linux-kernel - Re: [PATCH 1/2] mm, oom: introduce oom reaper

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Date:	Fri, 18 Dec 2015 13:11:13 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Argangeli <andrea@...nel.org>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, oom: introduce oom reaper

On Thu 17-12-15 13:13:56, Andrew Morton wrote:
[...]
> Also, re-reading your description:
> 
> : It has been shown (e.g.  by Tetsuo Handa) that it is not that hard to
> : construct workloads which break the core assumption mentioned above and
> : the OOM victim might take unbounded amount of time to exit because it
> : might be blocked in the uninterruptible state waiting for on an event
> : (e.g.  lock) which is blocked by another task looping in the page
> : allocator.
> 
> So the allocating task has done an oom-kill and is waiting for memory
> to become available.  The killed task is stuck on some lock, unable to
> free memory.
> 
> But the problematic lock will sometimes be the killed tasks's mmap_sem,
> so the reaper won't reap anything.  This scenario requires that the
> mmap_sem is held for writing, which sounds like it will be uncommon. 

Yes, I have mentioned that in the changelog:
"
oom_reaper has to take mmap_sem on the target task for reading so the
solution is not 100% because the semaphore might be held or blocked for
write but the probability is reduced considerably wrt. basically any
lock blocking forward progress as described above.
"

Another thing is to do is to change down_write(mmap_sem) to
down_write_killable in most cases where we have a clear ENITR semantic.
This is on my todo list.

> hm.  sigh.  I hate the oom-killer.  Just buy some more memory already!

Tell me something about that...
-- 
Michal Hocko
SUSE Labs
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