1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ========================================= 3 =========================================
4 Overview of the Linux Virtual File System 4 Overview of the Linux Virtual File System
5 ========================================= 5 =========================================
6 6
7 Original author: Richard Gooch <rgooch@atnf.csi 7 Original author: Richard Gooch <rgooch@atnf.csiro.au>
8 8
9 - Copyright (C) 1999 Richard Gooch 9 - Copyright (C) 1999 Richard Gooch
10 - Copyright (C) 2005 Pekka Enberg 10 - Copyright (C) 2005 Pekka Enberg
11 11
12 12
13 Introduction 13 Introduction
14 ============ 14 ============
15 15
16 The Virtual File System (also known as the Vir 16 The Virtual File System (also known as the Virtual Filesystem Switch) is
17 the software layer in the kernel that provides 17 the software layer in the kernel that provides the filesystem interface
18 to userspace programs. It also provides an ab 18 to userspace programs. It also provides an abstraction within the
19 kernel which allows different filesystem imple 19 kernel which allows different filesystem implementations to coexist.
20 20
21 VFS system calls open(2), stat(2), read(2), wr 21 VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
22 are called from a process context. Filesystem 22 are called from a process context. Filesystem locking is described in
23 the document Documentation/filesystems/locking 23 the document Documentation/filesystems/locking.rst.
24 24
25 25
26 Directory Entry Cache (dcache) 26 Directory Entry Cache (dcache)
27 ------------------------------ 27 ------------------------------
28 28
29 The VFS implements the open(2), stat(2), chmod 29 The VFS implements the open(2), stat(2), chmod(2), and similar system
30 calls. The pathname argument that is passed t 30 calls. The pathname argument that is passed to them is used by the VFS
31 to search through the directory entry cache (a 31 to search through the directory entry cache (also known as the dentry
32 cache or dcache). This provides a very fast l 32 cache or dcache). This provides a very fast look-up mechanism to
33 translate a pathname (filename) into a specifi 33 translate a pathname (filename) into a specific dentry. Dentries live
34 in RAM and are never saved to disc: they exist 34 in RAM and are never saved to disc: they exist only for performance.
35 35
36 The dentry cache is meant to be a view into yo 36 The dentry cache is meant to be a view into your entire filespace. As
37 most computers cannot fit all dentries in the 37 most computers cannot fit all dentries in the RAM at the same time, some
38 bits of the cache are missing. In order to re 38 bits of the cache are missing. In order to resolve your pathname into a
39 dentry, the VFS may have to resort to creating 39 dentry, the VFS may have to resort to creating dentries along the way,
40 and then loading the inode. This is done by l 40 and then loading the inode. This is done by looking up the inode.
41 41
42 42
43 The Inode Object 43 The Inode Object
44 ---------------- 44 ----------------
45 45
46 An individual dentry usually has a pointer to 46 An individual dentry usually has a pointer to an inode. Inodes are
47 filesystem objects such as regular files, dire 47 filesystem objects such as regular files, directories, FIFOs and other
48 beasts. They live either on the disc (for blo 48 beasts. They live either on the disc (for block device filesystems) or
49 in the memory (for pseudo filesystems). Inode 49 in the memory (for pseudo filesystems). Inodes that live on the disc
50 are copied into the memory when required and c 50 are copied into the memory when required and changes to the inode are
51 written back to disc. A single inode can be p 51 written back to disc. A single inode can be pointed to by multiple
52 dentries (hard links, for example, do this). 52 dentries (hard links, for example, do this).
53 53
54 To look up an inode requires that the VFS call 54 To look up an inode requires that the VFS calls the lookup() method of
55 the parent directory inode. This method is in 55 the parent directory inode. This method is installed by the specific
56 filesystem implementation that the inode lives 56 filesystem implementation that the inode lives in. Once the VFS has the
57 required dentry (and hence the inode), we can 57 required dentry (and hence the inode), we can do all those boring things
58 like open(2) the file, or stat(2) it to peek a 58 like open(2) the file, or stat(2) it to peek at the inode data. The
59 stat(2) operation is fairly simple: once the V 59 stat(2) operation is fairly simple: once the VFS has the dentry, it
60 peeks at the inode data and passes some of it 60 peeks at the inode data and passes some of it back to userspace.
61 61
62 62
63 The File Object 63 The File Object
64 --------------- 64 ---------------
65 65
66 Opening a file requires another operation: all 66 Opening a file requires another operation: allocation of a file
67 structure (this is the kernel-side implementat 67 structure (this is the kernel-side implementation of file descriptors).
68 The freshly allocated file structure is initia 68 The freshly allocated file structure is initialized with a pointer to
69 the dentry and a set of file operation member 69 the dentry and a set of file operation member functions. These are
70 taken from the inode data. The open() file me 70 taken from the inode data. The open() file method is then called so the
71 specific filesystem implementation can do its 71 specific filesystem implementation can do its work. You can see that
72 this is another switch performed by the VFS. 72 this is another switch performed by the VFS. The file structure is
73 placed into the file descriptor table for the 73 placed into the file descriptor table for the process.
74 74
75 Reading, writing and closing files (and other 75 Reading, writing and closing files (and other assorted VFS operations)
76 is done by using the userspace file descriptor 76 is done by using the userspace file descriptor to grab the appropriate
77 file structure, and then calling the required 77 file structure, and then calling the required file structure method to
78 do whatever is required. For as long as the f 78 do whatever is required. For as long as the file is open, it keeps the
79 dentry in use, which in turn means that the VF 79 dentry in use, which in turn means that the VFS inode is still in use.
80 80
81 81
82 Registering and Mounting a Filesystem 82 Registering and Mounting a Filesystem
83 ===================================== 83 =====================================
84 84
85 To register and unregister a filesystem, use t 85 To register and unregister a filesystem, use the following API
86 functions: 86 functions:
87 87
88 .. code-block:: c 88 .. code-block:: c
89 89
90 #include <linux/fs.h> 90 #include <linux/fs.h>
91 91
92 extern int register_filesystem(struct 92 extern int register_filesystem(struct file_system_type *);
93 extern int unregister_filesystem(struc 93 extern int unregister_filesystem(struct file_system_type *);
94 94
95 The passed struct file_system_type describes y 95 The passed struct file_system_type describes your filesystem. When a
96 request is made to mount a filesystem onto a d 96 request is made to mount a filesystem onto a directory in your
97 namespace, the VFS will call the appropriate m 97 namespace, the VFS will call the appropriate mount() method for the
98 specific filesystem. New vfsmount referring t 98 specific filesystem. New vfsmount referring to the tree returned by
99 ->mount() will be attached to the mountpoint, 99 ->mount() will be attached to the mountpoint, so that when pathname
100 resolution reaches the mountpoint it will jump 100 resolution reaches the mountpoint it will jump into the root of that
101 vfsmount. 101 vfsmount.
102 102
103 You can see all filesystems that are registere 103 You can see all filesystems that are registered to the kernel in the
104 file /proc/filesystems. 104 file /proc/filesystems.
105 105
106 106
107 struct file_system_type 107 struct file_system_type
108 ----------------------- 108 -----------------------
109 109
110 This describes the filesystem. The following !! 110 This describes the filesystem. As of kernel 2.6.39, the following
111 members are defined: 111 members are defined:
112 112
113 .. code-block:: c 113 .. code-block:: c
114 114
115 struct file_system_type { 115 struct file_system_type {
116 const char *name; 116 const char *name;
117 int fs_flags; 117 int fs_flags;
118 int (*init_fs_context)(struct <<
119 const struct fs_parameter_spec <<
120 struct dentry *(*mount) (struc 118 struct dentry *(*mount) (struct file_system_type *, int,
121 const char *, void *); !! 119 const char *, void *);
122 void (*kill_sb) (struct super_ 120 void (*kill_sb) (struct super_block *);
123 struct module *owner; 121 struct module *owner;
124 struct file_system_type * next 122 struct file_system_type * next;
125 struct hlist_head fs_supers; !! 123 struct list_head fs_supers;
126 <<
127 struct lock_class_key s_lock_k 124 struct lock_class_key s_lock_key;
128 struct lock_class_key s_umount 125 struct lock_class_key s_umount_key;
129 struct lock_class_key s_vfs_re <<
130 struct lock_class_key s_writer <<
131 <<
132 struct lock_class_key i_lock_k <<
133 struct lock_class_key i_mutex_ <<
134 struct lock_class_key invalida <<
135 struct lock_class_key i_mutex_ <<
136 }; 126 };
137 127
138 ``name`` 128 ``name``
139 the name of the filesystem type, such 129 the name of the filesystem type, such as "ext2", "iso9660",
140 "msdos" and so on 130 "msdos" and so on
141 131
142 ``fs_flags`` 132 ``fs_flags``
143 various flags (i.e. FS_REQUIRES_DEV, F 133 various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
144 134
145 ``init_fs_context`` <<
146 Initializes 'struct fs_context' ->ops <<
147 filesystem-specific data. <<
148 <<
149 ``parameters`` <<
150 Pointer to the array of filesystem par <<
151 'struct fs_parameter_spec'. <<
152 More info in Documentation/filesystems <<
153 <<
154 ``mount`` 135 ``mount``
155 the method to call when a new instance 136 the method to call when a new instance of this filesystem should
156 be mounted 137 be mounted
157 138
158 ``kill_sb`` 139 ``kill_sb``
159 the method to call when an instance of 140 the method to call when an instance of this filesystem should be
160 shut down 141 shut down
161 142
162 143
163 ``owner`` 144 ``owner``
164 for internal VFS use: you should initi 145 for internal VFS use: you should initialize this to THIS_MODULE
165 in most cases. 146 in most cases.
166 147
167 ``next`` 148 ``next``
168 for internal VFS use: you should initi 149 for internal VFS use: you should initialize this to NULL
169 150
170 ``fs_supers`` !! 151 s_lock_key, s_umount_key: lockdep-specific
171 for internal VFS use: hlist of filesys <<
172 <<
173 s_lock_key, s_umount_key, s_vfs_rename_key, <<
174 i_lock_key, i_mutex_key, invalidate_lock_key <<
175 152
176 The mount() method has the following arguments 153 The mount() method has the following arguments:
177 154
178 ``struct file_system_type *fs_type`` 155 ``struct file_system_type *fs_type``
179 describes the filesystem, partly initi 156 describes the filesystem, partly initialized by the specific
180 filesystem code 157 filesystem code
181 158
182 ``int flags`` 159 ``int flags``
183 mount flags 160 mount flags
184 161
185 ``const char *dev_name`` 162 ``const char *dev_name``
186 the device name we are mounting. 163 the device name we are mounting.
187 164
188 ``void *data`` 165 ``void *data``
189 arbitrary mount options, usually comes 166 arbitrary mount options, usually comes as an ASCII string (see
190 "Mount Options" section) 167 "Mount Options" section)
191 168
192 The mount() method must return the root dentry 169 The mount() method must return the root dentry of the tree requested by
193 caller. An active reference to its superblock 170 caller. An active reference to its superblock must be grabbed and the
194 superblock must be locked. On failure it shou 171 superblock must be locked. On failure it should return ERR_PTR(error).
195 172
196 The arguments match those of mount(2) and thei 173 The arguments match those of mount(2) and their interpretation depends
197 on filesystem type. E.g. for block filesystem 174 on filesystem type. E.g. for block filesystems, dev_name is interpreted
198 as block device name, that device is opened an 175 as block device name, that device is opened and if it contains a
199 suitable filesystem image the method creates a 176 suitable filesystem image the method creates and initializes struct
200 super_block accordingly, returning its root de 177 super_block accordingly, returning its root dentry to caller.
201 178
202 ->mount() may choose to return a subtree of ex 179 ->mount() may choose to return a subtree of existing filesystem - it
203 doesn't have to create a new one. The main re 180 doesn't have to create a new one. The main result from the caller's
204 point of view is a reference to dentry at the 181 point of view is a reference to dentry at the root of (sub)tree to be
205 attached; creation of new superblock is a comm 182 attached; creation of new superblock is a common side effect.
206 183
207 The most interesting member of the superblock 184 The most interesting member of the superblock structure that the mount()
208 method fills in is the "s_op" field. This is 185 method fills in is the "s_op" field. This is a pointer to a "struct
209 super_operations" which describes the next lev 186 super_operations" which describes the next level of the filesystem
210 implementation. 187 implementation.
