summaryrefslogtreecommitdiffstats
path: root/Documentation/filesystems/vfs.rst
blob: 2c15e70531137565ed5ab6b20fe690b60dafedce (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
.. SPDX-License-Identifier: GPL-2.0

=========================================
Overview of the Linux Virtual File System
=========================================

Original author: Richard Gooch <rgooch@atnf.csiro.au>

- Copyright (C) 1999 Richard Gooch
- Copyright (C) 2005 Pekka Enberg


Introduction
============

The Virtual File System (also known as the Virtual Filesystem Switch) is
the software layer in the kernel that provides the filesystem interface
to userspace programs.  It also provides an abstraction within the
kernel which allows different filesystem implementations to coexist.

VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on
are called from a process context.  Filesystem locking is described in
the document Documentation/filesystems/locking.rst.


Directory Entry Cache (dcache)
------------------------------

The VFS implements the open(2), stat(2), chmod(2), and similar system
calls.  The pathname argument that is passed to them is used by the VFS
to search through the directory entry cache (also known as the dentry
cache or dcache).  This provides a very fast look-up mechanism to
translate a pathname (filename) into a specific dentry.  Dentries live
in RAM and are never saved to disc: they exist only for performance.

The dentry cache is meant to be a view into your entire filespace.  As
most computers cannot fit all dentries in the RAM at the same time, some
bits of the cache are missing.  In order to resolve your pathname into a
dentry, the VFS may have to resort to creating dentries along the way,
and then loading the inode.  This is done by looking up the inode.


The Inode Object
----------------

An individual dentry usually has a pointer to an inode.  Inodes are
filesystem objects such as regular files, directories, FIFOs and other
beasts.  They live either on the disc (for block device filesystems) or
in the memory (for pseudo filesystems).  Inodes that live on the disc
are copied into the memory when required and changes to the inode are
written back to disc.  A single inode can be pointed to by multiple
dentries (hard links, for example, do this).

To look up an inode requires that the VFS calls the lookup() method of
the parent directory inode.  This method is installed by the specific
filesystem implementation that the inode lives in.  Once the VFS has the
required dentry (and hence the inode), we can do all those boring things
like open(2) the file, or stat(2) it to peek at the inode data.  The
stat(2) operation is fairly simple: once the VFS has the dentry, it
peeks at the inode data and passes some of it back to userspace.


The File Object
---------------

Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file descriptors).
The freshly allocated file structure is initialized with a pointer to
the dentry and a set of file operation member functions.  These are
taken from the inode data.  The open() file method is then called so the
specific filesystem implementation can do its work.  You can see that
this is another switch performed by the VFS.  The file structure is
placed into the file descriptor table for the process.

Reading, writing and closing files (and other assorted VFS operations)
is done by using the userspace file descriptor to grab the appropriate
file structure, and then calling the required file structure method to
do whatever is required.  For as long as the file is open, it keeps the
dentry in use, which in turn means that the VFS inode is still in use.


Registering and Mounting a Filesystem
=====================================

To register and unregister a filesystem, use the following API
functions:

.. code-block:: c

	#include <linux/fs.h>

	extern int register_filesystem(struct file_system_type *);
	extern int unregister_filesystem(struct file_system_type *);

The passed struct file_system_type describes your filesystem.  When a
request is made to mount a filesystem onto a directory in your
namespace, the VFS will call the appropriate mount() method for the
specific filesystem.  New vfsmount referring to the tree returned by
->mount() will be attached to the mountpoint, so that when pathname
resolution reaches the mountpoint it will jump into the root of that
vfsmount.

You can see all filesystems that are registered to the kernel in the
file /proc/filesystems.


struct file_system_type
-----------------------

This describes the filesystem.  As of kernel 2.6.39, the following
members are defined:

.. code-block:: c

	struct file_system_type {
		const char *name;
		int fs_flags;
		struct dentry *(*mount) (struct file_system_type *, int,
					 const char *, void *);
		void (*kill_sb) (struct super_block *);
		struct module *owner;
		struct file_system_type * next;
		struct list_head fs_supers;
		struct lock_class_key s_lock_key;
		struct lock_class_key s_umount_key;
	};

``name``
	the name of the filesystem type, such as "ext2", "iso9660",
	"msdos" and so on

``fs_flags``
	various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)

``mount``
	the method to call when a new instance of this filesystem should
	be mounted

``kill_sb``
	the method to call when an instance of this filesystem should be
	shut down


``owner``
	for internal VFS use: you should initialize this to THIS_MODULE
	in most cases.

``next``
	for internal VFS use: you should initialize this to NULL

  s_lock_key, s_umount_key: lockdep-specific

The mount() method has the following arguments:

``struct file_system_type *fs_type``
	describes the filesystem, partly initialized by the specific
	filesystem code

``int flags``
	mount flags

``const char *dev_name``
	the device name we are mounting.

``void *data``
	arbitrary mount options, usually comes as an ASCII string (see
	"Mount Options" section)

The mount() method must return the root dentry of the tree requested by
caller.  An active reference to its superblock must be grabbed and the
superblock must be locked.  On failure it should return ERR_PTR(error).

The arguments match those of mount(2) and their interpretation depends
on filesystem type.  E.g. for block filesystems, dev_name is interpreted
as block device name, that device is opened and if it contains a
suitable filesystem image the method creates and initializes struct
super_block accordingly, returning its root dentry to caller.

->mount() may choose to return a subtree of existing filesystem - it
doesn't have to create a new one.  The main result from the caller's
point of view is a reference to dentry at the root of (sub)tree to be
attached; creation of new superblock is a common side effect.

The most interesting member of the superblock structure that the mount()
method fills in is the "s_op" field.  This is a pointer to a "struct
super_operations" which describes the next level of the filesystem
implementation.

Usually, a filesystem uses one of the generic mount() implementations
and provides a fill_super() callback instead.  The generic variants are:

``mount_bdev``
	mount a filesystem residing on a block device

``mount_nodev``
	mount a filesystem that is not backed by a device

``mount_single``
	mount a filesystem which shares the instance between all mounts

A fill_super() callback implementation has the following arguments:

``struct super_block *sb``
	the superblock structure.  The callback must initialize this
	properly.

