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

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Date:	Thu, 14 Jul 2011 23:38:22 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, 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.

On Thu 14-07-11 16:08:24, Jeff Moyer wrote:
> 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.
  There are no explicit unplugs in journalling code. But checking the code
in 2.6.37, I still see wait_on_buffer() calls sync_bh() which calls
blk_run_address_space() which ends up calling bdi->unplug_io_fn() so I
would say unplug is called anyway.

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