211 188
212 Usually, a filesystem uses one of the generic 189 Usually, a filesystem uses one of the generic mount() implementations
213 and provides a fill_super() callback instead. 190 and provides a fill_super() callback instead. The generic variants are:
214 191
215 ``mount_bdev`` 192 ``mount_bdev``
216 mount a filesystem residing on a block 193 mount a filesystem residing on a block device
217 194
218 ``mount_nodev`` 195 ``mount_nodev``
219 mount a filesystem that is not backed 196 mount a filesystem that is not backed by a device
220 197
221 ``mount_single`` 198 ``mount_single``
222 mount a filesystem which shares the in 199 mount a filesystem which shares the instance between all mounts
223 200
224 A fill_super() callback implementation has the 201 A fill_super() callback implementation has the following arguments:
225 202
226 ``struct super_block *sb`` 203 ``struct super_block *sb``
227 the superblock structure. The callbac 204 the superblock structure. The callback must initialize this
228 properly. 205 properly.
229 206
230 ``void *data`` 207 ``void *data``
231 arbitrary mount options, usually comes 208 arbitrary mount options, usually comes as an ASCII string (see
232 "Mount Options" section) 209 "Mount Options" section)
233 210
234 ``int silent`` 211 ``int silent``
235 whether or not to be silent on error 212 whether or not to be silent on error
236 213
237 214
238 The Superblock Object 215 The Superblock Object
239 ===================== 216 =====================
240 217
241 A superblock object represents a mounted files 218 A superblock object represents a mounted filesystem.
242 219
243 220
244 struct super_operations 221 struct super_operations
245 ----------------------- 222 -----------------------
246 223
247 This describes how the VFS can manipulate the 224 This describes how the VFS can manipulate the superblock of your
248 filesystem. The following members are defined !! 225 filesystem. As of kernel 2.6.22, the following members are defined:
249 226
250 .. code-block:: c 227 .. code-block:: c
251 228
252 struct super_operations { 229 struct super_operations {
253 struct inode *(*alloc_inode)(s 230 struct inode *(*alloc_inode)(struct super_block *sb);
254 void (*destroy_inode)(struct i 231 void (*destroy_inode)(struct inode *);
255 void (*free_inode)(struct inod <<
256 232
257 void (*dirty_inode) (struct in 233 void (*dirty_inode) (struct inode *, int flags);
258 int (*write_inode) (struct ino !! 234 int (*write_inode) (struct inode *, int);
259 int (*drop_inode) (struct inod !! 235 void (*drop_inode) (struct inode *);
260 void (*evict_inode) (struct in !! 236 void (*delete_inode) (struct inode *);
261 void (*put_super) (struct supe 237 void (*put_super) (struct super_block *);
262 int (*sync_fs)(struct super_bl 238 int (*sync_fs)(struct super_block *sb, int wait);
263 int (*freeze_super) (struct su <<
264 enum f <<
265 int (*freeze_fs) (struct super 239 int (*freeze_fs) (struct super_block *);
266 int (*thaw_super) (struct supe <<
267 enum f <<
268 int (*unfreeze_fs) (struct sup 240 int (*unfreeze_fs) (struct super_block *);
269 int (*statfs) (struct dentry * 241 int (*statfs) (struct dentry *, struct kstatfs *);
270 int (*remount_fs) (struct supe 242 int (*remount_fs) (struct super_block *, int *, char *);
>> 243 void (*clear_inode) (struct inode *);
271 void (*umount_begin) (struct s 244 void (*umount_begin) (struct super_block *);
272 245
273 int (*show_options)(struct seq 246 int (*show_options)(struct seq_file *, struct dentry *);
274 int (*show_devname)(struct seq <<
275 int (*show_path)(struct seq_fi <<
276 int (*show_stats)(struct seq_f <<
277 247
278 ssize_t (*quota_read)(struct s 248 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
279 ssize_t (*quota_write)(struct 249 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
280 struct dquot **(*get_dquots)(s !! 250 int (*nr_cached_objects)(struct super_block *);
281 !! 251 void (*free_cached_objects)(struct super_block *, int);
282 long (*nr_cached_objects)(stru <<
283 struct <<
284 long (*free_cached_objects)(st <<
285 struct <<
286 }; 252 };
287 253
288 All methods are called without any locks being 254 All methods are called without any locks being held, unless otherwise
289 noted. This means that most methods can block 255 noted. This means that most methods can block safely. All methods are
290 only called from a process context (i.e. not f 256 only called from a process context (i.e. not from an interrupt handler
291 or bottom half). 257 or bottom half).
292 258
293 ``alloc_inode`` 259 ``alloc_inode``
294 this method is called by alloc_inode() 260 this method is called by alloc_inode() to allocate memory for
295 struct inode and initialize it. If th 261 struct inode and initialize it. If this function is not
296 defined, a simple 'struct inode' is al 262 defined, a simple 'struct inode' is allocated. Normally
297 alloc_inode will be used to allocate a 263 alloc_inode will be used to allocate a larger structure which
298 contains a 'struct inode' embedded wit 264 contains a 'struct inode' embedded within it.
299 265
300 ``destroy_inode`` 266 ``destroy_inode``
301 this method is called by destroy_inode 267 this method is called by destroy_inode() to release resources
302 allocated for struct inode. It is onl 268 allocated for struct inode. It is only required if
303 ->alloc_inode was defined and simply u 269 ->alloc_inode was defined and simply undoes anything done by
304 ->alloc_inode. 270 ->alloc_inode.
305 271
306 ``free_inode`` <<
307 this method is called from RCU callbac <<
308 in ->destroy_inode to free 'struct ino <<
309 better to release memory in this metho <<
310 <<
311 ``dirty_inode`` 272 ``dirty_inode``
312 this method is called by the VFS when 273 this method is called by the VFS when an inode is marked dirty.
313 This is specifically for the inode its 274 This is specifically for the inode itself being marked dirty,
314 not its data. If the update needs to 275 not its data. If the update needs to be persisted by fdatasync(),
315 then I_DIRTY_DATASYNC will be set in t 276 then I_DIRTY_DATASYNC will be set in the flags argument.
316 I_DIRTY_TIME will be set in the flags 277 I_DIRTY_TIME will be set in the flags in case lazytime is enabled
317 and struct inode has times updated sin 278 and struct inode has times updated since the last ->dirty_inode
318 call. 279 call.
319 280
320 ``write_inode`` 281 ``write_inode``
321 this method is called when the VFS nee 282 this method is called when the VFS needs to write an inode to
322 disc. The second parameter indicates 283 disc. The second parameter indicates whether the write should
323 be synchronous or not, not all filesys 284 be synchronous or not, not all filesystems check this flag.
324 285
325 ``drop_inode`` 286 ``drop_inode``
326 called when the last access to the ino 287 called when the last access to the inode is dropped, with the
327 inode->i_lock spinlock held. 288 inode->i_lock spinlock held.
328 289
329 This method should be either NULL (nor 290 This method should be either NULL (normal UNIX filesystem
330 semantics) or "generic_delete_inode" ( 291 semantics) or "generic_delete_inode" (for filesystems that do
331 not want to cache inodes - causing "de 292 not want to cache inodes - causing "delete_inode" to always be
332 called regardless of the value of i_nl 293 called regardless of the value of i_nlink)
333 294
334 The "generic_delete_inode()" behavior 295 The "generic_delete_inode()" behavior is equivalent to the old
335 practice of using "force_delete" in th 296 practice of using "force_delete" in the put_inode() case, but
336 does not have the races that the "forc 297 does not have the races that the "force_delete()" approach had.
337 298
338 ``evict_inode`` !! 299 ``delete_inode``
339 called when the VFS wants to evict an !! 300 called when the VFS wants to delete an inode
340 *not* evict the pagecache or inode-ass <<
341 the method has to use truncate_inode_p <<
342 of those. Caller makes sure async writ <<
343 the inode while (or after) ->evict_ino <<
344 301
345 ``put_super`` 302 ``put_super``
346 called when the VFS wishes to free the 303 called when the VFS wishes to free the superblock
347 (i.e. unmount). This is called with t 304 (i.e. unmount). This is called with the superblock lock held
348 305
349 ``sync_fs`` 306 ``sync_fs``
350 called when VFS is writing out all dir 307 called when VFS is writing out all dirty data associated with a
351 superblock. The second parameter indi 308 superblock. The second parameter indicates whether the method
352 should wait until the write out has be 309 should wait until the write out has been completed. Optional.
353 310
354 ``freeze_super`` <<
355 Called instead of ->freeze_fs callback <<
356 Main difference is that ->freeze_super <<
357 down_write(&sb->s_umount). If filesyst <<
358 ->freeze_fs to be called too, then it <<
359 explicitly from this callback. Optiona <<
360 <<
361 ``freeze_fs`` 311 ``freeze_fs``
362 called when VFS is locking a filesyste 312 called when VFS is locking a filesystem and forcing it into a
363 consistent state. This method is curr 313 consistent state. This method is currently used by the Logical
364 Volume Manager (LVM) and ioctl(FIFREEZ !! 314 Volume Manager (LVM).
365 <<
366 ``thaw_super`` <<
367 called when VFS is unlocking a filesys <<
368 again after ->freeze_super. Optional. <<
369 315
370 ``unfreeze_fs`` 316 ``unfreeze_fs``
371 called when VFS is unlocking a filesys 317 called when VFS is unlocking a filesystem and making it writable
372 again after ->freeze_fs. Optional. !! 318 again.
373 319
374 ``statfs`` 320 ``statfs``
375 called when the VFS needs to get files 321 called when the VFS needs to get filesystem statistics.
376 322
377 ``remount_fs`` 323 ``remount_fs``
378 called when the filesystem is remounte 324 called when the filesystem is remounted. This is called with
379 the kernel lock held 325 the kernel lock held
380 326
>> 327 ``clear_inode``
>> 328 called then the VFS clears the inode. Optional
>> 329
381 ``umount_begin`` 330 ``umount_begin``
382 called when the VFS is unmounting a fi 331 called when the VFS is unmounting a filesystem.
383 332
384 ``show_options`` 333 ``show_options``
385 called by the VFS to show mount option !! 334 called by the VFS to show mount options for /proc/<pid>/mounts.
386 and /proc/<pid>/mountinfo. <<
387 (see "Mount Options" section) 335 (see "Mount Options" section)
388 336
389 ``show_devname`` <<
390 Optional. Called by the VFS to show de <<
391 /proc/<pid>/{mounts,mountinfo,mountsta <<
392 '(struct mount).mnt_devname' will be u <<
393 <<
394 ``show_path`` <<
395 Optional. Called by the VFS (for /proc <<
396 the mount root dentry path relative to <<
397 <<
398 ``show_stats`` <<
399 Optional. Called by the VFS (for /proc <<
400 filesystem-specific mount statistics. <<
401 <<
402 ``quota_read`` 337 ``quota_read``
403 called by the VFS to read from filesys 338 called by the VFS to read from filesystem quota file.
404 339
405 ``quota_write`` 340 ``quota_write``
406 called by the VFS to write to filesyst 341 called by the VFS to write to filesystem quota file.
407 342
408 ``get_dquots`` <<
409 called by quota to get 'struct dquot' <<
410 Optional. <<
411 <<
412 ``nr_cached_objects`` 343 ``nr_cached_objects``
413 called by the sb cache shrinking funct 344 called by the sb cache shrinking function for the filesystem to
414 return the number of freeable cached o 345 return the number of freeable cached objects it contains.
415 Optional. 346 Optional.
416 347
417 ``free_cache_objects`` 348 ``free_cache_objects``
418 called by the sb cache shrinking funct 349 called by the sb cache shrinking function for the filesystem to
419 scan the number of objects indicated t 350 scan the number of objects indicated to try to free them.
420 Optional, but any filesystem implement 351 Optional, but any filesystem implementing this method needs to
421 also implement ->nr_cached_objects for 352 also implement ->nr_cached_objects for it to be called
422 correctly. 353 correctly.
423 354
424 We can't do anything with any errors t 355 We can't do anything with any errors that the filesystem might
425 encountered, hence the void return typ 356 encountered, hence the void return type. This will never be
426 called if the VM is trying to reclaim 357 called if the VM is trying to reclaim under GFP_NOFS conditions,
427 hence this method does not need to han 358 hence this method does not need to handle that situation itself.
428 359
429 Implementations must include condition 360 Implementations must include conditional reschedule calls inside
430 any scanning loop that is done. This 361 any scanning loop that is done. This allows the VFS to
431 determine appropriate scan batch sizes 362 determine appropriate scan batch sizes without having to worry
432 about whether implementations will cau 363 about whether implementations will cause holdoff problems due to
433 large scan batch sizes. 364 large scan batch sizes.
434 365
435 Whoever sets up the inode is responsible for f 366 Whoever sets up the inode is responsible for filling in the "i_op"
436 field. This is a pointer to a "struct inode_o 367 field. This is a pointer to a "struct inode_operations" which describes
437 the methods that can be performed on individua 368 the methods that can be performed on individual inodes.
