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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/DocBook/journal-api.tmpl
downloadconfigs-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="LinuxJBDAPI">
+ <bookinfo>
+ <title>The Linux Journalling API</title>
+ <authorgroup>
+ <author>
+ <firstname>Roger</firstname>
+ <surname>Gammans</surname>
+ <affiliation>
+ <address>
+ <email>rgammans@computer-surgery.co.uk</email>
+ </address>
+ </affiliation>
+ </author>
+ </authorgroup>
+
+ <authorgroup>
+ <author>
+ <firstname>Stephen</firstname>
+ <surname>Tweedie</surname>
+ <affiliation>
+ <address>
+ <email>sct@redhat.com</email>
+ </address>
+ </affiliation>
+ </author>
+ </authorgroup>
+
+ <copyright>
+ <year>2002</year>
+ <holder>Roger Gammans</holder>
+ </copyright>
+
+<legalnotice>
+ <para>
+ This documentation is free software; you can redistribute
+ it and/or modify it under the terms of the GNU General Public
+ License as published by the Free Software Foundation; either
+ version 2 of the License, or (at your option) any later
+ version.
+ </para>
+
+ <para>
+ This program is distributed in the hope that it will be
+ useful, but WITHOUT ANY WARRANTY; without even the implied
+ warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ See the GNU General Public License for more details.
+ </para>
+
+ <para>
+ You should have received a copy of the GNU General Public
+ License along with this program; if not, write to the Free
+ Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ MA 02111-1307 USA
+ </para>
+
+ <para>
+ For more details see the file COPYING in the source
+ distribution of Linux.
+ </para>
+ </legalnotice>
+ </bookinfo>
+
+<toc></toc>
+
+ <chapter id="Overview">
+ <title>Overview</title>
+ <sect1>
+ <title>Details</title>
+<para>
+The journalling layer is easy to use. You need to
+first of all create a journal_t data structure. There are
+two calls to do this dependent on how you decide to allocate the physical
+media on which the journal resides. The journal_init_inode() call
+is for journals stored in filesystem inodes, or the journal_init_dev()
+call can be use for journal stored on a raw device (in a continuous range
+of blocks). A journal_t is a typedef for a struct pointer, so when
+you are finally finished make sure you call journal_destroy() on it
+to free up any used kernel memory.
+</para>
+
+<para>
+Once you have got your journal_t object you need to 'mount' or load the journal
+file, unless of course you haven't initialised it yet - in which case you
+need to call journal_create().
+</para>
+
+<para>
+Most of the time however your journal file will already have been created, but
+before you load it you must call journal_wipe() to empty the journal file.
+Hang on, you say , what if the filesystem wasn't cleanly umount()'d . Well, it is the
+job of the client file system to detect this and skip the call to journal_wipe().
+</para>
+
+<para>
+In either case the next call should be to journal_load() which prepares the
+journal file for use. Note that journal_wipe(..,0) calls journal_skip_recovery()
+for you if it detects any outstanding transactions in the journal and similarly
+journal_load() will call journal_recover() if necessary.
+I would advise reading fs/ext3/super.c for examples on this stage.
+[RGG: Why is the journal_wipe() call necessary - doesn't this needlessly
+complicate the API. Or isn't a good idea for the journal layer to hide
+dirty mounts from the client fs]
+</para>
+
+<para>
+Now you can go ahead and start modifying the underlying
+filesystem. Almost.
+</para>
+
+
+<para>
+
+You still need to actually journal your filesystem changes, this
+is done by wrapping them into transactions. Additionally you
+also need to wrap the modification of each of the the buffers
+with calls to the journal layer, so it knows what the modifications
+you are actually making are. To do this use journal_start() which
+returns a transaction handle.
+</para>
+
+<para>
+journal_start()
+and its counterpart journal_stop(), which indicates the end of a transaction
+are nestable calls, so you can reenter a transaction if necessary,
+but remember you must call journal_stop() the same number of times as
+journal_start() before the transaction is completed (or more accurately
+leaves the the update phase). Ext3/VFS makes use of this feature to simplify
+quota support.
+</para>
+
+<para>
+Inside each transaction you need to wrap the modifications to the
+individual buffers (blocks). Before you start to modify a buffer you
+need to call journal_get_{create,write,undo}_access() as appropriate,
+this allows the journalling layer to copy the unmodified data if it
+needs to. After all the buffer may be part of a previously uncommitted
+transaction.
+At this point you are at last ready to modify a buffer, and once
+you are have done so you need to call journal_dirty_{meta,}data().
+Or if you've asked for access to a buffer you now know is now longer
+required to be pushed back on the device you can call journal_forget()
+in much the same way as you might have used bforget() in the past.
+</para>
+
+<para>
+A journal_flush() may be called at any time to commit and checkpoint
+all your transactions.
+</para>
+
+<para>
+Then at umount time , in your put_super() (2.4) or write_super() (2.5)
+you can then call journal_destroy() to clean up your in-core journal object.
+</para>
+
+
+<para>
+Unfortunately there a couple of ways the journal layer can cause a deadlock.
+The first thing to note is that each task can only have
+a single outstanding transaction at any one time, remember nothing
+commits until the outermost journal_stop(). This means
+you must complete the transaction at the end of each file/inode/address
+etc. operation you perform, so that the journalling system isn't re-entered
+on another journal. Since transactions can't be nested/batched
+across differing journals, and another filesystem other than
+yours (say ext3) may be modified in a later syscall.
