linux-kernel - Re: [PATCH] mm,oom: Exclude TIF_MEMDIE processes from candidates.

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Date:	Thu, 7 Jan 2016 22:31:32 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, rientjes@...gle.com
Cc:	akpm@...ux-foundation.org, mgorman@...e.de,
	torvalds@...ux-foundation.org, oleg@...hat.com, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm,oom: Exclude TIF_MEMDIE processes from candidates.

Michal Hocko wrote:
> I do not think the placement in find_lock_task_mm is desirable nor
> correct. This function is used in multiple contexts outside of the oom
> proper. It only returns a locked task_struct for a thread that belongs
> to the process.

OK. Andrew, please drop from -mm tree for now.

> What you are seeing is clearly undesirable of course but I believe we
> should handle it at oom_kill_process layer. Blindly selecting a child
> process even when it doesn't sit on some memory or when it has already
> been killed is wrong. The heuristic is clearly too naive and so we
> should touch it rather than compensating it somewhere else. What about
> the following simple approach? It does two things and I will split it
> up if this looks like a desirable approach. Please note I haven't tested
> it because it is more of an idea than a finished thing. What do you think?

I think we need to filter at select_bad_process() and oom_kill_process().

When P has no children, P is chosen and TIF_MEMDIE is set on P. But P can
be chosen forever due to P->signal->oom_score_adj == OOM_SCORE_ADJ_MAX
even if the OOM reaper reclaimed P's mm. We need to ensure that
oom_kill_process() is not called with P if P already has TIF_MEMDIE.

(By the way, we are already assuming the OOM reaper kernel thread is
available. Changing to BUG_ON(IS_ERR(oom_reaper_th)) should be OK. ;-) )
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