438 369
439 370
440 struct xattr_handler !! 371 struct xattr_handlers
441 --------------------- 372 ---------------------
442 373
443 On filesystems that support extended attribute 374 On filesystems that support extended attributes (xattrs), the s_xattr
444 superblock field points to a NULL-terminated a 375 superblock field points to a NULL-terminated array of xattr handlers.
445 Extended attributes are name:value pairs. 376 Extended attributes are name:value pairs.
446 377
447 ``name`` 378 ``name``
448 Indicates that the handler matches att 379 Indicates that the handler matches attributes with the specified
449 name (such as "system.posix_acl_access 380 name (such as "system.posix_acl_access"); the prefix field must
450 be NULL. 381 be NULL.
451 382
452 ``prefix`` 383 ``prefix``
453 Indicates that the handler matches all 384 Indicates that the handler matches all attributes with the
454 specified name prefix (such as "user." 385 specified name prefix (such as "user."); the name field must be
455 NULL. 386 NULL.
456 387
457 ``list`` 388 ``list``
458 Determine if attributes matching this 389 Determine if attributes matching this xattr handler should be
459 listed for a particular dentry. Used 390 listed for a particular dentry. Used by some listxattr
460 implementations like generic_listxattr 391 implementations like generic_listxattr.
461 392
462 ``get`` 393 ``get``
463 Called by the VFS to get the value of 394 Called by the VFS to get the value of a particular extended
464 attribute. This method is called by t 395 attribute. This method is called by the getxattr(2) system
465 call. 396 call.
466 397
467 ``set`` 398 ``set``
468 Called by the VFS to set the value of 399 Called by the VFS to set the value of a particular extended
469 attribute. When the new value is NULL 400 attribute. When the new value is NULL, called to remove a
470 particular extended attribute. This m 401 particular extended attribute. This method is called by the
471 setxattr(2) and removexattr(2) system 402 setxattr(2) and removexattr(2) system calls.
472 403
473 When none of the xattr handlers of a filesyste 404 When none of the xattr handlers of a filesystem match the specified
474 attribute name or when a filesystem doesn't su 405 attribute name or when a filesystem doesn't support extended attributes,
475 the various ``*xattr(2)`` system calls return 406 the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
476 407
477 408
478 The Inode Object 409 The Inode Object
479 ================ 410 ================
480 411
481 An inode object represents an object within th 412 An inode object represents an object within the filesystem.
482 413
483 414
484 struct inode_operations 415 struct inode_operations
485 ----------------------- 416 -----------------------
486 417
487 This describes how the VFS can manipulate an i 418 This describes how the VFS can manipulate an inode in your filesystem.
488 As of kernel 2.6.22, the following members are 419 As of kernel 2.6.22, the following members are defined:
489 420
490 .. code-block:: c 421 .. code-block:: c
491 422
492 struct inode_operations { 423 struct inode_operations {
493 int (*create) (struct mnt_idma !! 424 int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
494 struct dentry * (*lookup) (str 425 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
495 int (*link) (struct dentry *,s 426 int (*link) (struct dentry *,struct inode *,struct dentry *);
496 int (*unlink) (struct inode *, 427 int (*unlink) (struct inode *,struct dentry *);
497 int (*symlink) (struct mnt_idm !! 428 int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
498 int (*mkdir) (struct mnt_idmap !! 429 int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
499 int (*rmdir) (struct inode *,s 430 int (*rmdir) (struct inode *,struct dentry *);
500 int (*mknod) (struct mnt_idmap !! 431 int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
501 int (*rename) (struct mnt_idma !! 432 int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
502 struct inode *, 433 struct inode *, struct dentry *, unsigned int);
503 int (*readlink) (struct dentry 434 int (*readlink) (struct dentry *, char __user *,int);
504 const char *(*get_link) (struc 435 const char *(*get_link) (struct dentry *, struct inode *,
505 struc 436 struct delayed_call *);
506 int (*permission) (struct mnt_ !! 437 int (*permission) (struct user_namespace *, struct inode *, int);
507 struct posix_acl * (*get_inode !! 438 struct posix_acl * (*get_acl)(struct inode *, int, bool);
508 int (*setattr) (struct mnt_idm !! 439 int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
509 int (*getattr) (struct mnt_idm !! 440 int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
510 ssize_t (*listxattr) (struct d 441 ssize_t (*listxattr) (struct dentry *, char *, size_t);
511 void (*update_time)(struct ino 442 void (*update_time)(struct inode *, struct timespec *, int);
512 int (*atomic_open)(struct inod 443 int (*atomic_open)(struct inode *, struct dentry *, struct file *,
513 unsigned op 444 unsigned open_flag, umode_t create_mode);
514 int (*tmpfile) (struct mnt_idm !! 445 int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t);
515 struct posix_acl * (*get_acl)( !! 446 int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
516 int (*set_acl)(struct mnt_idma !! 447 int (*fileattr_set)(struct user_namespace *mnt_userns,
517 int (*fileattr_set)(struct mnt <<
518 struct den 448 struct dentry *dentry, struct fileattr *fa);
519 int (*fileattr_get)(struct den 449 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
520 struct offset_ctx *(*get_offse <<
521 }; 450 };
522 451
523 Again, all methods are called without any lock 452 Again, all methods are called without any locks being held, unless
524 otherwise noted. 453 otherwise noted.
525 454
526 ``create`` 455 ``create``
527 called by the open(2) and creat(2) sys 456 called by the open(2) and creat(2) system calls. Only required
528 if you want to support regular files. 457 if you want to support regular files. The dentry you get should
529 not have an inode (i.e. it should be a 458 not have an inode (i.e. it should be a negative dentry). Here
530 you will probably call d_instantiate() 459 you will probably call d_instantiate() with the dentry and the
531 newly created inode 460 newly created inode
532 461
533 ``lookup`` 462 ``lookup``
534 called when the VFS needs to look up a 463 called when the VFS needs to look up an inode in a parent
535 directory. The name to look for is fo 464 directory. The name to look for is found in the dentry. This
536 method must call d_add() to insert the 465 method must call d_add() to insert the found inode into the
537 dentry. The "i_count" field in the in 466 dentry. The "i_count" field in the inode structure should be
538 incremented. If the named inode does 467 incremented. If the named inode does not exist a NULL inode
539 should be inserted into the dentry (th 468 should be inserted into the dentry (this is called a negative
540 dentry). Returning an error code from 469 dentry). Returning an error code from this routine must only be
541 done on a real error, otherwise creati 470 done on a real error, otherwise creating inodes with system
542 calls like create(2), mknod(2), mkdir( 471 calls like create(2), mknod(2), mkdir(2) and so on will fail.
543 If you wish to overload the dentry met 472 If you wish to overload the dentry methods then you should
544 initialise the "d_dop" field in the de 473 initialise the "d_dop" field in the dentry; this is a pointer to
545 a struct "dentry_operations". This me 474 a struct "dentry_operations". This method is called with the
546 directory inode semaphore held 475 directory inode semaphore held
547 476
548 ``link`` 477 ``link``
549 called by the link(2) system call. On 478 called by the link(2) system call. Only required if you want to
550 support hard links. You will probably 479 support hard links. You will probably need to call
551 d_instantiate() just as you would in t 480 d_instantiate() just as you would in the create() method
552 481
553 ``unlink`` 482 ``unlink``
554 called by the unlink(2) system call. 483 called by the unlink(2) system call. Only required if you want
555 to support deleting inodes 484 to support deleting inodes
556 485
557 ``symlink`` 486 ``symlink``
558 called by the symlink(2) system call. 487 called by the symlink(2) system call. Only required if you want
559 to support symlinks. You will probabl 488 to support symlinks. You will probably need to call
560 d_instantiate() just as you would in t 489 d_instantiate() just as you would in the create() method
561 490
562 ``mkdir`` 491 ``mkdir``
563 called by the mkdir(2) system call. O 492 called by the mkdir(2) system call. Only required if you want
564 to support creating subdirectories. Y 493 to support creating subdirectories. You will probably need to
565 call d_instantiate() just as you would 494 call d_instantiate() just as you would in the create() method
566 495
567 ``rmdir`` 496 ``rmdir``
568 called by the rmdir(2) system call. O 497 called by the rmdir(2) system call. Only required if you want
569 to support deleting subdirectories 498 to support deleting subdirectories
570 499
571 ``mknod`` 500 ``mknod``
572 called by the mknod(2) system call to 501 called by the mknod(2) system call to create a device (char,
573 block) inode or a named pipe (FIFO) or 502 block) inode or a named pipe (FIFO) or socket. Only required if
574 you want to support creating these typ 503 you want to support creating these types of inodes. You will
575 probably need to call d_instantiate() 504 probably need to call d_instantiate() just as you would in the
576 create() method 505 create() method
577 506
578 ``rename`` 507 ``rename``
579 called by the rename(2) system call to 508 called by the rename(2) system call to rename the object to have
580 the parent and name given by the secon 509 the parent and name given by the second inode and dentry.
581 510
582 The filesystem must return -EINVAL for 511 The filesystem must return -EINVAL for any unsupported or
583 unknown flags. Currently the followin 512 unknown flags. Currently the following flags are implemented:
584 (1) RENAME_NOREPLACE: this flag indica 513 (1) RENAME_NOREPLACE: this flag indicates that if the target of
585 the rename exists the rename should fa 514 the rename exists the rename should fail with -EEXIST instead of
586 replacing the target. The VFS already 515 replacing the target. The VFS already checks for existence, so
587 for local filesystems the RENAME_NOREP 516 for local filesystems the RENAME_NOREPLACE implementation is
588 equivalent to plain rename. 517 equivalent to plain rename.
589 (2) RENAME_EXCHANGE: exchange source a 518 (2) RENAME_EXCHANGE: exchange source and target. Both must
590 exist; this is checked by the VFS. Un 519 exist; this is checked by the VFS. Unlike plain rename, source
591 and target may be of different type. 520 and target may be of different type.
592 521
593 ``get_link`` 522 ``get_link``
594 called by the VFS to follow a symbolic 523 called by the VFS to follow a symbolic link to the inode it
595 points to. Only required if you want 524 points to. Only required if you want to support symbolic links.
596 This method returns the symlink body t 525 This method returns the symlink body to traverse (and possibly
597 resets the current position with nd_ju 526 resets the current position with nd_jump_link()). If the body
598 won't go away until the inode is gone, 527 won't go away until the inode is gone, nothing else is needed;
599 if it needs to be otherwise pinned, ar 528 if it needs to be otherwise pinned, arrange for its release by
600 having get_link(..., ..., done) do set 529 having get_link(..., ..., done) do set_delayed_call(done,
601 destructor, argument). In that case d 530 destructor, argument). In that case destructor(argument) will
602 be called once VFS is done with the bo 531 be called once VFS is done with the body you've returned. May
603 be called in RCU mode; that is indicat 532 be called in RCU mode; that is indicated by NULL dentry
604 argument. If request can't be handled 533 argument. If request can't be handled without leaving RCU mode,
605 have it return ERR_PTR(-ECHILD). 534 have it return ERR_PTR(-ECHILD).
606 535
607 If the filesystem stores the symlink t 536 If the filesystem stores the symlink target in ->i_link, the
608 VFS may use it directly without callin 537 VFS may use it directly without calling ->get_link(); however,
609 ->get_link() must still be provided. 538 ->get_link() must still be provided. ->i_link must not be
610 freed until after an RCU grace period. 539 freed until after an RCU grace period. Writing to ->i_link
611 post-iget() time requires a 'release' 540 post-iget() time requires a 'release' memory barrier.
612 541
613 ``readlink`` 542 ``readlink``
614 this is now just an override for use b 543 this is now just an override for use by readlink(2) for the
615 cases when ->get_link uses nd_jump_lin 544 cases when ->get_link uses nd_jump_link() or object is not in
616 fact a symlink. Normally filesystems 545 fact a symlink. Normally filesystems should only implement
617 ->get_link for symlinks and readlink(2 546 ->get_link for symlinks and readlink(2) will automatically use
618 that. 547 that.
619 548
620 ``permission`` 549 ``permission``
621 called by the VFS to check for access 550 called by the VFS to check for access rights on a POSIX-like
622 filesystem. 551 filesystem.
623 552
624 May be called in rcu-walk mode (mask & 553 May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
625 rcu-walk mode, the filesystem must che 554 rcu-walk mode, the filesystem must check the permission without
626 blocking or storing to the inode. 555 blocking or storing to the inode.
