linux-kernel - Re: [PATCH v2] memcg: oom: fix totalpages calculation for memory.swappiness==0

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Message-Id: <20121107141025.2ac62206.akpm@linux-foundation.org>
Date:	Wed, 7 Nov 2012 14:10:25 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] memcg: oom: fix totalpages calculation for
 memory.swappiness==0

On Tue, 16 Oct 2012 00:04:08 +0200
Michal Hocko <mhocko@...e.cz> wrote:

> As Kosaki correctly pointed out, the glogal reclaim doesn't have this
> issue because we _do_ swap on swappinnes==0 so the swap space has
> to be considered. So the v2 is just acks + changelog fix.
> 
> Changes since v1
> - drop a note about global swappiness affected as well from the
>   changelog
> - stable needs 3.2+ rather than 3.5+ because the fe35004f has been
>   backported to stable
> ---
> >From c2ae4849f09dbfda6b61472c6dd1fd8c2fe8ac81 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.cz>
> Date: Wed, 10 Oct 2012 15:46:54 +0200
> Subject: [PATCH] memcg: oom: fix totalpages calculation for
>  memory.swappiness==0
> 
> oom_badness takes totalpages argument which says how many pages are
> available and it uses it as a base for the score calculation. The value
> is calculated by mem_cgroup_get_limit which considers both limit and
> total_swap_pages (resp. memsw portion of it).
> 
> This is usually correct but since fe35004f (mm: avoid swapping out
> with swappiness==0) we do not swap when swappiness is 0 which means
> that we cannot really use up all the totalpages pages. This in turn
> confuses oom score calculation if the memcg limit is much smaller than
> the available swap because the used memory (capped by the limit) is
> negligible comparing to totalpages so the resulting score is too small
> if adj!=0 (typically task with CAP_SYS_ADMIN or non zero oom_score_adj).
> A wrong process might be selected as result.
> 
> The problem can be worked around by checking mem_cgroup_swappiness==0
> and not considering swap at all in such a case.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> Acked-by: David Rientjes <rientjes@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: stable [3.2+]

That's "Cc: <stable@...r.kernel.org>", please.

It's unobvious from the changelog that a -stable backport is really
needed.  The bug looks pretty obscure and has been there for a long
time.  Realistically, is anyone likely to hurt from this?

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