linux-ext4 - Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV

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Date:	Thu, 8 Mar 2012 15:37:41 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	"Ted Ts'o" <tytso@....edu>, Zach Brown <zab@...bo.net>,
	Eric Sandeen <sandeen@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if
 !CONFIG_BLKDEV_INTEGRITY

On Thu, Mar 08, 2012 at 12:20:26PM -0800, Boaz Harrosh wrote:
> On 03/08/2012 10:09 AM, Chris Mason wrote:
> > 
> > But, why are we writeback for a second or more?  Aren't there other
> > parts of this we would want to fix as well?
> > 
> > I'm not against only turning on stable pages when they are needed, but
> > the code that isn't the default tends to be somewhat less used.  So it
> > does increase testing burden when we do want stable pages, and it tends
> > to make for awkward bugs that are hard to reproduce because someone
> > neglects to mention it.
> > 
> > IMHO it's much more important to nail down the 2 second writeback
> > latency. That's not good.
> > 
> 
> I think I understand this one. It's do to the sync nature introduced
> by page_waiting in mkwrite.

Pages go from dirty to writeback for a few reasons.  Background
writeout, or O_DIRECT or someone running sync

background writeout shouldn't be queueing up so much work that
synchronous writeout has a 2 second delay.

If the latencies are coming from something that was run through
fsync...well there's not too much we can do about that.  The problem is
that our page_mkwrite call isn't starting the IO it is just waiting on
it, so we can't bump the priority on it.

-chris
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