627 556
628 If a situation is encountered that rcu 557 If a situation is encountered that rcu-walk cannot handle,
629 return 558 return
630 -ECHILD and it will be called again in 559 -ECHILD and it will be called again in ref-walk mode.
631 560
632 ``setattr`` 561 ``setattr``
633 called by the VFS to set attributes fo 562 called by the VFS to set attributes for a file. This method is
634 called by chmod(2) and related system 563 called by chmod(2) and related system calls.
635 564
636 ``getattr`` 565 ``getattr``
637 called by the VFS to get attributes of 566 called by the VFS to get attributes of a file. This method is
638 called by stat(2) and related system c 567 called by stat(2) and related system calls.
639 568
640 ``listxattr`` 569 ``listxattr``
641 called by the VFS to list all extended 570 called by the VFS to list all extended attributes for a given
642 file. This method is called by the li 571 file. This method is called by the listxattr(2) system call.
643 572
644 ``update_time`` 573 ``update_time``
645 called by the VFS to update a specific 574 called by the VFS to update a specific time or the i_version of
646 an inode. If this is not defined the 575 an inode. If this is not defined the VFS will update the inode
647 itself and call mark_inode_dirty_sync. 576 itself and call mark_inode_dirty_sync.
648 577
649 ``atomic_open`` 578 ``atomic_open``
650 called on the last component of an ope 579 called on the last component of an open. Using this optional
651 method the filesystem can look up, pos 580 method the filesystem can look up, possibly create and open the
652 file in one atomic operation. If it w 581 file in one atomic operation. If it wants to leave actual
653 opening to the caller (e.g. if the fil 582 opening to the caller (e.g. if the file turned out to be a
654 symlink, device, or just something fil 583 symlink, device, or just something filesystem won't do atomic
655 open for), it may signal this by retur 584 open for), it may signal this by returning finish_no_open(file,
656 dentry). This method is only called i 585 dentry). This method is only called if the last component is
657 negative or needs lookup. Cached posi 586 negative or needs lookup. Cached positive dentries are still
658 handled by f_op->open(). If the file 587 handled by f_op->open(). If the file was created, FMODE_CREATED
659 flag should be set in file->f_mode. I 588 flag should be set in file->f_mode. In case of O_EXCL the
660 method must only succeed if the file d 589 method must only succeed if the file didn't exist and hence
661 FMODE_CREATED shall always be set on s 590 FMODE_CREATED shall always be set on success.
662 591
663 ``tmpfile`` 592 ``tmpfile``
664 called in the end of O_TMPFILE open(). 593 called in the end of O_TMPFILE open(). Optional, equivalent to
665 atomically creating, opening and unlin 594 atomically creating, opening and unlinking a file in given
666 directory. On success needs to return !! 595 directory.
667 open; this can be done by calling fini <<
668 the end. <<
669 596
670 ``fileattr_get`` 597 ``fileattr_get``
671 called on ioctl(FS_IOC_GETFLAGS) and i 598 called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
672 retrieve miscellaneous file flags and 599 retrieve miscellaneous file flags and attributes. Also called
673 before the relevant SET operation to c 600 before the relevant SET operation to check what is being changed
674 (in this case with i_rwsem locked excl 601 (in this case with i_rwsem locked exclusive). If unset, then
675 fall back to f_op->ioctl(). 602 fall back to f_op->ioctl().
676 603
677 ``fileattr_set`` 604 ``fileattr_set``
678 called on ioctl(FS_IOC_SETFLAGS) and i 605 called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
679 change miscellaneous file flags and at 606 change miscellaneous file flags and attributes. Callers hold
680 i_rwsem exclusive. If unset, then fal 607 i_rwsem exclusive. If unset, then fall back to f_op->ioctl().
681 ``get_offset_ctx`` !! 608
682 called to get the offset context for a <<
683 filesystem must define this operation <<
684 simple_offset_dir_operations. <<
685 609
686 The Address Space Object 610 The Address Space Object
687 ======================== 611 ========================
688 612
689 The address space object is used to group and 613 The address space object is used to group and manage pages in the page
690 cache. It can be used to keep track of the pa 614 cache. It can be used to keep track of the pages in a file (or anything
691 else) and also track the mapping of sections o 615 else) and also track the mapping of sections of the file into process
692 address spaces. 616 address spaces.
693 617
694 There are a number of distinct yet related ser 618 There are a number of distinct yet related services that an
695 address-space can provide. These include comm 619 address-space can provide. These include communicating memory pressure,
696 page lookup by address, and keeping track of p 620 page lookup by address, and keeping track of pages tagged as Dirty or
697 Writeback. 621 Writeback.
698 622
699 The first can be used independently to the oth 623 The first can be used independently to the others. The VM can try to
700 either write dirty pages in order to clean the 624 either write dirty pages in order to clean them, or release clean pages
701 in order to reuse them. To do this it can cal 625 in order to reuse them. To do this it can call the ->writepage method
702 on dirty pages, and ->release_folio on clean f 626 on dirty pages, and ->release_folio on clean folios with the private
703 flag set. Clean pages without PagePrivate and 627 flag set. Clean pages without PagePrivate and with no external references
704 will be released without notice being given to 628 will be released without notice being given to the address_space.
705 629
706 To achieve this functionality, pages need to b 630 To achieve this functionality, pages need to be placed on an LRU with
707 lru_cache_add and mark_page_active needs to be 631 lru_cache_add and mark_page_active needs to be called whenever the page
708 is used. 632 is used.
709 633
710 Pages are normally kept in a radix tree index 634 Pages are normally kept in a radix tree index by ->index. This tree
711 maintains information about the PG_Dirty and P 635 maintains information about the PG_Dirty and PG_Writeback status of each
712 page, so that pages with either of these flags 636 page, so that pages with either of these flags can be found quickly.
713 637
714 The Dirty tag is primarily used by mpage_write 638 The Dirty tag is primarily used by mpage_writepages - the default
715 ->writepages method. It uses the tag to find 639 ->writepages method. It uses the tag to find dirty pages to call
716 ->writepage on. If mpage_writepages is not us 640 ->writepage on. If mpage_writepages is not used (i.e. the address
717 provides its own ->writepages) , the PAGECACHE 641 provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
718 unused. write_inode_now and sync_inode do use 642 unused. write_inode_now and sync_inode do use it (through
719 __sync_single_inode) to check if ->writepages 643 __sync_single_inode) to check if ->writepages has been successful in
720 writing out the whole address_space. 644 writing out the whole address_space.
721 645
722 The Writeback tag is used by filemap*wait* and 646 The Writeback tag is used by filemap*wait* and sync_page* functions, via
723 filemap_fdatawait_range, to wait for all write 647 filemap_fdatawait_range, to wait for all writeback to complete.
724 648
725 An address_space handler may attach extra info 649 An address_space handler may attach extra information to a page,
726 typically using the 'private' field in the 'st 650 typically using the 'private' field in the 'struct page'. If such
727 information is attached, the PG_Private flag s 651 information is attached, the PG_Private flag should be set. This will
728 cause various VM routines to make extra calls 652 cause various VM routines to make extra calls into the address_space
729 handler to deal with that data. 653 handler to deal with that data.
730 654
731 An address space acts as an intermediate betwe 655 An address space acts as an intermediate between storage and
732 application. Data is read into the address sp 656 application. Data is read into the address space a whole page at a
733 time, and provided to the application either b 657 time, and provided to the application either by copying of the page, or
734 by memory-mapping the page. Data is written i 658 by memory-mapping the page. Data is written into the address space by
735 the application, and then written-back to stor 659 the application, and then written-back to storage typically in whole
736 pages, however the address_space has finer con 660 pages, however the address_space has finer control of write sizes.
737 661
738 The read process essentially only requires 're 662 The read process essentially only requires 'read_folio'. The write
739 process is more complicated and uses write_beg 663 process is more complicated and uses write_begin/write_end or
740 dirty_folio to write data into the address_spa 664 dirty_folio to write data into the address_space, and writepage and
741 writepages to writeback data to storage. 665 writepages to writeback data to storage.
742 666
743 Adding and removing pages to/from an address_s 667 Adding and removing pages to/from an address_space is protected by the
744 inode's i_mutex. 668 inode's i_mutex.
745 669
746 When data is written to a page, the PG_Dirty f 670 When data is written to a page, the PG_Dirty flag should be set. It
747 typically remains set until writepage asks for 671 typically remains set until writepage asks for it to be written. This
748 should clear PG_Dirty and set PG_Writeback. I 672 should clear PG_Dirty and set PG_Writeback. It can be actually written
749 at any point after PG_Dirty is clear. Once it 673 at any point after PG_Dirty is clear. Once it is known to be safe,
750 PG_Writeback is cleared. 674 PG_Writeback is cleared.
751 675
752 Writeback makes use of a writeback_control str 676 Writeback makes use of a writeback_control structure to direct the
753 operations. This gives the writepage and writ 677 operations. This gives the writepage and writepages operations some
754 information about the nature of and reason for 678 information about the nature of and reason for the writeback request,
755 and the constraints under which it is being do 679 and the constraints under which it is being done. It is also used to
756 return information back to the caller about th 680 return information back to the caller about the result of a writepage or
757 writepages request. 681 writepages request.
758 682
759 683
760 Handling errors during writeback 684 Handling errors during writeback
761 -------------------------------- 685 --------------------------------
762 686
763 Most applications that do buffered I/O will pe 687 Most applications that do buffered I/O will periodically call a file
764 synchronization call (fsync, fdatasync, msync 688 synchronization call (fsync, fdatasync, msync or sync_file_range) to
765 ensure that data written has made it to the ba 689 ensure that data written has made it to the backing store. When there
766 is an error during writeback, they expect that 690 is an error during writeback, they expect that error to be reported when
767 a file sync request is made. After an error h 691 a file sync request is made. After an error has been reported on one
768 request, subsequent requests on the same file 692 request, subsequent requests on the same file descriptor should return
769 0, unless further writeback errors have occurr 693 0, unless further writeback errors have occurred since the previous file
770 synchronization. !! 694 syncronization.
771 695
772 Ideally, the kernel would report errors only o 696 Ideally, the kernel would report errors only on file descriptions on
773 which writes were done that subsequently faile 697 which writes were done that subsequently failed to be written back. The
774 generic pagecache infrastructure does not trac 698 generic pagecache infrastructure does not track the file descriptions
775 that have dirtied each individual page however 699 that have dirtied each individual page however, so determining which
776 file descriptors should get back an error is n 700 file descriptors should get back an error is not possible.
777 701
778 Instead, the generic writeback error tracking 702 Instead, the generic writeback error tracking infrastructure in the
779 kernel settles for reporting errors to fsync o 703 kernel settles for reporting errors to fsync on all file descriptions
780 that were open at the time that the error occu 704 that were open at the time that the error occurred. In a situation with
781 multiple writers, all of them will get back an 705 multiple writers, all of them will get back an error on a subsequent
782 fsync, even if all of the writes done through 706 fsync, even if all of the writes done through that particular file
783 descriptor succeeded (or even if there were no 707 descriptor succeeded (or even if there were no writes on that file
784 descriptor at all). 708 descriptor at all).
785 709
786 Filesystems that wish to use this infrastructu 710 Filesystems that wish to use this infrastructure should call
787 mapping_set_error to record the error in the a 711 mapping_set_error to record the error in the address_space when it
788 occurs. Then, after writing back data from th 712 occurs. Then, after writing back data from the pagecache in their
789 file->fsync operation, they should call file_c 713 file->fsync operation, they should call file_check_and_advance_wb_err to
790 ensure that the struct file's error cursor has 714 ensure that the struct file's error cursor has advanced to the correct
791 point in the stream of errors emitted by the b 715 point in the stream of errors emitted by the backing device(s).
