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

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Message-ID: <20121011122037.GE31863@cmpxchg.org>
Date:	Thu, 11 Oct 2012 08:20:37 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: oom: fix totalpages calculation for
 memory.swappiness==0

On Thu, Oct 11, 2012 at 10:57:39AM +0200, Michal Hocko wrote:
> 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 same issue exists for the global oom killer as well but it is not
> that problematic as the amount of the RAM is usually much bigger than
> the swap space.
> 
> 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>
> Cc: stable [3.5+]

I also don't think it's hackish, the limit depends very much on
whether reclaim can swap, so it's natural that swappiness shows up
here.

Acked-by: Johannes Weiner <hannes@...xchg.org>
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