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1 <?xml version="1.0" encoding="UTF-8"?>
2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4
5 <book id="Linux-filesystems-API">
6 <bookinfo>
7 <title>Linux Filesystems API</title>
8
9 <legalnotice>
10 <para>
11 This documentation is free software; you can redistribute
12 it and/or modify it under the terms of the GNU General Public
13 License as published by the Free Software Foundation; either
14 version 2 of the License, or (at your option) any later
15 version.
16 </para>
17
18 <para>
19 This program is distributed in the hope that it will be
20 useful, but WITHOUT ANY WARRANTY; without even the implied
21 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
22 See the GNU General Public License for more details.
23 </para>
24
25 <para>
26 You should have received a copy of the GNU General Public
27 License along with this program; if not, write to the Free
28 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
29 MA 02111-1307 USA
30 </para>
31
32 <para>
33 For more details see the file COPYING in the source
34 distribution of Linux.
35 </para>
36 </legalnotice>
37 </bookinfo>
38
39 <toc></toc>
40
41 <chapter id="vfs">
42 <title>The Linux VFS</title>
43 <sect1><title>The Filesystem types</title>
44 !Iinclude/linux/fs.h
45 </sect1>
46 <sect1><title>The Directory Cache</title>
47 !Efs/dcache.c
48 !Iinclude/linux/dcache.h
49 </sect1>
50 <sect1><title>Inode Handling</title>
51 !Efs/inode.c
52 !Efs/bad_inode.c
53 </sect1>
54 <sect1><title>Registration and Superblocks</title>
55 !Efs/super.c
56 </sect1>
57 <sect1><title>File Locks</title>
58 !Efs/locks.c
59 !Ifs/locks.c
60 </sect1>
61 <sect1><title>Other Functions</title>
62 !Efs/mpage.c
63 !Efs/namei.c
64 !Efs/buffer.c
65 !Efs/bio.c
66 !Efs/seq_file.c
67 !Efs/filesystems.c
68 !Efs/fs-writeback.c
69 !Efs/block_dev.c
70 </sect1>
71 </chapter>
72
73 <chapter id="proc">
74 <title>The proc filesystem</title>
75
76 <sect1><title>sysctl interface</title>
77 !Ekernel/sysctl.c
78 </sect1>
79
80 <sect1><title>proc filesystem interface</title>
81 !Ifs/proc/base.c
82 </sect1>
83 </chapter>
84
85 <chapter id="sysfs">
86 <title>The Filesystem for Exporting Kernel Objects</title>
87 !Efs/sysfs/file.c
88 !Efs/sysfs/symlink.c
89 !Efs/sysfs/bin.c
90 </chapter>
91
92 <chapter id="debugfs">
93 <title>The debugfs filesystem</title>
94
95 <sect1><title>debugfs interface</title>
96 !Efs/debugfs/inode.c
97 !Efs/debugfs/file.c
98 </sect1>
99 </chapter>
100
101 <chapter id="LinuxJDBAPI">
102 <chapterinfo>
103 <title>The Linux Journalling API</title>
104
105 <authorgroup>
106 <author>
107 <firstname>Roger</firstname>
108 <surname>Gammans</surname>
109 <affiliation>
110 <address>
111 <email>rgammans@computer-surgery.co.uk</email>
112 </address>
113 </affiliation>
114 </author>
115 </authorgroup>
116
117 <authorgroup>
118 <author>
119 <firstname>Stephen</firstname>
120 <surname>Tweedie</surname>
121 <affiliation>
122 <address>
123 <email>sct@redhat.com</email>
124 </address>
125 </affiliation>
126 </author>
127 </authorgroup>
128
129 <copyright>
130 <year>2002</year>
131 <holder>Roger Gammans</holder>
132 </copyright>
133 </chapterinfo>
134
135 <title>The Linux Journalling API</title>
136
137 <sect1>
138 <title>Overview</title>
139 <sect2>
140 <title>Details</title>
141 <para>
142 The journalling layer is easy to use. You need to
143 first of all create a journal_t data structure. There are
144 two calls to do this dependent on how you decide to allocate the physical
145 media on which the journal resides. The journal_init_inode() call
146 is for journals stored in filesystem inodes, or the journal_init_dev()
147 call can be use for journal stored on a raw device (in a continuous range
148 of blocks). A journal_t is a typedef for a struct pointer, so when
149 you are finally finished make sure you call journal_destroy() on it
150 to free up any used kernel memory.