792 716
793 717
794 struct address_space_operations 718 struct address_space_operations
795 ------------------------------- 719 -------------------------------
796 720
797 This describes how the VFS can manipulate mapp 721 This describes how the VFS can manipulate mapping of a file to page
798 cache in your filesystem. The following membe 722 cache in your filesystem. The following members are defined:
799 723
800 .. code-block:: c 724 .. code-block:: c
801 725
802 struct address_space_operations { 726 struct address_space_operations {
803 int (*writepage)(struct page * 727 int (*writepage)(struct page *page, struct writeback_control *wbc);
804 int (*read_folio)(struct file 728 int (*read_folio)(struct file *, struct folio *);
805 int (*writepages)(struct addre 729 int (*writepages)(struct address_space *, struct writeback_control *);
806 bool (*dirty_folio)(struct add 730 bool (*dirty_folio)(struct address_space *, struct folio *);
807 void (*readahead)(struct reada 731 void (*readahead)(struct readahead_control *);
808 int (*write_begin)(struct file 732 int (*write_begin)(struct file *, struct address_space *mapping,
809 loff_t pos, 733 loff_t pos, unsigned len,
810 struct page ** 734 struct page **pagep, void **fsdata);
811 int (*write_end)(struct file * 735 int (*write_end)(struct file *, struct address_space *mapping,
812 loff_t pos, u 736 loff_t pos, unsigned len, unsigned copied,
813 struct folio !! 737 struct page *page, void *fsdata);
814 sector_t (*bmap)(struct addres 738 sector_t (*bmap)(struct address_space *, sector_t);
815 void (*invalidate_folio) (stru 739 void (*invalidate_folio) (struct folio *, size_t start, size_t len);
816 bool (*release_folio)(struct f 740 bool (*release_folio)(struct folio *, gfp_t);
817 void (*free_folio)(struct foli 741 void (*free_folio)(struct folio *);
818 ssize_t (*direct_IO)(struct ki 742 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
819 int (*migrate_folio)(struct ma !! 743 /* isolate a page for migration */
820 struct folio * !! 744 bool (*isolate_page) (struct page *, isolate_mode_t);
>> 745 /* migrate the contents of a page to the specified target */
>> 746 int (*migratepage) (struct page *, struct page *);
>> 747 /* put migration-failed page back to right list */
>> 748 void (*putback_page) (struct page *);
821 int (*launder_folio) (struct f 749 int (*launder_folio) (struct folio *);
822 750
823 bool (*is_partially_uptodate) 751 bool (*is_partially_uptodate) (struct folio *, size_t from,
824 752 size_t count);
825 void (*is_dirty_writeback)(str 753 void (*is_dirty_writeback)(struct folio *, bool *, bool *);
826 int (*error_remove_folio)(stru !! 754 int (*error_remove_page) (struct mapping *mapping, struct page *page);
827 int (*swap_activate)(struct sw 755 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
828 int (*swap_deactivate)(struct 756 int (*swap_deactivate)(struct file *);
829 int (*swap_rw)(struct kiocb *i 757 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
830 }; 758 };
831 759
832 ``writepage`` 760 ``writepage``
833 called by the VM to write a dirty page 761 called by the VM to write a dirty page to backing store. This
834 may happen for data integrity reasons 762 may happen for data integrity reasons (i.e. 'sync'), or to free
835 up memory (flush). The difference can 763 up memory (flush). The difference can be seen in
836 wbc->sync_mode. The PG_Dirty flag has 764 wbc->sync_mode. The PG_Dirty flag has been cleared and
837 PageLocked is true. writepage should 765 PageLocked is true. writepage should start writeout, should set
838 PG_Writeback, and should make sure the 766 PG_Writeback, and should make sure the page is unlocked, either
839 synchronously or asynchronously when t 767 synchronously or asynchronously when the write operation
840 completes. 768 completes.
841 769
842 If wbc->sync_mode is WB_SYNC_NONE, ->w 770 If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
843 try too hard if there are problems, an 771 try too hard if there are problems, and may choose to write out
844 other pages from the mapping if that i 772 other pages from the mapping if that is easier (e.g. due to
845 internal dependencies). If it chooses 773 internal dependencies). If it chooses not to start writeout, it
846 should return AOP_WRITEPAGE_ACTIVATE s 774 should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
847 keep calling ->writepage on that page. 775 keep calling ->writepage on that page.
848 776
849 See the file "Locking" for more detail 777 See the file "Locking" for more details.
850 778
851 ``read_folio`` 779 ``read_folio``
852 Called by the page cache to read a fol !! 780 called by the VM to read a folio from backing store. The folio
853 The 'file' argument supplies authentic !! 781 will be locked when read_folio is called, and should be unlocked
854 filesystems, and is generally not used !! 782 and marked uptodate once the read completes. If ->read_folio
855 It may be NULL if the caller does not !! 783 discovers that it cannot perform the I/O at this time, it can
856 the kernel is performing a read for it !! 784 unlock the folio and return AOP_TRUNCATED_PAGE. In this case,
857 of a userspace process with an open fi !! 785 the folio will be looked up again, relocked and if that all succeeds,
858 !! 786 ->read_folio will be called again.
859 If the mapping does not support large <<
860 contain a single page. The folio will <<
861 is called. If the read completes succ <<
862 be marked uptodate. The filesystem sh <<
863 once the read has completed, whether i <<
864 The filesystem does not need to modify <<
865 the page cache holds a reference count <<
866 released until the folio is unlocked. <<
867 <<
868 Filesystems may implement ->read_folio <<
869 In normal operation, folios are read t <<
870 method. Only if this fails, or if the <<
871 the read to complete will the page cac <<
872 Filesystems should not attempt to perf <<
873 in the ->read_folio() operation. <<
874 <<
875 If the filesystem cannot perform the r <<
876 unlock the folio, do whatever action i <<
877 read will succeed in the future and re <<
878 In this case, the caller should look u <<
879 and call ->read_folio again. <<
880 <<
881 Callers may invoke the ->read_folio() <<
882 read_mapping_folio() will take care of <<
883 read to complete and handle cases such <<
884 787
885 ``writepages`` 788 ``writepages``
886 called by the VM to write out pages as 789 called by the VM to write out pages associated with the
887 address_space object. If wbc->sync_mo 790 address_space object. If wbc->sync_mode is WB_SYNC_ALL, then
888 the writeback_control will specify a r 791 the writeback_control will specify a range of pages that must be
889 written out. If it is WB_SYNC_NONE, t 792 written out. If it is WB_SYNC_NONE, then a nr_to_write is
890 given and that many pages should be wr 793 given and that many pages should be written if possible. If no
891 ->writepages is given, then mpage_writ 794 ->writepages is given, then mpage_writepages is used instead.
892 This will choose pages from the addres 795 This will choose pages from the address space that are tagged as
893 DIRTY and will pass them to ->writepag 796 DIRTY and will pass them to ->writepage.
894 797
895 ``dirty_folio`` 798 ``dirty_folio``
896 called by the VM to mark a folio as di 799 called by the VM to mark a folio as dirty. This is particularly
897 needed if an address space attaches pr 800 needed if an address space attaches private data to a folio, and
898 that data needs to be updated when a f 801 that data needs to be updated when a folio is dirtied. This is
899 called, for example, when a memory map 802 called, for example, when a memory mapped page gets modified.
900 If defined, it should set the folio di 803 If defined, it should set the folio dirty flag, and the
901 PAGECACHE_TAG_DIRTY search mark in i_p 804 PAGECACHE_TAG_DIRTY search mark in i_pages.
902 805
903 ``readahead`` 806 ``readahead``
904 Called by the VM to read pages associa 807 Called by the VM to read pages associated with the address_space
905 object. The pages are consecutive in 808 object. The pages are consecutive in the page cache and are
906 locked. The implementation should dec 809 locked. The implementation should decrement the page refcount
907 after starting I/O on each page. Usua 810 after starting I/O on each page. Usually the page will be
908 unlocked by the I/O completion handler 811 unlocked by the I/O completion handler. The set of pages are
909 divided into some sync pages followed 812 divided into some sync pages followed by some async pages,
910 rac->ra->async_size gives the number o 813 rac->ra->async_size gives the number of async pages. The
911 filesystem should attempt to read all 814 filesystem should attempt to read all sync pages but may decide
912 to stop once it reaches the async page 815 to stop once it reaches the async pages. If it does decide to
913 stop attempting I/O, it can simply ret 816 stop attempting I/O, it can simply return. The caller will
914 remove the remaining pages from the ad 817 remove the remaining pages from the address space, unlock them
915 and decrement the page refcount. Set 818 and decrement the page refcount. Set PageUptodate if the I/O
916 completes successfully. !! 819 completes successfully. Setting PageError on any page will be
>> 820 ignored; simply unlock the page if an I/O error occurs.
917 821
918 ``write_begin`` 822 ``write_begin``
919 Called by the generic buffered write c 823 Called by the generic buffered write code to ask the filesystem
920 to prepare to write len bytes at the g 824 to prepare to write len bytes at the given offset in the file.
921 The address_space should check that th 825 The address_space should check that the write will be able to
922 complete, by allocating space if neces 826 complete, by allocating space if necessary and doing any other
923 internal housekeeping. If the write w 827 internal housekeeping. If the write will update parts of any
924 basic-blocks on storage, then those bl 828 basic-blocks on storage, then those blocks should be pre-read
925 (if they haven't been read already) so 829 (if they haven't been read already) so that the updated blocks
926 can be written out properly. 830 can be written out properly.
927 831
928 The filesystem must return the locked !! 832 The filesystem must return the locked pagecache page for the
929 specified offset, in ``*foliop``, for !! 833 specified offset, in ``*pagep``, for the caller to write into.
930 834
931 It must be able to cope with short wri 835 It must be able to cope with short writes (where the length
932 passed to write_begin is greater than 836 passed to write_begin is greater than the number of bytes copied
933 into the folio). !! 837 into the page).
934 838
935 A void * may be returned in fsdata, wh 839 A void * may be returned in fsdata, which then gets passed into
936 write_end. 840 write_end.
937 841
938 Returns 0 on success; < 0 on failure ( 842 Returns 0 on success; < 0 on failure (which is the error code),
939 in which case write_end is not called. 843 in which case write_end is not called.
940 844
941 ``write_end`` 845 ``write_end``
942 After a successful write_begin, and da 846 After a successful write_begin, and data copy, write_end must be
943 called. len is the original len passe 847 called. len is the original len passed to write_begin, and
944 copied is the amount that was able to 848 copied is the amount that was able to be copied.
945 849
946 The filesystem must take care of unloc !! 850 The filesystem must take care of unlocking the page and
947 decrementing its refcount, and updatin !! 851 releasing it refcount, and updating i_size.
948 852
949 Returns < 0 on failure, otherwise the 853 Returns < 0 on failure, otherwise the number of bytes (<=
950 'copied') that were able to be copied 854 'copied') that were able to be copied into pagecache.
951 855
952 ``bmap`` 856 ``bmap``
953 called by the VFS to map a logical blo 857 called by the VFS to map a logical block offset within object to
954 physical block number. This method is 858 physical block number. This method is used by the FIBMAP ioctl
955 and for working with swap-files. To b 859 and for working with swap-files. To be able to swap to a file,
956 the file must have a stable mapping to 860 the file must have a stable mapping to a block device. The swap
957 system does not go through the filesys 861 system does not go through the filesystem but instead uses bmap
958 to find out where the blocks in the fi 862 to find out where the blocks in the file are and uses those
959 addresses directly. 863 addresses directly.
960 864
961 ``invalidate_folio`` 865 ``invalidate_folio``
962 If a folio has private data, then inva 866 If a folio has private data, then invalidate_folio will be
963 called when part or all of the folio i 867 called when part or all of the folio is to be removed from the
964 address space. This generally corresp 868 address space. This generally corresponds to either a
965 truncation, punch hole or a complete i 869 truncation, punch hole or a complete invalidation of the address
966 space (in the latter case 'offset' wil 870 space (in the latter case 'offset' will always be 0 and 'length'
967 will be folio_size()). Any private da 871 will be folio_size()). Any private data associated with the folio
968 should be updated to reflect this trun 872 should be updated to reflect this truncation. If offset is 0
969 and length is folio_size(), then the p 873 and length is folio_size(), then the private data should be
970 released, because the folio must be ab 874 released, because the folio must be able to be completely
971 discarded. This may be done by callin 875 discarded. This may be done by calling the ->release_folio
972 function, but in this case the release 876 function, but in this case the release MUST succeed.
973 877
974 ``release_folio`` 878 ``release_folio``
975 release_folio is called on folios with 879 release_folio is called on folios with private data to tell the
976 filesystem that the folio is about to 880 filesystem that the folio is about to be freed. ->release_folio
977 should remove any private data from th 881 should remove any private data from the folio and clear the
978 private flag. If release_folio() fail 882 private flag. If release_folio() fails, it should return false.
979 release_folio() is used in two distinc 883 release_folio() is used in two distinct though related cases.
980 The first is when the VM wants to free 884 The first is when the VM wants to free a clean folio with no
981 active users. If ->release_folio succ 885 active users. If ->release_folio succeeds, the folio will be
982 removed from the address_space and be 886 removed from the address_space and be freed.