``void *data``
	arbitrary mount options, usually comes as an ASCII string (see
	"Mount Options" section)

``int silent``
	whether or not to be silent on error


The Superblock Object
=====================

A superblock object represents a mounted filesystem.


struct super_operations
-----------------------

This describes how the VFS can manipulate the superblock of your
filesystem.  As of kernel 2.6.22, the following members are defined:

.. code-block:: c

	struct super_operations {
		struct inode *(*alloc_inode)(struct super_block *sb);
		void (*destroy_inode)(struct inode *);

		void (*dirty_inode) (struct inode *, int flags);
		int (*write_inode) (struct inode *, int);
		void (*drop_inode) (struct inode *);
		void (*delete_inode) (struct inode *);
		void (*put_super) (struct super_block *);
		int (*sync_fs)(struct super_block *sb, int wait);
		int (*freeze_fs) (struct super_block *);
		int (*unfreeze_fs) (struct super_block *);
		int (*statfs) (struct dentry *, struct kstatfs *);
		int (*remount_fs) (struct super_block *, int *, char *);
		void (*clear_inode) (struct inode *);
		void (*umount_begin) (struct super_block *);

		int (*show_options)(struct seq_file *, struct dentry *);

		ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
		ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
		int (*nr_cached_objects)(struct super_block *);
		void (*free_cached_objects)(struct super_block *, int);
	};

All methods are called without any locks being held, unless otherwise
noted.  This means that most methods can block safely.  All methods are
only called from a process context (i.e. not from an interrupt handler
or bottom half).

``alloc_inode``
	this method is called by alloc_inode() to allocate memory for
	struct inode and initialize it.  If this function is not
	defined, a simple 'struct inode' is allocated.  Normally
	alloc_inode will be used to allocate a larger structure which
	contains a 'struct inode' embedded within it.

``destroy_inode``
	this method is called by destroy_inode() to release resources
	allocated for struct inode.  It is only required if
	->alloc_inode was defined and simply undoes anything done by
	->alloc_inode.

``dirty_inode``
	this method is called by the VFS when an inode is marked dirty.
	This is specifically for the inode itself being marked dirty,
	not its data.  If the update needs to be persisted by fdatasync(),
	then I_DIRTY_DATASYNC will be set in the flags argument.
	I_DIRTY_TIME will be set in the flags in case lazytime is enabled
	and struct inode has times updated since the last ->dirty_inode
	call.

``write_inode``
	this method is called when the VFS needs to write an inode to
	disc.  The second parameter indicates whether the write should
	be synchronous or not, not all filesystems check this flag.

``drop_inode``
	called when the last access to the inode is dropped, with the
	inode->i_lock spinlock held.

	This method should be either NULL (normal UNIX filesystem
	semantics) or "generic_delete_inode" (for filesystems that do
	not want to cache inodes - causing "delete_inode" to always be
	called regardless of the value of i_nlink)

	The "generic_delete_inode()" behavior is equivalent to the old
	practice of using "force_delete" in the put_inode() case, but
	does not have the races that the "force_delete()" approach had.

``delete_inode``
	called when the VFS wants to delete an inode

``put_super``
	called when the VFS wishes to free the superblock
	(i.e. unmount).  This is called with the superblock lock held

``sync_fs``
	called when VFS is writing out all dirty data associated with a
	superblock.  The second parameter indicates whether the method
	should wait until the write out has been completed.  Optional.

``freeze_fs``
	called when VFS is locking a filesystem and forcing it into a
	consistent state.  This method is currently used by the Logical
	Volume Manager (LVM).

``unfreeze_fs``
	called when VFS is unlocking a filesystem and making it writable
	again.

``statfs``
	called when the VFS needs to get filesystem statistics.

``remount_fs``
	called when the filesystem is remounted.  This is called with
	the kernel lock held

``clear_inode``
	called then the VFS clears the inode.  Optional

``umount_begin``
	called when the VFS is unmounting a filesystem.

``show_options``
	called by the VFS to show mount options for /proc/<pid>/mounts.
	(see "Mount Options" section)

``quota_read``
	called by the VFS to read from filesystem quota file.

``quota_write``
	called by the VFS to write to filesystem quota file.

``nr_cached_objects``
	called by the sb cache shrinking function for the filesystem to
	return the number of freeable cached objects it contains.
	Optional.

``free_cache_objects``
	called by the sb cache shrinking function for the filesystem to
	scan the number of objects indicated to try to free them.
	Optional, but any filesystem implementing this method needs to
	also implement ->nr_cached_objects for it to be called
	correctly.

	We can't do anything with any errors that the filesystem might
	encountered, hence the void return type.  This will never be
	called if the VM is trying to reclaim under GFP_NOFS conditions,
	hence this method does not need to handle that situation itself.

	Implementations must include conditional reschedule calls inside
	any scanning loop that is done.  This allows the VFS to
	determine appropriate scan batch sizes without having to worry
	about whether implementations will cause holdoff problems due to
	large scan batch sizes.

Whoever sets up the inode is responsible for filling in the "i_op"
field.  This is a pointer to a "struct inode_operations" which describes
the methods that can be performed on individual inodes.


struct xattr_handlers
---------------------

On filesystems that support extended attributes (xattrs), the s_xattr
superblock field points to a NULL-terminated array of xattr handlers.
Extended attributes are name:value pairs.

``name``
	Indicates that the handler matches attributes with the specified
	name (such as "system.posix_acl_access"); the prefix field must
	be NULL.

``prefix``
	Indicates that the handler matches all attributes with the
	specified name prefix (such as "user."); the name field must be
	NULL.

``list``
	Determine if attributes matching this xattr handler should be
	listed for a particular dentry.  Used by some listxattr
	implementations like generic_listxattr.

``get``
	Called by the VFS to get the value of a particular extended
	attribute.  This method is called by the getxattr(2) system
	call.

``set``
	Called by the VFS to set the value of a particular extended
	attribute.  When the new value is NULL, called to remove a
	particular extended attribute.  This method is called by the
	setxattr(2) and removexattr(2) system calls.

When none of the xattr handlers of a filesystem match the specified
attribute name or when a filesystem doesn't support extended attributes,
the various ``*xattr(2)`` system calls return -EOPNOTSUPP.


