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

[Michal Hocko <mhocko@suse.cz>]

On Wed 07-11-12 14:10:25, Andrew Morton wrote:
> On Tue, 16 Oct 2012 00:04:08 +0200
> Michal Hocko <mhocko@suse.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@suse.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@suse.cz>
> > Acked-by: David Rientjes <rientjes@google.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Cc: stable [3.2+]
> 
> That's "Cc: <stable@vger.kernel.org>", please.

Will do next time.

> 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.

Yes but it is not _that_ long since fe35004f made it into stable trees
(e.g. 3.2.29).
The reason why we probably do not see many reports is because people
didn't get used to swappiness==0 really works these days - especially
with memcg where it means _really_ no swapping.

> Realistically, is anyone likely to hurt from this?

The primary motivation for the fix was a real report by a customer.
-- 
Michal Hocko
SUSE Labs

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