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Message-ID: <20160129152307.GF32174@dhcp22.suse.cz>
Date: Fri, 29 Jan 2016 16:23:08 +0100
From: Michal Hocko <mhocko@...nel.org>
To: David Rientjes <rientjes@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Mel Gorman <mgorman@...e.de>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
Hillf Danton <hillf.zj@...baba-inc.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before
out_of_memory
On Thu 28-01-16 15:19:08, David Rientjes wrote:
> On Thu, 28 Jan 2016, Johannes Weiner wrote:
>
> > The check has to happen while holding the OOM lock, otherwise we'll
> > end up killing much more than necessary when there are many racing
> > allocations.
> >
>
> Right, we need to try with ALLOC_WMARK_HIGH after oom_lock has been
> acquired.
>
> The situation is still somewhat fragile, however, but I think it's
> tangential to this patch series. If the ALLOC_WMARK_HIGH allocation fails
> because an oom victim hasn't freed its memory yet, and then the TIF_MEMDIE
> thread isn't visible during the oom killer's tasklist scan because it has
> exited, we still end up killing more than we should. The likelihood of
> this happening grows with the length of the tasklist.
Yes exactly the point I made in the original thread which brought the
question about ALLOC_WMARK_HIGH originally. The race window after the
last attempt is much larger than between the last wmark check and the
attempt.
> Perhaps we should try testing watermarks after a victim has been selected
> and immediately before killing? (Aside: we actually carry an internal
> patch to test mem_cgroup_margin() in the memcg oom path after selecting a
> victim because we have been hit with this before in the memcg path.)
>
> I would think that retrying with ALLOC_WMARK_HIGH would be enough memory
> to deem that we aren't going to immediately reenter an oom condition so
> the deferred killing is a waste of time.
>
> The downside is how sloppy this would be because it's blurring the line
> between oom killer and page allocator. We'd need the oom killer to return
> the selected victim to the page allocator, try the allocation, and then
> call oom_kill_process() if necessary.
Yes the layer violation is definitely not nice.
--
Michal Hocko
SUSE Labs
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