The Inode Object
================

An inode object represents an object within the filesystem.


struct inode_operations
-----------------------

This describes how the VFS can manipulate an inode in your filesystem.
As of kernel 2.6.22, the following members are defined:

.. code-block:: c

	struct inode_operations {
		int (*create) (struct user_namespace *, struct inode *,struct dentry *, umode_t, bool);
		struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
		int (*link) (struct dentry *,struct inode *,struct dentry *);
		int (*unlink) (struct inode *,struct dentry *);
		int (*symlink) (struct user_namespace *, struct inode *,struct dentry *,const char *);
		int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
		int (*rmdir) (struct inode *,struct dentry *);
		int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
		int (*rename) (struct user_namespace *, struct inode *, struct dentry *,
			       struct inode *, struct dentry *, unsigned int);
		int (*readlink) (struct dentry *, char __user *,int);
		const char *(*get_link) (struct dentry *, struct inode *,
					 struct delayed_call *);
		int (*permission) (struct user_namespace *, struct inode *, int);
		struct posix_acl * (*get_inode_acl)(struct inode *, int, bool);
		int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
		int (*getattr) (struct user_namespace *, const struct path *, struct kstat *, u32, unsigned int);
		ssize_t (*listxattr) (struct dentry *, char *, size_t);
		void (*update_time)(struct inode *, struct timespec *, int);
		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
				   unsigned open_flag, umode_t create_mode);
		int (*tmpfile) (struct user_namespace *, struct inode *, struct file *, umode_t);
		struct posix_acl * (*get_acl)(struct user_namespace *, struct dentry *, int);
	        int (*set_acl)(struct user_namespace *, struct dentry *, struct posix_acl *, int);
		int (*fileattr_set)(struct user_namespace *mnt_userns,
				    struct dentry *dentry, struct fileattr *fa);
		int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
	};

Again, all methods are called without any locks being held, unless
otherwise noted.

``create``
	called by the open(2) and creat(2) system calls.  Only required
	if you want to support regular files.  The dentry you get should
	not have an inode (i.e. it should be a negative dentry).  Here
	you will probably call d_instantiate() with the dentry and the
	newly created inode

``lookup``
	called when the VFS needs to look up an inode in a parent
	directory.  The name to look for is found in the dentry.  This
	method must call d_add() to insert the found inode into the
	dentry.  The "i_count" field in the inode structure should be
	incremented.  If the named inode does not exist a NULL inode
	should be inserted into the dentry (this is called a negative
	dentry).  Returning an error code from this routine must only be
	done on a real error, otherwise creating inodes with system
	calls like create(2), mknod(2), mkdir(2) and so on will fail.
	If you wish to overload the dentry methods then you should
	initialise the "d_dop" field in the dentry; this is a pointer to
	a struct "dentry_operations".  This method is called with the
	directory inode semaphore held

``link``
	called by the link(2) system call.  Only required if you want to
	support hard links.  You will probably need to call
	d_instantiate() just as you would in the create() method

``unlink``
	called by the unlink(2) system call.  Only required if you want
	to support deleting inodes

``symlink``
	called by the symlink(2) system call.  Only required if you want
	to support symlinks.  You will probably need to call
	d_instantiate() just as you would in the create() method

``mkdir``
	called by the mkdir(2) system call.  Only required if you want
	to support creating subdirectories.  You will probably need to
	call d_instantiate() just as you would in the create() method

``rmdir``
	called by the rmdir(2) system call.  Only required if you want
	to support deleting subdirectories

``mknod``
	called by the mknod(2) system call to create a device (char,
	block) inode or a named pipe (FIFO) or socket.  Only required if
	you want to support creating these types of inodes.  You will
	probably need to call d_instantiate() just as you would in the
	create() method

``rename``
	called by the rename(2) system call to rename the object to have
	the parent and name given by the second inode and dentry.

	The filesystem must return -EINVAL for any unsupported or
	unknown flags.  Currently the following flags are implemented:
	(1) RENAME_NOREPLACE: this flag indicates that if the target of
	the rename exists the rename should fail with -EEXIST instead of
	replacing the target.  The VFS already checks for existence, so
	for local filesystems the RENAME_NOREPLACE implementation is
	equivalent to plain rename.
	(2) RENAME_EXCHANGE: exchange source and target.  Both must
	exist; this is checked by the VFS.  Unlike plain rename, source
	and target may be of different type.

``get_link``
	called by the VFS to follow a symbolic link to the inode it
	points to.  Only required if you want to support symbolic links.
	This method returns the symlink body to traverse (and possibly
	resets the current position with nd_jump_link()).  If the body
	won't go away until the inode is gone, nothing else is needed;
	if it needs to be otherwise pinned, arrange for its release by
	having get_link(..., ..., done) do set_delayed_call(done,
	destructor, argument).  In that case destructor(argument) will
	be called once VFS is done with the body you've returned.  May
	be called in RCU mode; that is indicated by NULL dentry
	argument.  If request can't be handled without leaving RCU mode,
	have it return ERR_PTR(-ECHILD).

	If the filesystem stores the symlink target in ->i_link, the
	VFS may use it directly without calling ->get_link(); however,
	->get_link() must still be provided.  ->i_link must not be
	freed until after an RCU grace period.  Writing to ->i_link
	post-iget() time requires a 'release' memory barrier.

``readlink``
	this is now just an override for use by readlink(2) for the
	cases when ->get_link uses nd_jump_link() or object is not in
	fact a symlink.  Normally filesystems should only implement
	->get_link for symlinks and readlink(2) will automatically use
	that.

``permission``
	called by the VFS to check for access rights on a POSIX-like
	filesystem.

	May be called in rcu-walk mode (mask & MAY_NOT_BLOCK).  If in
	rcu-walk mode, the filesystem must check the permission without
	blocking or storing to the inode.

	If a situation is encountered that rcu-walk cannot handle,
	return
	-ECHILD and it will be called again in ref-walk mode.

``setattr``
	called by the VFS to set attributes for a file.  This method is
	called by chmod(2) and related system calls.

``getattr``
	called by the VFS to get attributes of a file.  This method is
	called by stat(2) and related system calls.