983 887
984 The second case is when a request has 888 The second case is when a request has been made to invalidate
985 some or all folios in an address_space 889 some or all folios in an address_space. This can happen
986 through the fadvise(POSIX_FADV_DONTNEE 890 through the fadvise(POSIX_FADV_DONTNEED) system call or by the
987 filesystem explicitly requesting it as 891 filesystem explicitly requesting it as nfs and 9p do (when they
988 believe the cache may be out of date w 892 believe the cache may be out of date with storage) by calling
989 invalidate_inode_pages2(). If the fil 893 invalidate_inode_pages2(). If the filesystem makes such a call,
990 and needs to be certain that all folio 894 and needs to be certain that all folios are invalidated, then
991 its release_folio will need to ensure 895 its release_folio will need to ensure this. Possibly it can
992 clear the uptodate flag if it cannot f 896 clear the uptodate flag if it cannot free private data yet.
993 897
994 ``free_folio`` 898 ``free_folio``
995 free_folio is called once the folio is 899 free_folio is called once the folio is no longer visible in the
996 page cache in order to allow the clean 900 page cache in order to allow the cleanup of any private data.
997 Since it may be called by the memory r 901 Since it may be called by the memory reclaimer, it should not
998 assume that the original address_space 902 assume that the original address_space mapping still exists, and
999 it should not block. 903 it should not block.
1000 904
1001 ``direct_IO`` 905 ``direct_IO``
1002 called by the generic read/write rout 906 called by the generic read/write routines to perform direct_IO -
1003 that is IO requests which bypass the 907 that is IO requests which bypass the page cache and transfer
1004 data directly between the storage and 908 data directly between the storage and the application's address
1005 space. 909 space.
1006 910
1007 ``migrate_folio`` !! 911 ``isolate_page``
>> 912 Called by the VM when isolating a movable non-lru page. If page
>> 913 is successfully isolated, VM marks the page as PG_isolated via
>> 914 __SetPageIsolated.
>> 915
>> 916 ``migrate_page``
1008 This is used to compact the physical 917 This is used to compact the physical memory usage. If the VM
1009 wants to relocate a folio (maybe from !! 918 wants to relocate a page (maybe off a memory card that is
1010 signalling imminent failure) it will !! 919 signalling imminent failure) it will pass a new page and an old
1011 folio to this function. migrate_foli !! 920 page to this function. migrate_page should transfer any private
1012 data across and update any references !! 921 data across and update any references that it has to the page.
>> 922
>> 923 ``putback_page``
>> 924 Called by the VM when isolated page's migration fails.
1013 925
1014 ``launder_folio`` 926 ``launder_folio``
1015 Called before freeing a folio - it wr 927 Called before freeing a folio - it writes back the dirty folio.
1016 To prevent redirtying the folio, it i 928 To prevent redirtying the folio, it is kept locked during the
1017 whole operation. 929 whole operation.
1018 930
1019 ``is_partially_uptodate`` 931 ``is_partially_uptodate``
1020 Called by the VM when reading a file 932 Called by the VM when reading a file through the pagecache when
1021 the underlying blocksize is smaller t 933 the underlying blocksize is smaller than the size of the folio.
1022 If the required block is up to date t 934 If the required block is up to date then the read can complete
1023 without needing I/O to bring the whol 935 without needing I/O to bring the whole page up to date.
1024 936
1025 ``is_dirty_writeback`` 937 ``is_dirty_writeback``
1026 Called by the VM when attempting to r 938 Called by the VM when attempting to reclaim a folio. The VM uses
1027 dirty and writeback information to de 939 dirty and writeback information to determine if it needs to
1028 stall to allow flushers a chance to c 940 stall to allow flushers a chance to complete some IO.
1029 Ordinarily it can use folio_test_dirt 941 Ordinarily it can use folio_test_dirty and folio_test_writeback but
1030 some filesystems have more complex st 942 some filesystems have more complex state (unstable folios in NFS
1031 prevent reclaim) or do not set those 943 prevent reclaim) or do not set those flags due to locking
1032 problems. This callback allows a fil 944 problems. This callback allows a filesystem to indicate to the
1033 VM if a folio should be treated as di 945 VM if a folio should be treated as dirty or writeback for the
1034 purposes of stalling. 946 purposes of stalling.
1035 947
1036 ``error_remove_folio`` !! 948 ``error_remove_page``
1037 normally set to generic_error_remove_ !! 949 normally set to generic_error_remove_page if truncation is ok
1038 for this address space. Used for mem 950 for this address space. Used for memory failure handling.
1039 Setting this implies you deal with pa 951 Setting this implies you deal with pages going away under you,
1040 unless you have them locked or refere 952 unless you have them locked or reference counts increased.
1041 953
1042 ``swap_activate`` 954 ``swap_activate``
1043 955
1044 Called to prepare the given file for 956 Called to prepare the given file for swap. It should perform
1045 any validation and preparation necess 957 any validation and preparation necessary to ensure that writes
1046 can be performed with minimal memory 958 can be performed with minimal memory allocation. It should call
1047 add_swap_extent(), or the helper ioma 959 add_swap_extent(), or the helper iomap_swapfile_activate(), and
1048 return the number of extents added. 960 return the number of extents added. If IO should be submitted
1049 through ->swap_rw(), it should set SW 961 through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
1050 be submitted directly to the block de 962 be submitted directly to the block device ``sis->bdev``.
1051 963
1052 ``swap_deactivate`` 964 ``swap_deactivate``
1053 Called during swapoff on files where 965 Called during swapoff on files where swap_activate was
1054 successful. 966 successful.
1055 967
1056 ``swap_rw`` 968 ``swap_rw``
1057 Called to read or write swap pages wh 969 Called to read or write swap pages when SWP_FS_OPS is set.
1058 970
1059 The File Object 971 The File Object
1060 =============== 972 ===============
1061 973
1062 A file object represents a file opened by a p 974 A file object represents a file opened by a process. This is also known
1063 as an "open file description" in POSIX parlan 975 as an "open file description" in POSIX parlance.
1064 976
1065 977
1066 struct file_operations 978 struct file_operations
1067 ---------------------- 979 ----------------------
1068 980
1069 This describes how the VFS can manipulate an 981 This describes how the VFS can manipulate an open file. As of kernel
1070 4.18, the following members are defined: 982 4.18, the following members are defined:
1071 983
1072 .. code-block:: c 984 .. code-block:: c
1073 985
1074 struct file_operations { 986 struct file_operations {
1075 struct module *owner; 987 struct module *owner;
1076 loff_t (*llseek) (struct file 988 loff_t (*llseek) (struct file *, loff_t, int);
1077 ssize_t (*read) (struct file 989 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1078 ssize_t (*write) (struct file 990 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1079 ssize_t (*read_iter) (struct 991 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1080 ssize_t (*write_iter) (struct 992 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1081 int (*iopoll)(struct kiocb *k 993 int (*iopoll)(struct kiocb *kiocb, bool spin);
>> 994 int (*iterate) (struct file *, struct dir_context *);
1082 int (*iterate_shared) (struct 995 int (*iterate_shared) (struct file *, struct dir_context *);
1083 __poll_t (*poll) (struct file 996 __poll_t (*poll) (struct file *, struct poll_table_struct *);
1084 long (*unlocked_ioctl) (struc 997 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1085 long (*compat_ioctl) (struct 998 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1086 int (*mmap) (struct file *, s 999 int (*mmap) (struct file *, struct vm_area_struct *);
1087 int (*open) (struct inode *, 1000 int (*open) (struct inode *, struct file *);
1088 int (*flush) (struct file *, 1001 int (*flush) (struct file *, fl_owner_t id);
1089 int (*release) (struct inode 1002 int (*release) (struct inode *, struct file *);
1090 int (*fsync) (struct file *, 1003 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1091 int (*fasync) (int, struct fi 1004 int (*fasync) (int, struct file *, int);
1092 int (*lock) (struct file *, i 1005 int (*lock) (struct file *, int, struct file_lock *);
>> 1006 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1093 unsigned long (*get_unmapped_ 1007 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1094 int (*check_flags)(int); 1008 int (*check_flags)(int);
1095 int (*flock) (struct file *, 1009 int (*flock) (struct file *, int, struct file_lock *);
1096 ssize_t (*splice_write)(struc 1010 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
1097 ssize_t (*splice_read)(struct 1011 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
1098 int (*setlease)(struct file * 1012 int (*setlease)(struct file *, long, struct file_lock **, void **);
1099 long (*fallocate)(struct file 1013 long (*fallocate)(struct file *file, int mode, loff_t offset,
1100 loff_t len) 1014 loff_t len);
1101 void (*show_fdinfo)(struct se 1015 void (*show_fdinfo)(struct seq_file *m, struct file *f);
1102 #ifndef CONFIG_MMU 1016 #ifndef CONFIG_MMU
1103 unsigned (*mmap_capabilities) 1017 unsigned (*mmap_capabilities)(struct file *);
1104 #endif 1018 #endif
1105 ssize_t (*copy_file_range)(st 1019 ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
1106 loff_t (*remap_file_range)(st 1020 loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
1107 st 1021 struct file *file_out, loff_t pos_out,
1108 lo 1022 loff_t len, unsigned int remap_flags);
1109 int (*fadvise)(struct file *, 1023 int (*fadvise)(struct file *, loff_t, loff_t, int);
1110 }; 1024 };
1111 1025
1112 Again, all methods are called without any loc 1026 Again, all methods are called without any locks being held, unless
1113 otherwise noted. 1027 otherwise noted.
1114 1028
1115 ``llseek`` 1029 ``llseek``
1116 called when the VFS needs to move the 1030 called when the VFS needs to move the file position index
1117 1031
1118 ``read`` 1032 ``read``
1119 called by read(2) and related system 1033 called by read(2) and related system calls
1120 1034
1121 ``read_iter`` 1035 ``read_iter``
1122 possibly asynchronous read with iov_i 1036 possibly asynchronous read with iov_iter as destination
1123 1037
1124 ``write`` 1038 ``write``
1125 called by write(2) and related system 1039 called by write(2) and related system calls
1126 1040
1127 ``write_iter`` 1041 ``write_iter``
1128 possibly asynchronous write with iov_ 1042 possibly asynchronous write with iov_iter as source
1129 1043
1130 ``iopoll`` 1044 ``iopoll``
1131 called when aio wants to poll for com 1045 called when aio wants to poll for completions on HIPRI iocbs
1132 1046
1133 ``iterate_shared`` !! 1047 ``iterate``
1134 called when the VFS needs to read the 1048 called when the VFS needs to read the directory contents
1135 1049
>> 1050 ``iterate_shared``
>> 1051 called when the VFS needs to read the directory contents when
>> 1052 filesystem supports concurrent dir iterators
>> 1053
1136 ``poll`` 1054 ``poll``
1137 called by the VFS when a process want 1055 called by the VFS when a process wants to check if there is
1138 activity on this file and (optionally 1056 activity on this file and (optionally) go to sleep until there
1139 is activity. Called by the select(2) 1057 is activity. Called by the select(2) and poll(2) system calls
1140 1058
1141 ``unlocked_ioctl`` 1059 ``unlocked_ioctl``
1142 called by the ioctl(2) system call. 1060 called by the ioctl(2) system call.
1143 1061
1144 ``compat_ioctl`` 1062 ``compat_ioctl``
1145 called by the ioctl(2) system call wh 1063 called by the ioctl(2) system call when 32 bit system calls are
1146 used on 64 bit kernels. 1064 used on 64 bit kernels.
1147 1065
1148 ``mmap`` 1066 ``mmap``
1149 called by the mmap(2) system call 1067 called by the mmap(2) system call
1150 1068
1151 ``open`` 1069 ``open``
1152 called by the VFS when an inode shoul 1070 called by the VFS when an inode should be opened. When the VFS
1153 opens a file, it creates a new "struc 1071 opens a file, it creates a new "struct file". It then calls the
1154 open method for the newly allocated f 1072 open method for the newly allocated file structure. You might
1155 think that the open method really bel 1073 think that the open method really belongs in "struct
1156 inode_operations", and you may be rig 1074 inode_operations", and you may be right. I think it's done the
1157 way it is because it makes filesystem 1075 way it is because it makes filesystems simpler to implement.
1158 The open() method is a good place to 1076 The open() method is a good place to initialize the
1159 "private_data" member in the file str 1077 "private_data" member in the file structure if you want to point
1160 to a device structure 1078 to a device structure
1161 1079
1162 ``flush`` 1080 ``flush``
1163 called by the close(2) system call to 1081 called by the close(2) system call to flush a file
1164 1082
1165 ``release`` 1083 ``release``
1166 called when the last reference to an 1084 called when the last reference to an open file is closed
1167 1085
1168 ``fsync`` 1086 ``fsync``
1169 called by the fsync(2) system call. 1087 called by the fsync(2) system call. Also see the section above
1170 entitled "Handling errors during writ 1088 entitled "Handling errors during writeback".
