DocBook/filesystems/LinuxJDBAPI.html

<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 6. The Linux Journalling API</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="Linux Filesystems API"><link rel="up" href="index.html" title="Linux Filesystems API"><link rel="prev" href="API-debugfs-create-devm-seqfile.html" title="debugfs_create_devm_seqfile"><link rel="next" href="data_types.html" title="Data Types"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 6. The Linux Journalling API</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="API-debugfs-create-devm-seqfile.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="data_types.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="LinuxJDBAPI"></a>Chapter 6. The Linux Journalling API</h1></div><div><div class="authorgroup"><div class="author"><h3 class="author"><span class="firstname">Roger</span> <span class="surname">Gammans</span></h3><div class="affiliation"><div class="address"><p><br>
      <code class="email">&lt;<a class="email" href="mailto:rgammans@computer-surgery.co.uk">rgammans@computer-surgery.co.uk</a>&gt;</code><br>
     </p></div></div></div></div></div><div><div class="authorgroup"><div class="author"><h3 class="author"><span class="firstname">Stephen</span> <span class="surname">Tweedie</span></h3><div class="affiliation"><div class="address"><p><br>
      <code class="email">&lt;<a class="email" href="mailto:sct@redhat.com">sct@redhat.com</a>&gt;</code><br>
     </p></div></div></div></div></div><div><p class="copyright">Copyright © 2002 Roger Gammans</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl class="toc"><dt><span class="sect1"><a href="LinuxJDBAPI.html#journaling_overview">Overview</a></span></dt><dd><dl><dt><span class="sect2"><a href="LinuxJDBAPI.html#journaling_details">Details</a></span></dt><dt><span class="sect2"><a href="LinuxJDBAPI.html#jbd_summary">Summary</a></span></dt></dl></dd><dt><span class="sect1"><a href="data_types.html">Data Types</a></span></dt><dd><dl><dt><span class="sect2"><a href="data_types.html#structures">Structures</a></span></dt></dl></dd><dt><span class="sect1"><a href="functions.html">Functions</a></span></dt><dd><dl><dt><span class="sect2"><a href="functions.html#journal_level">Journal Level</a></span></dt><dt><span class="sect2"><a href="functions.html#transaction_level">Transasction Level</a></span></dt></dl></dd><dt><span class="sect1"><a href="see_also.html">See also</a></span></dt></dl></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="journaling_overview"></a>Overview</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="LinuxJDBAPI.html#journaling_details">Details</a></span></dt><dt><span class="sect2"><a href="LinuxJDBAPI.html#jbd_summary">Summary</a></span></dt></dl></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="journaling_details"></a>Details</h3></div></div></div><p>
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 jbd2_journal_init_inode() call
is for journals stored in filesystem inodes, or the jbd2_journal_init_dev()
call can be used 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 jbd2_journal_destroy() on it
to free up any used kernel memory.
</p><p>
Once you have got your journal_t object you need to 'mount' or load the journal
file. The journalling layer expects the space for the journal was already
allocated and initialized properly by the userspace tools.  When loading the
journal you must call jbd2_journal_load() to process journal contents.  If the
client file system detects the journal contents does not need to be processed
(or even need not have valid contents), it may call jbd2_journal_wipe() to
clear the journal contents before calling jbd2_journal_load().
</p><p>
Note that jbd2_journal_wipe(..,0) calls jbd2_journal_skip_recovery() for you if
it detects any outstanding transactions in the journal and similarly
jbd2_journal_load() will call jbd2_journal_recover() if necessary.  I would
advise reading ext4_load_journal() in fs/ext4/super.c for examples on this
stage.
</p><p>
Now you can go ahead and start modifying the underlying
filesystem. Almost.
</p><p>

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 buffers
with calls to the journal layer, so it knows what the modifications
you are actually making are. To do this use jbd2_journal_start() which
returns a transaction handle.
</p><p>
jbd2_journal_start()
and its counterpart jbd2_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 jbd2_journal_stop() the same number of times as
jbd2_journal_start() before the transaction is completed (or more accurately
leaves the update phase). Ext4/VFS makes use of this feature to simplify
handling of inode dirtying, quota support, etc.
</p><p>
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 jbd2_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 jbd2_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 jbd2_journal_forget()
in much the same way as you might have used bforget() in the past.
</p><p>
A jbd2_journal_flush() may be called at any time to commit and checkpoint
all your transactions.
</p><p>
Then at umount time , in your put_super() you can then call jbd2_journal_destroy()
to clean up your in-core journal object.
</p><p>
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 jbd2_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 ext4) may be modified in a later syscall.
</p><p>
The second case to bear in mind is that jbd2_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 jbd2_journal_stop(). So to avoid
deadlocks you must treat jbd2_journal_start/stop() as if they
were semaphores and include them in your semaphore ordering rules to prevent
deadlocks. Note that jbd2_journal_extend() has similar blocking behaviour to
jbd2_journal_start() so you can deadlock here just as easily as on
jbd2_journal_start().
</p><p>
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 ext4_jbd.h to see the basis on which
ext4 uses to make these decisions.
</p><p>
Another wriggle to watch out for is your on-disk block allocation strategy.
Why? Because, if you do a delete, you need to ensure you haven't reused any
of the freed blocks until the transaction freeing these blocks commits. If you
reused these blocks and crash happens, there is no way to restore the contents
of the reallocated blocks at the end of the last fully committed transaction.

One simple way of doing this is to mark blocks as free in internal in-memory
block allocation structures only after the transaction freeing them commits.
Ext4 uses journal commit callback for this purpose.
</p><p>
With journal commit callbacks you can ask the journalling layer to call a
callback function when the transaction is finally committed to disk, so that
you can do some of your own management. You ask the journalling layer for
calling the callback by simply setting journal-&gt;j_commit_callback function
pointer and that function is called after each transaction commit. You can also
use transaction-&gt;t_private_list for attaching entries to a transaction that
need processing when the transaction commits.
</p><p>
JBD2 also provides a way to block all transaction updates via
jbd2_journal_{un,}lock_updates(). Ext4 uses this when it wants a window with a
clean and stable fs for a moment.  E.g.
</p><pre class="programlisting">

	jbd2_journal_lock_updates() //stop new stuff happening..
	jbd2_journal_flush()        // checkpoint everything.
	..do stuff on stable fs
	jbd2_journal_unlock_updates() // carry on with filesystem use.
</pre><p>
The opportunities for abuse and DOS attacks with this should be obvious,
if you allow unprivileged userspace to trigger codepaths containing these
calls.
</p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="jbd_summary"></a>Summary</h3></div></div></div><p>
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.
</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="API-debugfs-create-devm-seqfile.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="data_types.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span class="phrase">debugfs_create_devm_seqfile</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Data Types</td></tr></table></div></body></html>