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

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Date:	Thu, 14 Jul 2011 21:46:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Tao Ma <tm@....ma>, Vivek Goyal <vgoyal@...hat.com>,
	linux-ext4@...r.kernel.org, stable@...nel.org,
	Jan Kara <jack@...e.cz>, 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.

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...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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