1171 1089
1172 ``fasync`` 1090 ``fasync``
1173 called by the fcntl(2) system call wh 1091 called by the fcntl(2) system call when asynchronous
1174 (non-blocking) mode is enabled for a 1092 (non-blocking) mode is enabled for a file
1175 1093
1176 ``lock`` 1094 ``lock``
1177 called by the fcntl(2) system call fo 1095 called by the fcntl(2) system call for F_GETLK, F_SETLK, and
1178 F_SETLKW commands 1096 F_SETLKW commands
1179 1097
1180 ``get_unmapped_area`` 1098 ``get_unmapped_area``
1181 called by the mmap(2) system call 1099 called by the mmap(2) system call
1182 1100
1183 ``check_flags`` 1101 ``check_flags``
1184 called by the fcntl(2) system call fo 1102 called by the fcntl(2) system call for F_SETFL command
1185 1103
1186 ``flock`` 1104 ``flock``
1187 called by the flock(2) system call 1105 called by the flock(2) system call
1188 1106
1189 ``splice_write`` 1107 ``splice_write``
1190 called by the VFS to splice data from 1108 called by the VFS to splice data from a pipe to a file. This
1191 method is used by the splice(2) syste 1109 method is used by the splice(2) system call
1192 1110
1193 ``splice_read`` 1111 ``splice_read``
1194 called by the VFS to splice data from 1112 called by the VFS to splice data from file to a pipe. This
1195 method is used by the splice(2) syste 1113 method is used by the splice(2) system call
1196 1114
1197 ``setlease`` 1115 ``setlease``
1198 called by the VFS to set or release a 1116 called by the VFS to set or release a file lock lease. setlease
1199 implementations should call generic_s 1117 implementations should call generic_setlease to record or remove
1200 the lease in the inode after setting 1118 the lease in the inode after setting it.
1201 1119
1202 ``fallocate`` 1120 ``fallocate``
1203 called by the VFS to preallocate bloc 1121 called by the VFS to preallocate blocks or punch a hole.
1204 1122
1205 ``copy_file_range`` 1123 ``copy_file_range``
1206 called by the copy_file_range(2) syst 1124 called by the copy_file_range(2) system call.
1207 1125
1208 ``remap_file_range`` 1126 ``remap_file_range``
1209 called by the ioctl(2) system call fo 1127 called by the ioctl(2) system call for FICLONERANGE and FICLONE
1210 and FIDEDUPERANGE commands to remap f 1128 and FIDEDUPERANGE commands to remap file ranges. An
1211 implementation should remap len bytes 1129 implementation should remap len bytes at pos_in of the source
1212 file into the dest file at pos_out. 1130 file into the dest file at pos_out. Implementations must handle
1213 callers passing in len == 0; this mea 1131 callers passing in len == 0; this means "remap to the end of the
1214 source file". The return value shoul 1132 source file". The return value should the number of bytes
1215 remapped, or the usual negative error 1133 remapped, or the usual negative error code if errors occurred
1216 before any bytes were remapped. The 1134 before any bytes were remapped. The remap_flags parameter
1217 accepts REMAP_FILE_* flags. If REMAP 1135 accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
1218 implementation must only remap if the 1136 implementation must only remap if the requested file ranges have
1219 identical contents. If REMAP_FILE_CA 1137 identical contents. If REMAP_FILE_CAN_SHORTEN is set, the caller is
1220 ok with the implementation shortening 1138 ok with the implementation shortening the request length to
1221 satisfy alignment or EOF requirements 1139 satisfy alignment or EOF requirements (or any other reason).
1222 1140
1223 ``fadvise`` 1141 ``fadvise``
1224 possibly called by the fadvise64() sy 1142 possibly called by the fadvise64() system call.
1225 1143
1226 Note that the file operations are implemented 1144 Note that the file operations are implemented by the specific
1227 filesystem in which the inode resides. When 1145 filesystem in which the inode resides. When opening a device node
1228 (character or block special) most filesystems 1146 (character or block special) most filesystems will call special
1229 support routines in the VFS which will locate 1147 support routines in the VFS which will locate the required device
1230 driver information. These support routines r 1148 driver information. These support routines replace the filesystem file
1231 operations with those for the device driver, 1149 operations with those for the device driver, and then proceed to call
1232 the new open() method for the file. This is 1150 the new open() method for the file. This is how opening a device file
1233 in the filesystem eventually ends up calling 1151 in the filesystem eventually ends up calling the device driver open()
1234 method. 1152 method.
1235 1153
1236 1154
1237 Directory Entry Cache (dcache) 1155 Directory Entry Cache (dcache)
1238 ============================== 1156 ==============================
1239 1157
1240 1158
1241 struct dentry_operations 1159 struct dentry_operations
1242 ------------------------ 1160 ------------------------
1243 1161
1244 This describes how a filesystem can overload 1162 This describes how a filesystem can overload the standard dentry
1245 operations. Dentries and the dcache are the 1163 operations. Dentries and the dcache are the domain of the VFS and the
1246 individual filesystem implementations. Devic 1164 individual filesystem implementations. Device drivers have no business
1247 here. These methods may be set to NULL, as t 1165 here. These methods may be set to NULL, as they are either optional or
1248 the VFS uses a default. As of kernel 2.6.22, 1166 the VFS uses a default. As of kernel 2.6.22, the following members are
1249 defined: 1167 defined:
1250 1168
1251 .. code-block:: c 1169 .. code-block:: c
1252 1170
1253 struct dentry_operations { 1171 struct dentry_operations {
1254 int (*d_revalidate)(struct de 1172 int (*d_revalidate)(struct dentry *, unsigned int);
1255 int (*d_weak_revalidate)(stru 1173 int (*d_weak_revalidate)(struct dentry *, unsigned int);
1256 int (*d_hash)(const struct de 1174 int (*d_hash)(const struct dentry *, struct qstr *);
1257 int (*d_compare)(const struct 1175 int (*d_compare)(const struct dentry *,
1258 unsigned int 1176 unsigned int, const char *, const struct qstr *);
1259 int (*d_delete)(const struct 1177 int (*d_delete)(const struct dentry *);
1260 int (*d_init)(struct dentry * 1178 int (*d_init)(struct dentry *);
1261 void (*d_release)(struct dent 1179 void (*d_release)(struct dentry *);
1262 void (*d_iput)(struct dentry 1180 void (*d_iput)(struct dentry *, struct inode *);
1263 char *(*d_dname)(struct dentr 1181 char *(*d_dname)(struct dentry *, char *, int);
1264 struct vfsmount *(*d_automoun 1182 struct vfsmount *(*d_automount)(struct path *);
1265 int (*d_manage)(const struct 1183 int (*d_manage)(const struct path *, bool);
1266 struct dentry *(*d_real)(stru !! 1184 struct dentry *(*d_real)(struct dentry *, const struct inode *);
1267 }; 1185 };
1268 1186
1269 ``d_revalidate`` 1187 ``d_revalidate``
1270 called when the VFS needs to revalida 1188 called when the VFS needs to revalidate a dentry. This is
1271 called whenever a name look-up finds 1189 called whenever a name look-up finds a dentry in the dcache.
1272 Most local filesystems leave this as 1190 Most local filesystems leave this as NULL, because all their
1273 dentries in the dcache are valid. Ne 1191 dentries in the dcache are valid. Network filesystems are
1274 different since things can change on 1192 different since things can change on the server without the
1275 client necessarily being aware of it. 1193 client necessarily being aware of it.
1276 1194
1277 This function should return a positiv 1195 This function should return a positive value if the dentry is
1278 still valid, and zero or a negative e 1196 still valid, and zero or a negative error code if it isn't.
1279 1197
1280 d_revalidate may be called in rcu-wal 1198 d_revalidate may be called in rcu-walk mode (flags &
1281 LOOKUP_RCU). If in rcu-walk mode, th 1199 LOOKUP_RCU). If in rcu-walk mode, the filesystem must
1282 revalidate the dentry without blockin 1200 revalidate the dentry without blocking or storing to the dentry,
1283 d_parent and d_inode should not be us 1201 d_parent and d_inode should not be used without care (because
1284 they can change and, in d_inode case, 1202 they can change and, in d_inode case, even become NULL under
1285 us). 1203 us).
1286 1204
1287 If a situation is encountered that rc 1205 If a situation is encountered that rcu-walk cannot handle,
1288 return 1206 return
1289 -ECHILD and it will be called again i 1207 -ECHILD and it will be called again in ref-walk mode.
1290 1208
1291 ``d_weak_revalidate`` !! 1209 ``_weak_revalidate``
1292 called when the VFS needs to revalida 1210 called when the VFS needs to revalidate a "jumped" dentry. This
1293 is called when a path-walk ends at de 1211 is called when a path-walk ends at dentry that was not acquired
1294 by doing a lookup in the parent direc 1212 by doing a lookup in the parent directory. This includes "/",
1295 "." and "..", as well as procfs-style 1213 "." and "..", as well as procfs-style symlinks and mountpoint
1296 traversal. 1214 traversal.
1297 1215
1298 In this case, we are less concerned w 1216 In this case, we are less concerned with whether the dentry is
1299 still fully correct, but rather that 1217 still fully correct, but rather that the inode is still valid.
1300 As with d_revalidate, most local file 1218 As with d_revalidate, most local filesystems will set this to
1301 NULL since their dcache entries are a 1219 NULL since their dcache entries are always valid.
1302 1220
1303 This function has the same return cod 1221 This function has the same return code semantics as
1304 d_revalidate. 1222 d_revalidate.
1305 1223
1306 d_weak_revalidate is only called afte 1224 d_weak_revalidate is only called after leaving rcu-walk mode.
1307 1225
1308 ``d_hash`` 1226 ``d_hash``
1309 called when the VFS adds a dentry to 1227 called when the VFS adds a dentry to the hash table. The first
1310 dentry passed to d_hash is the parent 1228 dentry passed to d_hash is the parent directory that the name is
1311 to be hashed into. 1229 to be hashed into.
1312 1230
1313 Same locking and synchronisation rule 1231 Same locking and synchronisation rules as d_compare regarding
1314 what is safe to dereference etc. 1232 what is safe to dereference etc.
1315 1233
1316 ``d_compare`` 1234 ``d_compare``
1317 called to compare a dentry name with 1235 called to compare a dentry name with a given name. The first
1318 dentry is the parent of the dentry to 1236 dentry is the parent of the dentry to be compared, the second is
1319 the child dentry. len and name strin 1237 the child dentry. len and name string are properties of the
1320 dentry to be compared. qstr is the n 1238 dentry to be compared. qstr is the name to compare it with.
1321 1239
1322 Must be constant and idempotent, and 1240 Must be constant and idempotent, and should not take locks if
1323 possible, and should not or store int 1241 possible, and should not or store into the dentry. Should not
1324 dereference pointers outside the dent 1242 dereference pointers outside the dentry without lots of care
1325 (eg. d_parent, d_inode, d_name shoul 1243 (eg. d_parent, d_inode, d_name should not be used).
1326 1244
1327 However, our vfsmount is pinned, and 1245 However, our vfsmount is pinned, and RCU held, so the dentries
1328 and inodes won't disappear, neither w 1246 and inodes won't disappear, neither will our sb or filesystem
1329 module. ->d_sb may be used. 1247 module. ->d_sb may be used.
1330 1248
1331 It is a tricky calling convention bec 1249 It is a tricky calling convention because it needs to be called
1332 under "rcu-walk", ie. without any loc 1250 under "rcu-walk", ie. without any locks or references on things.
1333 1251
1334 ``d_delete`` 1252 ``d_delete``
1335 called when the last reference to a d 1253 called when the last reference to a dentry is dropped and the
1336 dcache is deciding whether or not to 1254 dcache is deciding whether or not to cache it. Return 1 to
1337 delete immediately, or 0 to cache the 1255 delete immediately, or 0 to cache the dentry. Default is NULL
1338 which means to always cache a reachab 1256 which means to always cache a reachable dentry. d_delete must
1339 be constant and idempotent. 1257 be constant and idempotent.
1340 1258
1341 ``d_init`` 1259 ``d_init``
1342 called when a dentry is allocated 1260 called when a dentry is allocated
1343 1261
1344 ``d_release`` 1262 ``d_release``
1345 called when a dentry is really deallo 1263 called when a dentry is really deallocated
1346 1264
1347 ``d_iput`` 1265 ``d_iput``
1348 called when a dentry loses its inode 1266 called when a dentry loses its inode (just prior to its being
1349 deallocated). The default when this 1267 deallocated). The default when this is NULL is that the VFS
1350 calls iput(). If you define this met 1268 calls iput(). If you define this method, you must call iput()
1351 yourself 1269 yourself
1352 1270
1353 ``d_dname`` 1271 ``d_dname``
1354 called when the pathname of a dentry 1272 called when the pathname of a dentry should be generated.