151 </para>
152
153 <para>
154 Once you have got your journal_t object you need to 'mount' or load the journal
155 file, unless of course you haven't initialised it yet - in which case you
156 need to call journal_create().
157 </para>
158
159 <para>
160 Most of the time however your journal file will already have been created, but
161 before you load it you must call journal_wipe() to empty the journal file.
162 Hang on, you say , what if the filesystem wasn't cleanly umount()'d . Well, it is the
163 job of the client file system to detect this and skip the call to journal_wipe().
164 </para>
165
166 <para>
167 In either case the next call should be to journal_load() which prepares the
168 journal file for use. Note that journal_wipe(..,0) calls journal_skip_recovery()
169 for you if it detects any outstanding transactions in the journal and similarly
170 journal_load() will call journal_recover() if necessary.
171 I would advise reading fs/ext3/super.c for examples on this stage.
172 [RGG: Why is the journal_wipe() call necessary - doesn't this needlessly
173 complicate the API. Or isn't a good idea for the journal layer to hide
174 dirty mounts from the client fs]
175 </para>
176
177 <para>
178 Now you can go ahead and start modifying the underlying
179 filesystem. Almost.
180 </para>
181
182 <para>
183
184 You still need to actually journal your filesystem changes, this
185 is done by wrapping them into transactions. Additionally you
186 also need to wrap the modification of each of the buffers
187 with calls to the journal layer, so it knows what the modifications
188 you are actually making are. To do this use journal_start() which
189 returns a transaction handle.
190 </para>
191
192 <para>
193 journal_start()
194 and its counterpart journal_stop(), which indicates the end of a transaction
195 are nestable calls, so you can reenter a transaction if necessary,
196 but remember you must call journal_stop() the same number of times as
197 journal_start() before the transaction is completed (or more accurately
198 leaves the update phase). Ext3/VFS makes use of this feature to simplify
199 quota support.
200 </para>
201
202 <para>
203 Inside each transaction you need to wrap the modifications to the
204 individual buffers (blocks). Before you start to modify a buffer you
205 need to call journal_get_{create,write,undo}_access() as appropriate,
206 this allows the journalling layer to copy the unmodified data if it
207 needs to. After all the buffer may be part of a previously uncommitted
208 transaction.
209 At this point you are at last ready to modify a buffer, and once
210 you are have done so you need to call journal_dirty_{meta,}data().
211 Or if you've asked for access to a buffer you now know is now longer
212 required to be pushed back on the device you can call journal_forget()
213 in much the same way as you might have used bforget() in the past.
214 </para>
215
216 <para>
217 A journal_flush() may be called at any time to commit and checkpoint
218 all your transactions.
219 </para>
220
221 <para>
222 Then at umount time , in your put_super() (2.4) or write_super() (2.5)
223 you can then call journal_destroy() to clean up your in-core journal object.
224 </para>
225
226 <para>
227 Unfortunately there a couple of ways the journal layer can cause a deadlock.
228 The first thing to note is that each task can only have
229 a single outstanding transaction at any one time, remember nothing
230 commits until the outermost journal_stop(). This means
231 you must complete the transaction at the end of each file/inode/address
232 etc. operation you perform, so that the journalling system isn't re-entered
233 on another journal. Since transactions can't be nested/batched
234 across differing journals, and another filesystem other than
235 yours (say ext3) may be modified in a later syscall.
236 </para>
237
238 <para>
239 The second case to bear in mind is that journal_start() can
240 block if there isn't enough space in the journal for your transaction
241 (based on the passed nblocks param) - when it blocks it merely(!) needs to
242 wait for transactions to complete and be committed from other tasks,
243 so essentially we are waiting for journal_stop(). So to avoid
244 deadlocks you must treat journal_start/stop() as if they
245 were semaphores and include them in your semaphore ordering rules to prevent
246 deadlocks. Note that journal_extend() has similar blocking behaviour to
247 journal_start() so you can deadlock here just as easily as on journal_start().
248 </para>
249
250 <para>
251 Try to reserve the right number of blocks the first time. ;-). This will
252 be the maximum number of blocks you are going to touch in this transaction.
253 I advise having a look at at least ext3_jbd.h to see the basis on which
254 ext3 uses to make these decisions.
