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Randy Dunlapbbb5bbb2006-10-29 22:46:40 -08001<?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
Randy Dunlap733b72c2006-11-02 22:07:02 -0800101 <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>
142The journalling layer is easy to use. You need to
143first of all create a journal_t data structure. There are
144two calls to do this dependent on how you decide to allocate the physical
145media on which the journal resides. The journal_init_inode() call
146is for journals stored in filesystem inodes, or the journal_init_dev()
147call can be use for journal stored on a raw device (in a continuous range
148of blocks). A journal_t is a typedef for a struct pointer, so when
149you are finally finished make sure you call journal_destroy() on it
150to free up any used kernel memory.
151</para>
152
153<para>
154Once you have got your journal_t object you need to 'mount' or load the journal
155file, unless of course you haven't initialised it yet - in which case you
156need to call journal_create().
157</para>
158
159<para>
160Most of the time however your journal file will already have been created, but
161before you load it you must call journal_wipe() to empty the journal file.
162Hang on, you say , what if the filesystem wasn't cleanly umount()'d . Well, it is the
163job of the client file system to detect this and skip the call to journal_wipe().
164</para>
165
166<para>
167In either case the next call should be to journal_load() which prepares the
168journal file for use. Note that journal_wipe(..,0) calls journal_skip_recovery()
169for you if it detects any outstanding transactions in the journal and similarly
170journal_load() will call journal_recover() if necessary.
171I 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
173complicate the API. Or isn't a good idea for the journal layer to hide
174dirty mounts from the client fs]
175</para>
176
177<para>
178Now you can go ahead and start modifying the underlying
179filesystem. Almost.
180</para>
181
182<para>
183
184You still need to actually journal your filesystem changes, this
185is done by wrapping them into transactions. Additionally you
186also need to wrap the modification of each of the buffers
187with calls to the journal layer, so it knows what the modifications
188you are actually making are. To do this use journal_start() which
189returns a transaction handle.
190</para>
191
192<para>
193journal_start()
194and its counterpart journal_stop(), which indicates the end of a transaction
195are nestable calls, so you can reenter a transaction if necessary,
196but remember you must call journal_stop() the same number of times as
197journal_start() before the transaction is completed (or more accurately
198leaves the update phase). Ext3/VFS makes use of this feature to simplify
199quota support.
200</para>
201
202<para>
203Inside each transaction you need to wrap the modifications to the
204individual buffers (blocks). Before you start to modify a buffer you
205need to call journal_get_{create,write,undo}_access() as appropriate,
206this allows the journalling layer to copy the unmodified data if it
207needs to. After all the buffer may be part of a previously uncommitted
208transaction.
209At this point you are at last ready to modify a buffer, and once
210you are have done so you need to call journal_dirty_{meta,}data().
211Or if you've asked for access to a buffer you now know is now longer
212required to be pushed back on the device you can call journal_forget()
213in much the same way as you might have used bforget() in the past.
214</para>
215
216<para>
217A journal_flush() may be called at any time to commit and checkpoint
218all your transactions.
219</para>
220
221<para>
222Then at umount time , in your put_super() (2.4) or write_super() (2.5)
223you can then call journal_destroy() to clean up your in-core journal object.
224</para>
225
226<para>
227Unfortunately there a couple of ways the journal layer can cause a deadlock.
228The first thing to note is that each task can only have
229a single outstanding transaction at any one time, remember nothing
230commits until the outermost journal_stop(). This means
231you must complete the transaction at the end of each file/inode/address
232etc. operation you perform, so that the journalling system isn't re-entered
233on another journal. Since transactions can't be nested/batched
234across differing journals, and another filesystem other than
235yours (say ext3) may be modified in a later syscall.
236</para>
237
238<para>
239The second case to bear in mind is that journal_start() can
240block 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
242wait for transactions to complete and be committed from other tasks,
243so essentially we are waiting for journal_stop(). So to avoid
244deadlocks you must treat journal_start/stop() as if they
245were semaphores and include them in your semaphore ordering rules to prevent
246deadlocks. Note that journal_extend() has similar blocking behaviour to
247journal_start() so you can deadlock here just as easily as on journal_start().
248</para>
249
250<para>
251Try to reserve the right number of blocks the first time. ;-). This will
252be the maximum number of blocks you are going to touch in this transaction.
253I advise having a look at at least ext3_jbd.h to see the basis on which
254ext3 uses to make these decisions.
255</para>
256
257<para>
258Another wriggle to watch out for is your on-disk block allocation strategy.
259why? Because, if you undo a delete, you need to ensure you haven't reused any
260of the freed blocks in a later transaction. One simple way of doing this
261is make sure any blocks you allocate only have checkpointed transactions
262listed against them. Ext3 does this in ext3_test_allocatable().
263</para>
264
265<para>
266Lock is also providing through journal_{un,}lock_updates(),
267ext3 uses this when it wants a window with a clean and stable fs for a moment.
268eg.
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>
280The opportunities for abuse and DOS attacks with this should be obvious,
281if you allow unprivileged userspace to trigger codepaths containing these
282calls.
283</para>
284
285<para>
286A new feature of jbd since 2.5.25 is commit callbacks with the new
287journal_callback_set() function you can now ask the journalling layer
288to call you back when the transaction is finally committed to disk, so that
289you can do some of your own management. The key to this is the journal_callback
290struct, this maintains the internal callback information but you can
291extend 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>
304this would be useful if you needed to know when data was committed to a
305particular inode.
306</para>
307
308 </sect2>
309
310 <sect2>
311 <title>Summary</title>
312<para>
313Using the journal is a matter of wrapping the different context changes,
314being each mount, each modification (transaction) and each changed buffer
315to tell the journalling layer about them.
316</para>
317
318<para>
319Here is a some pseudo code to give you an idea of how it works, as
320an 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
Randy Dunlapbbb5bbb2006-10-29 22:46:40 -0800401</book>