1355 Useful for some pseudo filesystems (s 1273 Useful for some pseudo filesystems (sockfs, pipefs, ...) to
1356 delay pathname generation. (Instead 1274 delay pathname generation. (Instead of doing it when dentry is
1357 created, it's done only when the path 1275 created, it's done only when the path is needed.). Real
1358 filesystems probably dont want to use 1276 filesystems probably dont want to use it, because their dentries
1359 are present in global dcache hash, so 1277 are present in global dcache hash, so their hash should be an
1360 invariant. As no lock is held, d_dna 1278 invariant. As no lock is held, d_dname() should not try to
1361 modify the dentry itself, unless appr 1279 modify the dentry itself, unless appropriate SMP safety is used.
1362 CAUTION : d_path() logic is quite tri 1280 CAUTION : d_path() logic is quite tricky. The correct way to
1363 return for example "Hello" is to put 1281 return for example "Hello" is to put it at the end of the
1364 buffer, and returns a pointer to the 1282 buffer, and returns a pointer to the first char.
1365 dynamic_dname() helper function is pr 1283 dynamic_dname() helper function is provided to take care of
1366 this. 1284 this.
1367 1285
1368 Example : 1286 Example :
1369 1287
1370 .. code-block:: c 1288 .. code-block:: c
1371 1289
1372 static char *pipefs_dname(struct dent 1290 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
1373 { 1291 {
1374 return dynamic_dname(dentry, 1292 return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
1375 dentry->d_ino 1293 dentry->d_inode->i_ino);
1376 } 1294 }
1377 1295
1378 ``d_automount`` 1296 ``d_automount``
1379 called when an automount dentry is to 1297 called when an automount dentry is to be traversed (optional).
1380 This should create a new VFS mount re 1298 This should create a new VFS mount record and return the record
1381 to the caller. The caller is supplie 1299 to the caller. The caller is supplied with a path parameter
1382 giving the automount directory to des 1300 giving the automount directory to describe the automount target
1383 and the parent VFS mount record to pr 1301 and the parent VFS mount record to provide inheritable mount
1384 parameters. NULL should be returned 1302 parameters. NULL should be returned if someone else managed to
1385 make the automount first. If the vfs 1303 make the automount first. If the vfsmount creation failed, then
1386 an error code should be returned. If 1304 an error code should be returned. If -EISDIR is returned, then
1387 the directory will be treated as an o 1305 the directory will be treated as an ordinary directory and
1388 returned to pathwalk to continue walk 1306 returned to pathwalk to continue walking.
1389 1307
1390 If a vfsmount is returned, the caller 1308 If a vfsmount is returned, the caller will attempt to mount it
1391 on the mountpoint and will remove the 1309 on the mountpoint and will remove the vfsmount from its
1392 expiration list in the case of failur 1310 expiration list in the case of failure. The vfsmount should be
1393 returned with 2 refs on it to prevent 1311 returned with 2 refs on it to prevent automatic expiration - the
1394 caller will clean up the additional r 1312 caller will clean up the additional ref.
1395 1313
1396 This function is only used if DCACHE_ 1314 This function is only used if DCACHE_NEED_AUTOMOUNT is set on
1397 the dentry. This is set by __d_insta 1315 the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
1398 set on the inode being added. 1316 set on the inode being added.
1399 1317
1400 ``d_manage`` 1318 ``d_manage``
1401 called to allow the filesystem to man 1319 called to allow the filesystem to manage the transition from a
1402 dentry (optional). This allows autof 1320 dentry (optional). This allows autofs, for example, to hold up
1403 clients waiting to explore behind a ' 1321 clients waiting to explore behind a 'mountpoint' while letting
1404 the daemon go past and construct the 1322 the daemon go past and construct the subtree there. 0 should be
1405 returned to let the calling process c 1323 returned to let the calling process continue. -EISDIR can be
1406 returned to tell pathwalk to use this 1324 returned to tell pathwalk to use this directory as an ordinary
1407 directory and to ignore anything moun 1325 directory and to ignore anything mounted on it and not to check
1408 the automount flag. Any other error 1326 the automount flag. Any other error code will abort pathwalk
1409 completely. 1327 completely.
1410 1328
1411 If the 'rcu_walk' parameter is true, 1329 If the 'rcu_walk' parameter is true, then the caller is doing a
1412 pathwalk in RCU-walk mode. Sleeping 1330 pathwalk in RCU-walk mode. Sleeping is not permitted in this
1413 mode, and the caller can be asked to 1331 mode, and the caller can be asked to leave it and call again by
1414 returning -ECHILD. -EISDIR may also 1332 returning -ECHILD. -EISDIR may also be returned to tell
1415 pathwalk to ignore d_automount or any 1333 pathwalk to ignore d_automount or any mounts.
1416 1334
1417 This function is only used if DCACHE_ 1335 This function is only used if DCACHE_MANAGE_TRANSIT is set on
1418 the dentry being transited from. 1336 the dentry being transited from.
1419 1337
1420 ``d_real`` 1338 ``d_real``
1421 overlay/union type filesystems implem !! 1339 overlay/union type filesystems implement this method to return
1422 of the underlying dentries of a regul !! 1340 one of the underlying dentries hidden by the overlay. It is
1423 !! 1341 used in two different modes:
1424 The 'type' argument takes the values !! 1342
1425 for returning the real underlying den !! 1343 Called from file_dentry() it returns the real dentry matching
1426 hosting the file's data or metadata r !! 1344 the inode argument. The real dentry may be from a lower layer
>> 1345 already copied up, but still referenced from the file. This
>> 1346 mode is selected with a non-NULL inode argument.
1427 1347
1428 For non-regular files, the 'dentry' a !! 1348 With NULL inode the topmost real underlying dentry is returned.
1429 1349
1430 Each dentry has a pointer to its parent dentr 1350 Each dentry has a pointer to its parent dentry, as well as a hash list
1431 of child dentries. Child dentries are basica 1351 of child dentries. Child dentries are basically like files in a
1432 directory. 1352 directory.
1433 1353
1434 1354
1435 Directory Entry Cache API 1355 Directory Entry Cache API
1436 -------------------------- 1356 --------------------------
1437 1357
1438 There are a number of functions defined which 1358 There are a number of functions defined which permit a filesystem to
1439 manipulate dentries: 1359 manipulate dentries:
1440 1360
1441 ``dget`` 1361 ``dget``
1442 open a new handle for an existing den 1362 open a new handle for an existing dentry (this just increments
1443 the usage count) 1363 the usage count)
1444 1364
1445 ``dput`` 1365 ``dput``
1446 close a handle for a dentry (decremen 1366 close a handle for a dentry (decrements the usage count). If
1447 the usage count drops to 0, and the d 1367 the usage count drops to 0, and the dentry is still in its
1448 parent's hash, the "d_delete" method 1368 parent's hash, the "d_delete" method is called to check whether
1449 it should be cached. If it should no 1369 it should be cached. If it should not be cached, or if the
1450 dentry is not hashed, it is deleted. 1370 dentry is not hashed, it is deleted. Otherwise cached dentries
1451 are put into an LRU list to be reclai 1371 are put into an LRU list to be reclaimed on memory shortage.
1452 1372
1453 ``d_drop`` 1373 ``d_drop``
1454 this unhashes a dentry from its paren 1374 this unhashes a dentry from its parents hash list. A subsequent
1455 call to dput() will deallocate the de 1375 call to dput() will deallocate the dentry if its usage count
1456 drops to 0 1376 drops to 0
1457 1377
1458 ``d_delete`` 1378 ``d_delete``
1459 delete a dentry. If there are no oth 1379 delete a dentry. If there are no other open references to the
1460 dentry then the dentry is turned into 1380 dentry then the dentry is turned into a negative dentry (the
1461 d_iput() method is called). If there 1381 d_iput() method is called). If there are other references, then
1462 d_drop() is called instead 1382 d_drop() is called instead
1463 1383
1464 ``d_add`` 1384 ``d_add``
1465 add a dentry to its parents hash list 1385 add a dentry to its parents hash list and then calls
1466 d_instantiate() 1386 d_instantiate()
1467 1387
1468 ``d_instantiate`` 1388 ``d_instantiate``
1469 add a dentry to the alias hash list f 1389 add a dentry to the alias hash list for the inode and updates
1470 the "d_inode" member. The "i_count" 1390 the "d_inode" member. The "i_count" member in the inode
1471 structure should be set/incremented. 1391 structure should be set/incremented. If the inode pointer is
1472 NULL, the dentry is called a "negativ 1392 NULL, the dentry is called a "negative dentry". This function
1473 is commonly called when an inode is c 1393 is commonly called when an inode is created for an existing
1474 negative dentry 1394 negative dentry
1475 1395
1476 ``d_lookup`` 1396 ``d_lookup``
1477 look up a dentry given its parent and 1397 look up a dentry given its parent and path name component It
1478 looks up the child of that given name 1398 looks up the child of that given name from the dcache hash
1479 table. If it is found, the reference 1399 table. If it is found, the reference count is incremented and
1480 the dentry is returned. The caller m 1400 the dentry is returned. The caller must use dput() to free the
1481 dentry when it finishes using it. 1401 dentry when it finishes using it.
1482 1402
1483 1403
1484 Mount Options 1404 Mount Options
1485 ============= 1405 =============
1486 1406
1487 1407
1488 Parsing options 1408 Parsing options
1489 --------------- 1409 ---------------
1490 1410
1491 On mount and remount the filesystem is passed 1411 On mount and remount the filesystem is passed a string containing a
1492 comma separated list of mount options. The o 1412 comma separated list of mount options. The options can have either of
1493 these forms: 1413 these forms:
1494 1414
1495 option 1415 option
1496 option=value 1416 option=value
1497 1417
1498 The <linux/parser.h> header defines an API th 1418 The <linux/parser.h> header defines an API that helps parse these
1499 options. There are plenty of examples on how 1419 options. There are plenty of examples on how to use it in existing
1500 filesystems. 1420 filesystems.
1501 1421
1502 1422
1503 Showing options 1423 Showing options
1504 --------------- 1424 ---------------
1505 1425
1506 If a filesystem accepts mount options, it mus 1426 If a filesystem accepts mount options, it must define show_options() to
1507 show all the currently active options. The r 1427 show all the currently active options. The rules are:
1508 1428
1509 - options MUST be shown which are not defau 1429 - options MUST be shown which are not default or their values differ
1510 from the default 1430 from the default
1511 1431
1512 - options MAY be shown which are enabled by 1432 - options MAY be shown which are enabled by default or have their
1513 default value 1433 default value
1514 1434
1515 Options used only internally between a mount 1435 Options used only internally between a mount helper and the kernel (such
1516 as file descriptors), or which only have an e 1436 as file descriptors), or which only have an effect during the mounting
1517 (such as ones controlling the creation of a j 1437 (such as ones controlling the creation of a journal) are exempt from the
1518 above rules. 1438 above rules.
1519 1439
1520 The underlying reason for the above rules is 1440 The underlying reason for the above rules is to make sure, that a mount
1521 can be accurately replicated (e.g. umounting 1441 can be accurately replicated (e.g. umounting and mounting again) based
1522 on the information found in /proc/mounts. 1442 on the information found in /proc/mounts.
1523 1443
1524 1444
1525 Resources 1445 Resources
1526 ========= 1446 =========
1527 1447
1528 (Note some of these resources are not up-to-d 1448 (Note some of these resources are not up-to-date with the latest kernel
1529 version.) 1449 version.)
1530 1450
1531 Creating Linux virtual filesystems. 2002 1451 Creating Linux virtual filesystems. 2002
1532 <https://lwn.net/Articles/13325/> 1452 <https://lwn.net/Articles/13325/>
1533 1453
1534 The Linux Virtual File-system Layer by Neil B 1454 The Linux Virtual File-system Layer by Neil Brown. 1999
1535 <http://www.cse.unsw.edu.au/~neilb/oss/li 1455 <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
1536 1456
1537 A tour of the Linux VFS by Michael K. Johnson 1457 A tour of the Linux VFS by Michael K. Johnson. 1996
1538 <https://www.tldp.org/LDP/khg/HyperNews/g 1458 <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
1539 1459
1540 A small trail through the Linux kernel by And 1460 A small trail through the Linux kernel by Andries Brouwer. 2001
1541 <https://www.win.tue.nl/~aeb/linux/vfs/tr 1461 <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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