, unsigned i <<
552 int (*fadvise)(struct file *, loff_t, <<
553 535
554 locking rules: 536 locking rules:
555 All may block. 537 All may block.
556 538
557 ->llseek() locking has moved from llseek to th 539 ->llseek() locking has moved from llseek to the individual llseek
558 implementations. If your fs is not using gene 540 implementations. If your fs is not using generic_file_llseek, you
559 need to acquire and release the appropriate lo 541 need to acquire and release the appropriate locks in your ->llseek().
560 For many filesystems, it is probably safe to a 542 For many filesystems, it is probably safe to acquire the inode
561 mutex or just to use i_size_read() instead. 543 mutex or just to use i_size_read() instead.
562 Note: this does not protect the file->f_pos ag 544 Note: this does not protect the file->f_pos against concurrent modifications
563 since this is something the userspace has to t 545 since this is something the userspace has to take care about.
564 546
565 ->iterate_shared() is called with i_rwsem held !! 547 ->iterate() is called with i_rwsem exclusive.
566 file f_pos_lock held exclusively !! 548
>> 549 ->iterate_shared() is called with i_rwsem at least shared.
567 550
568 ->fasync() is responsible for maintaining the 551 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
569 Most instances call fasync_helper(), which doe 552 Most instances call fasync_helper(), which does that maintenance, so it's
570 not normally something one needs to worry abou 553 not normally something one needs to worry about. Return values > 0 will be
571 mapped to zero in the VFS layer. 554 mapped to zero in the VFS layer.
572 555
573 ->readdir() and ->ioctl() on directories must 556 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
574 move ->readdir() to inode_operations and use a 557 move ->readdir() to inode_operations and use a separate method for directory
575 ->ioctl() or kill the latter completely. One o 558 ->ioctl() or kill the latter completely. One of the problems is that for
576 anything that resembles union-mount we won't h 559 anything that resembles union-mount we won't have a struct file for all
577 components. And there are other reasons why th 560 components. And there are other reasons why the current interface is a mess...
578 561
579 ->read on directories probably must go away - 562 ->read on directories probably must go away - we should just enforce -EISDIR
580 in sys_read() and friends. 563 in sys_read() and friends.
581 564
582 ->setlease operations should call generic_setl 565 ->setlease operations should call generic_setlease() before or after setting
583 the lease within the individual filesystem to 566 the lease within the individual filesystem to record the result of the
584 operation 567 operation
585 568
586 ->fallocate implementation must be really care <<
587 consistency when punching holes or performing <<
588 page cache contents. Usually the filesystem ne <<
589 truncate_inode_pages_range() to invalidate rel <<
590 However the filesystem usually also needs to u <<
591 view of file offset -> disk block mapping. Unt <<
592 filesystem needs to block page faults and read <<
593 cache contents from the disk. Since VFS acquir <<
594 shared mode when loading pages from disk (file <<
595 readahead paths), the fallocate implementation <<
596 prevent reloading. <<
597 <<
598 ->copy_file_range and ->remap_file_range imple <<
599 against modifications of file data while the o <<
600 blocking changes through write(2) and similar <<
601 used. To block changes to file contents via a <<
602 operation, the filesystem must take mapping->i <<
603 with ->page_mkwrite. <<
604 <<
605 dquot_operations 569 dquot_operations
606 ================ 570 ================
607 571
608 prototypes:: 572 prototypes::
609 573
610 int (*write_dquot) (struct dquot *); 574 int (*write_dquot) (struct dquot *);
611 int (*acquire_dquot) (struct dquot *); 575 int (*acquire_dquot) (struct dquot *);
612 int (*release_dquot) (struct dquot *); 576 int (*release_dquot) (struct dquot *);
613 int (*mark_dirty) (struct dquot *); 577 int (*mark_dirty) (struct dquot *);
614 int (*write_info) (struct super_block 578 int (*write_info) (struct super_block *, int);
615 579
616 These operations are intended to be more or le 580 These operations are intended to be more or less wrapping functions that ensure
617 a proper locking wrt the filesystem and call t 581 a proper locking wrt the filesystem and call the generic quota operations.
618 582
619 What filesystem should expect from the generic 583 What filesystem should expect from the generic quota functions:
620 584
621 ============== ============ ============== 585 ============== ============ =========================
622 ops FS recursion Held locks whe 586 ops FS recursion Held locks when called
623 ============== ============ ============== 587 ============== ============ =========================
624 write_dquot: yes dqonoff_sem or 588 write_dquot: yes dqonoff_sem or dqptr_sem
625 acquire_dquot: yes dqonoff_sem or 589 acquire_dquot: yes dqonoff_sem or dqptr_sem
626 release_dquot: yes dqonoff_sem or 590 release_dquot: yes dqonoff_sem or dqptr_sem
627 mark_dirty: no - 591 mark_dirty: no -
628 write_info: yes dqonoff_sem 592 write_info: yes dqonoff_sem
629 ============== ============ ============== 593 ============== ============ =========================
630 594
631 FS recursion means calling ->quota_read() and 595 FS recursion means calling ->quota_read() and ->quota_write() from superblock
632 operations. 596 operations.
