linux-kernel - Re: [PATCH 3/5] page allocator: Wait on both sync and async congestion after direct reclaim

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Date:	Fri, 13 Nov 2009 15:41:46 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frans Pop <elendil@...net.nl>, Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>, linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/5] page allocator: Wait on both sync and async 
	congestion after direct reclaim

Hi Jens,

On Fri, Nov 13, 2009 at 3:32 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>> Suggest an alternative that brings congestion_wait() more in line with
>> 2.6.30 behaviour then.
>
> I don't have a good explanation as to why the delays have changed,
> unfortunately. Are we sure that they have between .30 and .31? The
> dm-crypt case is overly complex and lots of changes could have broken
> that house of cards.

Hand-waving or not, we have end user reports stating that reverting
commit 8aa7e847d834ed937a9ad37a0f2ad5b8584c1ab0 ("Fix
congestion_wait() sync/async vs read/write confusion") fixes their
(rather serious) OOM regression. The commit in question _does_
introduce a functional change and if this was your average regression,
people would be kicking and screaming to get it reverted.

So is there a reason we shouldn't send a partial revert of the commit
(switching to BLK_RW_SYNC) to Linus until the "real" issue gets
resolved? Yes, I realize it's ugly voodoo magic but dammit, it used to
work!

                        Pekka
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