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