linux-ext4 - Re: [PATCH] jbd/2[stable only]: Use WRITE_SYNC_PLUG in journal_commit

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Date:	Thu, 14 Jul 2011 16:08:24 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Tao Ma <tm@....ma>, Vivek Goyal <vgoyal@...hat.com>,
	linux-ext4@...r.kernel.org, stable@...nel.org,
	Corrado Zoccolo <czoccolo@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] jbd/2[stable only]: Use WRITE_SYNC_PLUG in journal_commit_transaction.

Jan Kara <jack@...e.cz> writes:

> On Thu 14-07-11 12:30:32, Jeff Moyer wrote:
>> Tao Ma <tm@....ma> writes:
>> >> - WRITE_SYNC_PLUG will plug the queue and expects explicity unplug. Who
>> >>   is doing unplug in this case?
>> > See the comments I removed, "we rely on sync_buffer() doing the unplug
>> > for us". I removed them cause we all use pluged write now.
>> 
>> Your logic is upside-down.  The code currently only uses the _PLUG
>> variant when t_synchronous_commit is set, meaning somebody *will* call
>> sync_buffer.  Simply setting WRITE_SYNC_PLUG doens't mean the upper
>> layer is going to issue the unplug.  Of course, I'm not 100% sure of the
>> journaling process, so it may very well be that there always is an
>> unplug.  Can Jan or someone comment on that?  Anyway, you could test
>> this theory by seeing if your kernel generates any timer unplugs in the
>> blktrace output.
>   So I'm not expert in plugging code but from what I understand when we do
> wait_on_buffer() (which calls io_schedule()) which will do
> blk_flush_plug()), the queue will get unplugged and IO starts. And we wait
> for all buffers we submit so we are guaranteed wait_on_buffer() will be
> called...

Sorry, I should have been more specific.  As Vivek mentioned, we're
talking about older kernels (pre the blk plugging series).  So, the
question is, if journal_commit_transaction is called with
t_synchronous_commit not set, will the underlying device ever be
unplugged by the journal code?  My guess is there's no explicit unplug,
so it's not correct to replace a WRITE_SYNC with a WRITE_SYNC_PLUG.

Cheers,
Jeff
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