``listxattr``
	called by the VFS to list all extended attributes for a given
	file.  This method is called by the listxattr(2) system call.

``update_time``
	called by the VFS to update a specific time or the i_version of
	an inode.  If this is not defined the VFS will update the inode
	itself and call mark_inode_dirty_sync.

``atomic_open``
	called on the last component of an open.  Using this optional
	method the filesystem can look up, possibly create and open the
	file in one atomic operation.  If it wants to leave actual
	opening to the caller (e.g. if the file turned out to be a
	symlink, device, or just something filesystem won't do atomic
	open for), it may signal this by returning finish_no_open(file,
	dentry).  This method is only called if the last component is
	negative or needs lookup.  Cached positive dentries are still
	handled by f_op->open().  If the file was created, FMODE_CREATED
	flag should be set in file->f_mode.  In case of O_EXCL the
	method must only succeed if the file didn't exist and hence
	FMODE_CREATED shall always be set on success.

``tmpfile``
	called in the end of O_TMPFILE open().  Optional, equivalent to
	atomically creating, opening and unlinking a file in given
	directory.  On success needs to return with the file already
	open; this can be done by calling finish_open_simple() right at
	the end.

``fileattr_get``
	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
	retrieve miscellaneous file flags and attributes.  Also called
	before the relevant SET operation to check what is being changed
	(in this case with i_rwsem locked exclusive).  If unset, then
	fall back to f_op->ioctl().

``fileattr_set``
	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
	change miscellaneous file flags and attributes.  Callers hold
	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().


The Address Space Object
========================

The address space object is used to group and manage pages in the page
cache.  It can be used to keep track of the pages in a file (or anything
else) and also track the mapping of sections of the file into process
address spaces.

There are a number of distinct yet related services that an
address-space can provide.  These include communicating memory pressure,
page lookup by address, and keeping track of pages tagged as Dirty or
Writeback.

The first can be used independently to the others.  The VM can try to
either write dirty pages in order to clean them, or release clean pages
in order to reuse them.  To do this it can call the ->writepage method
on dirty pages, and ->release_folio on clean folios with the private
flag set.  Clean pages without PagePrivate and with no external references
will be released without notice being given to the address_space.

To achieve this functionality, pages need to be placed on an LRU with
lru_cache_add and mark_page_active needs to be called whenever the page
is used.

Pages are normally kept in a radix tree index by ->index.  This tree
maintains information about the PG_Dirty and PG_Writeback status of each
page, so that pages with either of these flags can be found quickly.

The Dirty tag is primarily used by mpage_writepages - the default
->writepages method.  It uses the tag to find dirty pages to call
->writepage on.  If mpage_writepages is not used (i.e. the address
provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is almost
unused.  write_inode_now and sync_inode do use it (through
__sync_single_inode) to check if ->writepages has been successful in
writing out the whole address_space.

The Writeback tag is used by filemap*wait* and sync_page* functions, via
filemap_fdatawait_range, to wait for all writeback to complete.

An address_space handler may attach extra information to a page,
typically using the 'private' field in the 'struct page'.  If such
information is attached, the PG_Private flag should be set.  This will
cause various VM routines to make extra calls into the address_space
handler to deal with that data.

An address space acts as an intermediate between storage and
application.  Data is read into the address space a whole page at a
time, and provided to the application either by copying of the page, or
by memory-mapping the page.  Data is written into the address space by
the application, and then written-back to storage typically in whole
pages, however the address_space has finer control of write sizes.

The read process essentially only requires 'read_folio'.  The write
process is more complicated and uses write_begin/write_end or
dirty_folio to write data into the address_space, and writepage and
writepages to writeback data to storage.

Adding and removing pages to/from an address_space is protected by the
inode's i_mutex.

When data is written to a page, the PG_Dirty flag should be set.  It
typically remains set until writepage asks for it to be written.  This
should clear PG_Dirty and set PG_Writeback.  It can be actually written
at any point after PG_Dirty is clear.  Once it is known to be safe,
PG_Writeback is cleared.

Writeback makes use of a writeback_control structure to direct the
operations.  This gives the writepage and writepages operations some
information about the nature of and reason for the writeback request,
and the constraints under which it is being done.  It is also used to
return information back to the caller about the result of a writepage or
writepages request.


Handling errors during writeback
--------------------------------

Most applications that do buffered I/O will periodically call a file
synchronization call (fsync, fdatasync, msync or sync_file_range) to
ensure that data written has made it to the backing store.  When there
is an error during writeback, they expect that error to be reported when
a file sync request is made.  After an error has been reported on one
request, subsequent requests on the same file descriptor should return
0, unless further writeback errors have occurred since the previous file
syncronization.

Ideally, the kernel would report errors only on file descriptions on
which writes were done that subsequently failed to be written back.  The
generic pagecache infrastructure does not track the file descriptions
that have dirtied each individual page however, so determining which
file descriptors should get back an error is not possible.

Instead, the generic writeback error tracking infrastructure in the
kernel settles for reporting errors to fsync on all file descriptions
that were open at the time that the error occurred.  In a situation with
multiple writers, all of them will get back an error on a subsequent
fsync, even if all of the writes done through that particular file
descriptor succeeded (or even if there were no writes on that file
descriptor at all).

