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