255 </para>
256
257 <para>
258 Another wriggle to watch out for is your on-disk block allocation strategy.
259 why? Because, if you undo a delete, you need to ensure you haven't reused any
260 of the freed blocks in a later transaction. One simple way of doing this
261 is make sure any blocks you allocate only have checkpointed transactions
262 listed against them. Ext3 does this in ext3_test_allocatable().
263 </para>
264
265 <para>
266 Lock is also providing through journal_{un,}lock_updates(),
267 ext3 uses this when it wants a window with a clean and stable fs for a moment.
268 eg.
269 </para>
270
271 <programlisting>
272
273 journal_lock_updates() //stop new stuff happening..
274 journal_flush() // checkpoint everything.
275 ..do stuff on stable fs
276 journal_unlock_updates() // carry on with filesystem use.
277 </programlisting>
278
279 <para>
280 The opportunities for abuse and DOS attacks with this should be obvious,
281 if you allow unprivileged userspace to trigger codepaths containing these
282 calls.
283 </para>
284
285 <para>
286 A new feature of jbd since 2.5.25 is commit callbacks with the new
287 journal_callback_set() function you can now ask the journalling layer
288 to call you back when the transaction is finally committed to disk, so that
289 you can do some of your own management. The key to this is the journal_callback
290 struct, this maintains the internal callback information but you can
291 extend it like this:-
292 </para>
293 <programlisting>
294 struct myfs_callback_s {
295 //Data structure element required by jbd..
296 struct journal_callback for_jbd;
297 // Stuff for myfs allocated together.
298 myfs_inode* i_commited;
299
300 }
301 </programlisting>
302
303 <para>
304 this would be useful if you needed to know when data was committed to a
305 particular inode.
306 </para>
307
308 </sect2>
309
310 <sect2>
311 <title>Summary</title>
312 <para>
313 Using the journal is a matter of wrapping the different context changes,
314 being each mount, each modification (transaction) and each changed buffer
315 to tell the journalling layer about them.
316 </para>
317
318 <para>
319 Here is a some pseudo code to give you an idea of how it works, as
320 an example.
321 </para>
322
323 <programlisting>
324 journal_t* my_jnrl = journal_create();
325 journal_init_{dev,inode}(jnrl,...)
326 if (clean) journal_wipe();
327 journal_load();
328
329 foreach(transaction) { /*transactions must be
330 completed before
331 a syscall returns to
332 userspace*/
333
334 handle_t * xct=journal_start(my_jnrl);
335 foreach(bh) {
336 journal_get_{create,write,undo}_access(xact,bh);
337 if ( myfs_modify(bh) ) { /* returns true
338 if makes changes */
339 journal_dirty_{meta,}data(xact,bh);
340 } else {
341 journal_forget(bh);
342 }
343 }
344 journal_stop(xct);
345 }
346 journal_destroy(my_jrnl);
347 </programlisting>
348 </sect2>
349
350 </sect1>
351
352 <sect1>
353 <title>Data Types</title>
354 <para>
355 The journalling layer uses typedefs to 'hide' the concrete definitions
356 of the structures used. As a client of the JBD layer you can
357 just rely on the using the pointer as a magic cookie of some sort.
358
359 Obviously the hiding is not enforced as this is 'C'.
360 </para>
361 <sect2><title>Structures</title>
362 !Iinclude/linux/jbd.h
363 </sect2>
364 </sect1>
365
366 <sect1>
367 <title>Functions</title>
368 <para>
369 The functions here are split into two groups those that
370 affect a journal as a whole, and those which are used to
371 manage transactions
372 </para>
373 <sect2><title>Journal Level</title>
374 !Efs/jbd/journal.c
375 !Ifs/jbd/recovery.c
376 </sect2>
377 <sect2><title>Transasction Level</title>
378 !Efs/jbd/transaction.c
379 </sect2>
380 </sect1>
381 <sect1>
382 <title>See also</title>
383 <para>
384 <citation>
385 <ulink url="ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/journal-design.ps.gz">
386 Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen Tweedie
387 </ulink>
388 </citation>
389 </para>
390 <para>
391 <citation>
392 <ulink url="http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html">
393 Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen Tweedie
394 </ulink>
395 </citation>
396 </para>
397 </sect1>
398
399 </chapter>
400
401 </book>