Filesystems that wish to use this infrastructure should call
mapping_set_error to record the error in the address_space when it
occurs.  Then, after writing back data from the pagecache in their
file->fsync operation, they should call file_check_and_advance_wb_err to
ensure that the struct file's error cursor has advanced to the correct
point in the stream of errors emitted by the backing device(s).


struct address_space_operations
-------------------------------

This describes how the VFS can manipulate mapping of a file to page
cache in your filesystem.  The following members are defined:

.. code-block:: c

	struct address_space_operations {
		int (*writepage)(struct page *page, struct writeback_control *wbc);
		int (*read_folio)(struct file *, struct folio *);
		int (*writepages)(struct address_space *, struct writeback_control *);
		bool (*dirty_folio)(struct address_space *, struct folio *);
		void (*readahead)(struct readahead_control *);
		int (*write_begin)(struct file *, struct address_space *mapping,
				   loff_t pos, unsigned len,
				struct page **pagep, void **fsdata);
		int (*write_end)(struct file *, struct address_space *mapping,
				 loff_t pos, unsigned len, unsigned copied,
				 struct page *page, void *fsdata);
		sector_t (*bmap)(struct address_space *, sector_t);
		void (*invalidate_folio) (struct folio *, size_t start, size_t len);
		bool (*release_folio)(struct folio *, gfp_t);
		void (*free_folio)(struct folio *);
		ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
		int (*migrate_folio)(struct mapping *, struct folio *dst,
				struct folio *src, enum migrate_mode);
		int (*launder_folio) (struct folio *);

		bool (*is_partially_uptodate) (struct folio *, size_t from,
					       size_t count);
		void (*is_dirty_writeback)(struct folio *, bool *, bool *);
		int (*error_remove_page) (struct mapping *mapping, struct page *page);
		int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
		int (*swap_deactivate)(struct file *);
		int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
	};

``writepage``
	called by the VM to write a dirty page to backing store.  This
	may happen for data integrity reasons (i.e. 'sync'), or to free
	up memory (flush).  The difference can be seen in
	wbc->sync_mode.  The PG_Dirty flag has been cleared and
	PageLocked is true.  writepage should start writeout, should set
	PG_Writeback, and should make sure the page is unlocked, either
	synchronously or asynchronously when the write operation
	completes.

	If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
	try too hard if there are problems, and may choose to write out
	other pages from the mapping if that is easier (e.g. due to
	internal dependencies).  If it chooses not to start writeout, it
	should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
	keep calling ->writepage on that page.

	See the file "Locking" for more details.

``read_folio``
	Called by the page cache to read a folio from the backing store.
	The 'file' argument supplies authentication information to network
	filesystems, and is generally not used by block based filesystems.
	It may be NULL if the caller does not have an open file (eg if
	the kernel is performing a read for itself rather than on behalf
	of a userspace process with an open file).

	If the mapping does not support large folios, the folio will
	contain a single page.	The folio will be locked when read_folio
	is called.  If the read completes successfully, the folio should
	be marked uptodate.  The filesystem should unlock the folio
	once the read has completed, whether it was successful or not.
	The filesystem does not need to modify the refcount on the folio;
	the page cache holds a reference count and that will not be
	released until the folio is unlocked.

	Filesystems may implement ->read_folio() synchronously.
	In normal operation, folios are read through the ->readahead()
	method.  Only if this fails, or if the caller needs to wait for
	the read to complete will the page cache call ->read_folio().
	Filesystems should not attempt to perform their own readahead
	in the ->read_folio() operation.

	If the filesystem cannot perform the read at this time, it can
	unlock the folio, do whatever action it needs to ensure that the
	read will succeed in the future and return AOP_TRUNCATED_PAGE.
	In this case, the caller should look up the folio, lock it,
	and call ->read_folio again.

	Callers may invoke the ->read_folio() method directly, but using
	read_mapping_folio() will take care of locking, waiting for the
	read to complete and handle cases such as AOP_TRUNCATED_PAGE.

``writepages``
	called by the VM to write out pages associated with the
	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
	the writeback_control will specify a range of pages that must be
	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
	given and that many pages should be written if possible.  If no
	->writepages is given, then mpage_writepages is used instead.
	This will choose pages from the address space that are tagged as
	DIRTY and will pass them to ->writepage.

``dirty_folio``
	called by the VM to mark a folio as dirty.  This is particularly
	needed if an address space attaches private data to a folio, and
	that data needs to be updated when a folio is dirtied.  This is
	called, for example, when a memory mapped page gets modified.
	If defined, it should set the folio dirty flag, and the
	PAGECACHE_TAG_DIRTY search mark in i_pages.

``readahead``
	Called by the VM to read pages associated with the address_space
	object.  The pages are consecutive in the page cache and are
	locked.  The implementation should decrement the page refcount
	after starting I/O on each page.  Usually the page will be
	unlocked by the I/O completion handler.  The set of pages are
	divided into some sync pages followed by some async pages,
	rac->ra->async_size gives the number of async pages.  The
	filesystem should attempt to read all sync pages but may decide
	to stop once it reaches the async pages.  If it does decide to
	stop attempting I/O, it can simply return.  The caller will
	remove the remaining pages from the address space, unlock them
	and decrement the page refcount.  Set PageUptodate if the I/O
	completes successfully.  Setting PageError on any page will be
	ignored; simply unlock the page if an I/O error occurs.

``write_begin``
	Called by the generic buffered write code to ask the filesystem
	to prepare to write len bytes at the given offset in the file.
	The address_space should check that the write will be able to
	complete, by allocating space if necessary and doing any other
	internal housekeeping.  If the write will update parts of any
	basic-blocks on storage, then those blocks should be pre-read
	(if they haven't been read already) so that the updated blocks
	can be written out properly.

	The filesystem must return the locked pagecache page for the
	specified offset, in ``*pagep``, for the caller to write into.

	It must be able to cope with short writes (where the length
	passed to write_begin is greater than the number of bytes copied
	into the page).

	A void * may be returned in fsdata, which then gets passed into
	write_end.

	Returns 0 on success; < 0 on failure (which is the error code),
	in which case write_end is not called.

``write_end``
	After a successful write_begin, and data copy, write_end must be
	called.  len is the original len passed to write_begin, and
	copied is the amount that was able to be copied.

	The filesystem must take care of unlocking the page and
	releasing it refcount, and updating i_size.

	Returns < 0 on failure, otherwise the number of bytes (<=
	'copied') that were able to be copied into pagecache.

``bmap``
	called by the VFS to map a logical block offset within object to
	physical block number.  This method is used by the FIBMAP ioctl
	and for working with swap-files.  To be able to swap to a file,
	the file must have a stable mapping to a block device.  The swap
	system does not go through the filesystem but instead uses bmap
	to find out where the blocks in the file are and uses those
	addresses directly.