+</para>
+
+<para>
+The second case to bear in mind is that journal_start() can
+block if there isn't enough space in the journal for your transaction
+(based on the passed nblocks param) - when it blocks it merely(!) needs to
+wait for transactions to complete and be committed from other tasks,
+so essentially we are waiting for journal_stop(). So to avoid
+deadlocks you must treat journal_start/stop() as if they
+were semaphores and include them in your semaphore ordering rules to prevent
+deadlocks. Note that journal_extend() has similar blocking behaviour to
+journal_start() so you can deadlock here just as easily as on journal_start().
+</para>
+
+<para>
+Try to reserve the right number of blocks the first time. ;-). This will
+be the maximum number of blocks you are going to touch in this transaction.
+I advise having a look at at least ext3_jbd.h to see the basis on which
+ext3 uses to make these decisions.
+</para>
+
+<para>
+Another wriggle to watch out for is your on-disk block allocation strategy.
+why? Because, if you undo a delete, you need to ensure you haven't reused any
+of the freed blocks in a later transaction. One simple way of doing this
+is make sure any blocks you allocate only have checkpointed transactions
+listed against them. Ext3 does this in ext3_test_allocatable().
+</para>
+
+<para>
+Lock is also providing through journal_{un,}lock_updates(),
+ext3 uses this when it wants a window with a clean and stable fs for a moment.
+eg.
+</para>
+
+<programlisting>
+
+ journal_lock_updates() //stop new stuff happening..
+ journal_flush() // checkpoint everything.
+ ..do stuff on stable fs
+ journal_unlock_updates() // carry on with filesystem use.
+</programlisting>
+
+<para>
+The opportunities for abuse and DOS attacks with this should be obvious,
+if you allow unprivileged userspace to trigger codepaths containing these
+calls.
+</para>
+
+<para>
+A new feature of jbd since 2.5.25 is commit callbacks with the new
+journal_callback_set() function you can now ask the journalling layer
+to call you back when the transaction is finally committed to disk, so that
+you can do some of your own management. The key to this is the journal_callback
+struct, this maintains the internal callback information but you can
+extend it like this:-
+</para>
+<programlisting>
+ struct myfs_callback_s {
+ //Data structure element required by jbd..
+ struct journal_callback for_jbd;
+ // Stuff for myfs allocated together.
+ myfs_inode* i_commited;
+
+ }
+</programlisting>
+
+<para>
+this would be useful if you needed to know when data was committed to a
+particular inode.
+</para>
+
+</sect1>
+
+<sect1>
+<title>Summary</title>
+<para>
+Using the journal is a matter of wrapping the different context changes,
+being each mount, each modification (transaction) and each changed buffer
+to tell the journalling layer about them.
+</para>
+
+<para>
+Here is a some pseudo code to give you an idea of how it works, as
+an example.
+</para>
+
+<programlisting>
+ journal_t* my_jnrl = journal_create();
+ journal_init_{dev,inode}(jnrl,...)
+ if (clean) journal_wipe();
+ journal_load();
+
+ foreach(transaction) { /*transactions must be
+ completed before
+ a syscall returns to
+ userspace*/
+
+ handle_t * xct=journal_start(my_jnrl);
+ foreach(bh) {
+ journal_get_{create,write,undo}_access(xact,bh);
+ if ( myfs_modify(bh) ) { /* returns true
+ if makes changes */
+ journal_dirty_{meta,}data(xact,bh);
+ } else {
+ journal_forget(bh);
+ }
+ }
+ journal_stop(xct);
+ }
+ journal_destroy(my_jrnl);
+</programlisting>
+</sect1>
+
+</chapter>
+
+ <chapter id="adt">
+ <title>Data Types</title>
+ <para>
+ The journalling layer uses typedefs to 'hide' the concrete definitions
+ of the structures used. As a client of the JBD layer you can
+ just rely on the using the pointer as a magic cookie of some sort.
+
+ Obviously the hiding is not enforced as this is 'C'.
+ </para>
+ <sect1><title>Structures</title>
+!Iinclude/linux/jbd.h
+ </sect1>
+</chapter>
+
+ <chapter id="calls">
+ <title>Functions</title>
+ <para>
+ The functions here are split into two groups those that
+ affect a journal as a whole, and those which are used to
+ manage transactions
+</para>
+ <sect1><title>Journal Level</title>
+!Efs/jbd/journal.c
+!Efs/jbd/recovery.c
+ </sect1>
+ <sect1><title>Transasction Level</title>
+!Efs/jbd/transaction.c
+ </sect1>
+</chapter>
+<chapter>
+ <title>See also</title>
+ <para>
+ <citation>
+ <ulink url="ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/journal-design.ps.gz">
+ Journaling the Linux ext2fs Filesystem,LinuxExpo 98, Stephen Tweedie
+ </ulink>
+ </citation>
+ </para>
+ <para>
+ <citation>
+ <ulink url="http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html">
+ Ext3 Journalling FileSystem , OLS 2000, Dr. Stephen Tweedie
+ </ulink>
+ </citation>
+ </para>
+</chapter>
+
+</book>