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