``invalidate_folio``
	If a folio has private data, then invalidate_folio will be
	called when part or all of the folio is to be removed from the
	address space.  This generally corresponds to either a
	truncation, punch hole or a complete invalidation of the address
	space (in the latter case 'offset' will always be 0 and 'length'
	will be folio_size()).  Any private data associated with the folio
	should be updated to reflect this truncation.  If offset is 0
	and length is folio_size(), then the private data should be
	released, because the folio must be able to be completely
	discarded.  This may be done by calling the ->release_folio
	function, but in this case the release MUST succeed.

``release_folio``
	release_folio is called on folios with private data to tell the
	filesystem that the folio is about to be freed.  ->release_folio
	should remove any private data from the folio and clear the
	private flag.  If release_folio() fails, it should return false.
	release_folio() is used in two distinct though related cases.
	The first is when the VM wants to free a clean folio with no
	active users.  If ->release_folio succeeds, the folio will be
	removed from the address_space and be freed.

	The second case is when a request has been made to invalidate
	some or all folios in an address_space.  This can happen
	through the fadvise(POSIX_FADV_DONTNEED) system call or by the
	filesystem explicitly requesting it as nfs and 9p do (when they
	believe the cache may be out of date with storage) by calling
	invalidate_inode_pages2().  If the filesystem makes such a call,
	and needs to be certain that all folios are invalidated, then
	its release_folio will need to ensure this.  Possibly it can
	clear the uptodate flag if it cannot free private data yet.

``free_folio``
	free_folio is called once the folio is no longer visible in the
	page cache in order to allow the cleanup of any private data.
	Since it may be called by the memory reclaimer, it should not
	assume that the original address_space mapping still exists, and
	it should not block.

``direct_IO``
	called by the generic read/write routines to perform direct_IO -
	that is IO requests which bypass the page cache and transfer
	data directly between the storage and the application's address
	space.

``migrate_folio``
	This is used to compact the physical memory usage.  If the VM
	wants to relocate a folio (maybe from a memory device that is
	signalling imminent failure) it will pass a new folio and an old
	folio to this function.  migrate_folio should transfer any private
	data across and update any references that it has to the folio.

``launder_folio``
	Called before freeing a folio - it writes back the dirty folio.
	To prevent redirtying the folio, it is kept locked during the
	whole operation.

``is_partially_uptodate``
	Called by the VM when reading a file through the pagecache when
	the underlying blocksize is smaller than the size of the folio.
	If the required block is up to date then the read can complete
	without needing I/O to bring the whole page up to date.

``is_dirty_writeback``
	Called by the VM when attempting to reclaim a folio.  The VM uses
	dirty and writeback information to determine if it needs to
	stall to allow flushers a chance to complete some IO.
	Ordinarily it can use folio_test_dirty and folio_test_writeback but
	some filesystems have more complex state (unstable folios in NFS
	prevent reclaim) or do not set those flags due to locking
	problems.  This callback allows a filesystem to indicate to the
	VM if a folio should be treated as dirty or writeback for the
	purposes of stalling.

``error_remove_page``
	normally set to generic_error_remove_page if truncation is ok
	for this address space.  Used for memory failure handling.
	Setting this implies you deal with pages going away under you,
	unless you have them locked or reference counts increased.

``swap_activate``

	Called to prepare the given file for swap.  It should perform
	any validation and preparation necessary to ensure that writes
	can be performed with minimal memory allocation.  It should call
	add_swap_extent(), or the helper iomap_swapfile_activate(), and
	return the number of extents added.  If IO should be submitted
	through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
	be submitted directly to the block device ``sis->bdev``.

``swap_deactivate``
	Called during swapoff on files where swap_activate was
	successful.

``swap_rw``
	Called to read or write swap pages when SWP_FS_OPS is set.

The File Object
===============

A file object represents a file opened by a process.  This is also known
as an "open file description" in POSIX parlance.


struct file_operations
----------------------

This describes how the VFS can manipulate an open file.  As of kernel
4.18, the following members are defined:

.. code-block:: c

	struct file_operations {
		struct module *owner;
		loff_t (*llseek) (struct file *, loff_t, int);
		ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
		ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
		ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
		ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
		int (*iopoll)(struct kiocb *kiocb, bool spin);
		int (*iterate) (struct file *, struct dir_context *);
		int (*iterate_shared) (struct file *, struct dir_context *);
		__poll_t (*poll) (struct file *, struct poll_table_struct *);
		long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
		long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
		int (*mmap) (struct file *, struct vm_area_struct *);
		int (*open) (struct inode *, struct file *);
		int (*flush) (struct file *, fl_owner_t id);
		int (*release) (struct inode *, struct file *);
		int (*fsync) (struct file *, loff_t, loff_t, int datasync);
		int (*fasync) (int, struct file *, int);
		int (*lock) (struct file *, int, struct file_lock *);
		ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
		unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
		int (*check_flags)(int);
		int (*flock) (struct file *, int, struct file_lock *);
		ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
		ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
		int (*setlease)(struct file *, long, struct file_lock **, void **);
		long (*fallocate)(struct file *file, int mode, loff_t offset,
				  loff_t len);
		void (*show_fdinfo)(struct seq_file *m, struct file *f);
	#ifndef CONFIG_MMU
		unsigned (*mmap_capabilities)(struct file *);
	#endif
		ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
		loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
					   struct file *file_out, loff_t pos_out,
					   loff_t len, unsigned int remap_flags);
		int (*fadvise)(struct file *, loff_t, loff_t, int);
	};

Again, all methods are called without any locks being held, unless
otherwise noted.

``llseek``
	called when the VFS needs to move the file position index

``read``
	called by read(2) and related system calls

``read_iter``
	possibly asynchronous read with iov_iter as destination

``write``
	called by write(2) and related system calls

``write_iter``
	possibly asynchronous write with iov_iter as source

``iopoll``
	called when aio wants to poll for completions on HIPRI iocbs

``iterate``
	called when the VFS needs to read the directory contents

``iterate_shared``
	called when the VFS needs to read the directory contents when
	filesystem supports concurrent dir iterators

``poll``
	called by the VFS when a process wants to check if there is
	activity on this file and (optionally) go to sleep until there
	is activity.  Called by the select(2) and poll(2) system calls

``unlocked_ioctl``
	called by the ioctl(2) system call.

``compat_ioctl``
	called by the ioctl(2) system call when 32 bit system calls are
	 used on 64 bit kernels.