633 597
634 More details about quota locking can be found 598 More details about quota locking can be found in fs/dquot.c.
635 599
636 vm_operations_struct 600 vm_operations_struct
637 ==================== 601 ====================
638 602
639 prototypes:: 603 prototypes::
640 604
641 void (*open)(struct vm_area_struct *); !! 605 void (*open)(struct vm_area_struct*);
642 void (*close)(struct vm_area_struct *) !! 606 void (*close)(struct vm_area_struct*);
643 vm_fault_t (*fault)(struct vm_fault *) !! 607 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
644 vm_fault_t (*huge_fault)(struct vm_fau <<
645 vm_fault_t (*map_pages)(struct vm_faul <<
646 vm_fault_t (*page_mkwrite)(struct vm_a 608 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
647 vm_fault_t (*pfn_mkwrite)(struct vm_ar 609 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
648 int (*access)(struct vm_area_struct *, 610 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
649 611
650 locking rules: 612 locking rules:
651 613
652 ============= ========== ============== !! 614 ============= ========= ===========================
653 ops mmap_lock PageLocked(pag 615 ops mmap_lock PageLocked(page)
654 ============= ========== ============== !! 616 ============= ========= ===========================
655 open: write !! 617 open: yes
656 close: read/write !! 618 close: yes
657 fault: read can return wit !! 619 fault: yes can return with page locked
658 huge_fault: maybe-read !! 620 map_pages: yes
659 map_pages: maybe-read !! 621 page_mkwrite: yes can return with page locked
660 page_mkwrite: read can return wit !! 622 pfn_mkwrite: yes
661 pfn_mkwrite: read !! 623 access: yes
662 access: read !! 624 ============= ========= ===========================
663 ============= ========== ============== !! 625
664 !! 626 ->fault() is called when a previously not present pte is about
665 ->fault() is called when a previously not pres !! 627 to be faulted in. The filesystem must find and return the page associated
666 in. The filesystem must find and return the pa !! 628 with the passed in "pgoff" in the vm_fault structure. If it is possible that
667 "pgoff" in the vm_fault structure. If it is po !! 629 the page may be truncated and/or invalidated, then the filesystem must lock
668 truncated and/or invalidated, then the filesys !! 630 the page, then ensure it is not already truncated (the page lock will block
669 then ensure the page is not already truncated <<
670 subsequent truncate), and then return with VM_ 631 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
671 locked. The VM will unlock the page. 632 locked. The VM will unlock the page.
672 633
673 ->huge_fault() is called when there is no PUD <<
674 gives the filesystem the opportunity to instal <<
675 Filesystems can also use the ->fault method to <<
676 so implementing this function may not be neces <<
677 filesystems should not call filemap_fault() fr <<
678 The mmap_lock may not be held when this method <<
679 <<
680 ->map_pages() is called when VM asks to map ea 634 ->map_pages() is called when VM asks to map easy accessible pages.
681 Filesystem should find and map pages associate 635 Filesystem should find and map pages associated with offsets from "start_pgoff"
682 till "end_pgoff". ->map_pages() is called with !! 636 till "end_pgoff". ->map_pages() is called with page table locked and must
683 not block. If it's not possible to reach a pa 637 not block. If it's not possible to reach a page without blocking,
684 filesystem should skip it. Filesystem should u !! 638 filesystem should skip it. Filesystem should use do_set_pte() to setup
685 page table entry. Pointer to entry associated 639 page table entry. Pointer to entry associated with the page is passed in
686 "pte" field in vm_fault structure. Pointers to 640 "pte" field in vm_fault structure. Pointers to entries for other offsets
687 should be calculated relative to "pte". 641 should be calculated relative to "pte".
688 642
689 ->page_mkwrite() is called when a previously r !! 643 ->page_mkwrite() is called when a previously read-only pte is
690 writeable. The filesystem again must ensure th !! 644 about to become writeable. The filesystem again must ensure that there are
691 truncate/invalidate races or races with operat !! 645 no truncate/invalidate races, and then return with the page locked. If
692 or ->copy_file_range, and then return with the !! 646 the page has been truncated, the filesystem should not look up a new page
693 mapping->invalidate_lock is suitable for prope !! 647 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
694 been truncated, the filesystem should not look !! 648 will cause the VM to retry the fault.
695 handler, but simply return with VM_FAULT_NOPAG <<
696 retry the fault. <<
697 649
698 ->pfn_mkwrite() is the same as page_mkwrite bu 650 ->pfn_mkwrite() is the same as page_mkwrite but when the pte is
699 VM_PFNMAP or VM_MIXEDMAP with a page-less entr 651 VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
700 VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR 652 VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
701 after this call is to make the pte read-write, 653 after this call is to make the pte read-write, unless pfn_mkwrite returns
702 an error. 654 an error.
703 655
704 ->access() is called when get_user_pages() fai 656 ->access() is called when get_user_pages() fails in
705 access_process_vm(), typically used to debug a 657 access_process_vm(), typically used to debug a process through
706 /proc/pid/mem or ptrace. This function is nee 658 /proc/pid/mem or ptrace. This function is needed only for
707 VM_IO | VM_PFNMAP VMAs. 659 VM_IO | VM_PFNMAP VMAs.
708 660
709 ---------------------------------------------- 661 --------------------------------------------------------------------------------
710 662
711 Dubious stuff 663 Dubious stuff
712 664
713 (if you break something or notice that it is b 665 (if you break something or notice that it is broken and do not fix it yourself
714 - at least put it here) 666 - at least put it here)
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