linux-kernel - Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before going OOM

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Message-ID: <20121213103420.GW1009@suse.de>
Date:	Thu, 13 Dec 2012 10:34:20 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before
 going OOM

On Wed, Dec 12, 2012 at 04:43:34PM -0500, Johannes Weiner wrote:
> When a reclaim scanner is doing its final scan before giving up and
> there is swap space available, pay no attention to swappiness
> preference anymore.  Just swap.
> 
> Note that this change won't make too big of a difference for general
> reclaim: anonymous pages are already force-scanned when there is only
> very little file cache left, and there very likely isn't when the
> reclaimer enters this final cycle.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Ok, I see the motivation for your patch but is the block inside still
wrong for what you want? After your patch the block looks like this

                if (sc->priority || noswap) {
                        scan >>= sc->priority;
                        if (!scan && force_scan)
                                scan = SWAP_CLUSTER_MAX;
                        scan = div64_u64(scan * fraction[file], denominator);
                }

if sc->priority == 0 and swappiness==0 then you enter this block but
fraction[0] for anonymous pages will also be 0 and because of the ordering
of statements there, scan will be

scan = scan * 0 / denominator

so you are still not reclaiming anonymous pages in the swappiness=0
case. What did I miss?

-- 
Mel Gorman
SUSE Labs
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