``mmap``
	called by the mmap(2) system call

``open``
	called by the VFS when an inode should be opened.  When the VFS
	opens a file, it creates a new "struct file".  It then calls the
	open method for the newly allocated file structure.  You might
	think that the open method really belongs in "struct
	inode_operations", and you may be right.  I think it's done the
	way it is because it makes filesystems simpler to implement.
	The open() method is a good place to initialize the
	"private_data" member in the file structure if you want to point
	to a device structure

``flush``
	called by the close(2) system call to flush a file

``release``
	called when the last reference to an open file is closed

``fsync``
	called by the fsync(2) system call.  Also see the section above
	entitled "Handling errors during writeback".

``fasync``
	called by the fcntl(2) system call when asynchronous
	(non-blocking) mode is enabled for a file

``lock``
	called by the fcntl(2) system call for F_GETLK, F_SETLK, and
	F_SETLKW commands

``get_unmapped_area``
	called by the mmap(2) system call

``check_flags``
	called by the fcntl(2) system call for F_SETFL command

``flock``
	called by the flock(2) system call

``splice_write``
	called by the VFS to splice data from a pipe to a file.  This
	method is used by the splice(2) system call

``splice_read``
	called by the VFS to splice data from file to a pipe.  This
	method is used by the splice(2) system call

``setlease``
	called by the VFS to set or release a file lock lease.  setlease
	implementations should call generic_setlease to record or remove
	the lease in the inode after setting it.

``fallocate``
	called by the VFS to preallocate blocks or punch a hole.

``copy_file_range``
	called by the copy_file_range(2) system call.

``remap_file_range``
	called by the ioctl(2) system call for FICLONERANGE and FICLONE
	and FIDEDUPERANGE commands to remap file ranges.  An
	implementation should remap len bytes at pos_in of the source
	file into the dest file at pos_out.  Implementations must handle
	callers passing in len == 0; this means "remap to the end of the
	source file".  The return value should the number of bytes
	remapped, or the usual negative error code if errors occurred
	before any bytes were remapped.  The remap_flags parameter
	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
	implementation must only remap if the requested file ranges have
	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
	ok with the implementation shortening the request length to
	satisfy alignment or EOF requirements (or any other reason).

``fadvise``
	possibly called by the fadvise64() system call.

Note that the file operations are implemented by the specific
filesystem in which the inode resides.  When opening a device node
(character or block special) most filesystems will call special
support routines in the VFS which will locate the required device
driver information.  These support routines replace the filesystem file
operations with those for the device driver, and then proceed to call
the new open() method for the file.  This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
method.


Directory Entry Cache (dcache)
==============================


struct dentry_operations
------------------------

This describes how a filesystem can overload the standard dentry
operations.  Dentries and the dcache are the domain of the VFS and the
individual filesystem implementations.  Device drivers have no business
here.  These methods may be set to NULL, as they are either optional or
the VFS uses a default.  As of kernel 2.6.22, the following members are
defined:

.. code-block:: c

	struct dentry_operations {
		int (*d_revalidate)(struct dentry *, unsigned int);
		int (*d_weak_revalidate)(struct dentry *, unsigned int);
		int (*d_hash)(const struct dentry *, struct qstr *);
		int (*d_compare)(const struct dentry *,
				 unsigned int, const char *, const struct qstr *);
		int (*d_delete)(const struct dentry *);
		int (*d_init)(struct dentry *);
		void (*d_release)(struct dentry *);
		void (*d_iput)(struct dentry *, struct inode *);
		char *(*d_dname)(struct dentry *, char *, int);
		struct vfsmount *(*d_automount)(struct path *);
		int (*d_manage)(const struct path *, bool);
		struct dentry *(*d_real)(struct dentry *, const struct inode *);
	};

``d_revalidate``
	called when the VFS needs to revalidate a dentry.  This is
	called whenever a name look-up finds a dentry in the dcache.
	Most local filesystems leave this as NULL, because all their
	dentries in the dcache are valid.  Network filesystems are
	different since things can change on the server without the
	client necessarily being aware of it.

	This function should return a positive value if the dentry is
	still valid, and zero or a negative error code if it isn't.

	d_revalidate may be called in rcu-walk mode (flags &
	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
	revalidate the dentry without blocking or storing to the dentry,
	d_parent and d_inode should not be used without care (because
	they can change and, in d_inode case, even become NULL under
	us).

	If a situation is encountered that rcu-walk cannot handle,
	return
	-ECHILD and it will be called again in ref-walk mode.

``_weak_revalidate``
	called when the VFS needs to revalidate a "jumped" dentry.  This
	is called when a path-walk ends at dentry that was not acquired
	by doing a lookup in the parent directory.  This includes "/",
	"." and "..", as well as procfs-style symlinks and mountpoint
	traversal.

	In this case, we are less concerned with whether the dentry is
	still fully correct, but rather that the inode is still valid.
	As with d_revalidate, most local filesystems will set this to
	NULL since their dcache entries are always valid.

	This function has the same return code semantics as
	d_revalidate.

	d_weak_revalidate is only called after leaving rcu-walk mode.

``d_hash``
	called when the VFS adds a dentry to the hash table.  The first
	dentry passed to d_hash is the parent directory that the name is
	to be hashed into.

	Same locking and synchronisation rules as d_compare regarding
	what is safe to dereference etc.

``d_compare``
	called to compare a dentry name with a given name.  The first
	dentry is the parent of the dentry to be compared, the second is
	the child dentry.  len and name string are properties of the
	dentry to be compared.  qstr is the name to compare it with.

	Must be constant and idempotent, and should not take locks if
	possible, and should not or store into the dentry.  Should not
	dereference pointers outside the dentry without lots of care
	(eg.  d_parent, d_inode, d_name should not be used).

	However, our vfsmount is pinned, and RCU held, so the dentries
	and inodes won't disappear, neither will our sb or filesystem
	module.  ->d_sb may be used.

	It is a tricky calling convention because it needs to be called
	under "rcu-walk", ie. without any locks or references on things.

``d_delete``
	called when the last reference to a dentry is dropped and the
	dcache is deciding whether or not to cache it.  Return 1 to
	delete immediately, or 0 to cache the dentry.  Default is NULL
	which means to always cache a reachable dentry.  d_delete must
	be constant and idempotent.

