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Date: Fri, 13 Nov 2009 15:41:46 +0200
Message-ID: <84144f020911130541p42c0b3d5lc307f97f22e2d356@mail.gmail.com>
Subject: Re: [PATCH 3/5] page allocator: Wait on both sync and async 
	congestion after direct reclaim
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: Mel Gorman <mel@csn.ul.ie>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Frans Pop <elendil@planet.nl>, Jiri Kosina <jkosina@suse.cz>,
       Sven Geggus <lists@fuchsschwanzdomain.de>,
       Karol Lewandowski <karol.k.lewandowski@gmail.com>,
       Tobias Oetiker <tobi@oetiker.ch>, linux-kernel@vger.kernel.org,
       "linux-mm@kvack.org" <linux-mm@kvack.org>,
       Rik van Riel <riel@redhat.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Stephan von Krawczynski <skraw@ithnet.com>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Kernel Testers List <kernel-testers@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>
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Hi Jens,

On Fri, Nov 13, 2009 at 3:32 PM, Jens Axboe <jens.axboe@oracle.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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