``d_init``
	called when a dentry is allocated

``d_release``
	called when a dentry is really deallocated

``d_iput``
	called when a dentry loses its inode (just prior to its being
	deallocated).  The default when this is NULL is that the VFS
	calls iput().  If you define this method, you must call iput()
	yourself

``d_dname``
	called when the pathname of a dentry should be generated.
	Useful for some pseudo filesystems (sockfs, pipefs, ...) to
	delay pathname generation.  (Instead of doing it when dentry is
	created, it's done only when the path is needed.).  Real
	filesystems probably dont want to use it, because their dentries
	are present in global dcache hash, so their hash should be an
	invariant.  As no lock is held, d_dname() should not try to
	modify the dentry itself, unless appropriate SMP safety is used.
	CAUTION : d_path() logic is quite tricky.  The correct way to
	return for example "Hello" is to put it at the end of the
	buffer, and returns a pointer to the first char.
	dynamic_dname() helper function is provided to take care of
	this.

	Example :

.. code-block:: c

	static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
	{
		return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
				dentry->d_inode->i_ino);
	}

``d_automount``
	called when an automount dentry is to be traversed (optional).
	This should create a new VFS mount record and return the record
	to the caller.  The caller is supplied with a path parameter
	giving the automount directory to describe the automount target
	and the parent VFS mount record to provide inheritable mount
	parameters.  NULL should be returned if someone else managed to
	make the automount first.  If the vfsmount creation failed, then
	an error code should be returned.  If -EISDIR is returned, then
	the directory will be treated as an ordinary directory and
	returned to pathwalk to continue walking.

	If a vfsmount is returned, the caller will attempt to mount it
	on the mountpoint and will remove the vfsmount from its
	expiration list in the case of failure.  The vfsmount should be
	returned with 2 refs on it to prevent automatic expiration - the
	caller will clean up the additional ref.

	This function is only used if DCACHE_NEED_AUTOMOUNT is set on
	the dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is
	set on the inode being added.

``d_manage``
	called to allow the filesystem to manage the transition from a
	dentry (optional).  This allows autofs, for example, to hold up
	clients waiting to explore behind a 'mountpoint' while letting
	the daemon go past and construct the subtree there.  0 should be
	returned to let the calling process continue.  -EISDIR can be
	returned to tell pathwalk to use this directory as an ordinary
	directory and to ignore anything mounted on it and not to check
	the automount flag.  Any other error code will abort pathwalk
	completely.

	If the 'rcu_walk' parameter is true, then the caller is doing a
	pathwalk in RCU-walk mode.  Sleeping is not permitted in this
	mode, and the caller can be asked to leave it and call again by
	returning -ECHILD.  -EISDIR may also be returned to tell
	pathwalk to ignore d_automount or any mounts.

	This function is only used if DCACHE_MANAGE_TRANSIT is set on
	the dentry being transited from.

``d_real``
	overlay/union type filesystems implement this method to return
	one of the underlying dentries hidden by the overlay.  It is
	used in two different modes:

	Called from file_dentry() it returns the real dentry matching
	the inode argument.  The real dentry may be from a lower layer
	already copied up, but still referenced from the file.  This
	mode is selected with a non-NULL inode argument.

	With NULL inode the topmost real underlying dentry is returned.

Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries.  Child dentries are basically like files in a
directory.


Directory Entry Cache API
--------------------------

There are a number of functions defined which permit a filesystem to
manipulate dentries:

``dget``
	open a new handle for an existing dentry (this just increments
	the usage count)

``dput``
	close a handle for a dentry (decrements the usage count).  If
	the usage count drops to 0, and the dentry is still in its
	parent's hash, the "d_delete" method is called to check whether
	it should be cached.  If it should not be cached, or if the
	dentry is not hashed, it is deleted.  Otherwise cached dentries
	are put into an LRU list to be reclaimed on memory shortage.

``d_drop``
	this unhashes a dentry from its parents hash list.  A subsequent
	call to dput() will deallocate the dentry if its usage count
	drops to 0

``d_delete``
	delete a dentry.  If there are no other open references to the
	dentry then the dentry is turned into a negative dentry (the
	d_iput() method is called).  If there are other references, then
	d_drop() is called instead

``d_add``
	add a dentry to its parents hash list and then calls
	d_instantiate()

``d_instantiate``
	add a dentry to the alias hash list for the inode and updates
	the "d_inode" member.  The "i_count" member in the inode
	structure should be set/incremented.  If the inode pointer is
	NULL, the dentry is called a "negative dentry".  This function
	is commonly called when an inode is created for an existing
	negative dentry

``d_lookup``
	look up a dentry given its parent and path name component It
	looks up the child of that given name from the dcache hash
	table.  If it is found, the reference count is incremented and
	the dentry is returned.  The caller must use dput() to free the
	dentry when it finishes using it.


Mount Options
=============


Parsing options
---------------

On mount and remount the filesystem is passed a string containing a
comma separated list of mount options.  The options can have either of
these forms:

  option
  option=value

The <linux/parser.h> header defines an API that helps parse these
options.  There are plenty of examples on how to use it in existing
filesystems.


Showing options
---------------

If a filesystem accepts mount options, it must define show_options() to
show all the currently active options.  The rules are:

  - options MUST be shown which are not default or their values differ
    from the default

  - options MAY be shown which are enabled by default or have their
    default value

Options used only internally between a mount helper and the kernel (such
as file descriptors), or which only have an effect during the mounting
(such as ones controlling the creation of a journal) are exempt from the
above rules.

The underlying reason for the above rules is to make sure, that a mount
can be accurately replicated (e.g. umounting and mounting again) based
on the information found in /proc/mounts.


Resources
=========

(Note some of these resources are not up-to-date with the latest kernel
 version.)

Creating Linux virtual filesystems. 2002
    <https://lwn.net/Articles/13325/>

The Linux Virtual File-system Layer by Neil Brown. 1999
    <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>

A tour of the Linux VFS by Michael K. Johnson. 1996
    <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>

A small trail through the Linux kernel by Andries Brouwer. 2001
    <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>