linux-kernel - Re: [PATCH 4/6] mm,oom: exclude oom_task_origin processes if they are OOM-unkillable.

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Date:	Wed, 17 Feb 2016 22:36:47 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, mgorman@...e.de,
	oleg@...hat.com, torvalds@...ux-foundation.org, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] mm,oom: exclude oom_task_origin processes if they are OOM-unkillable.

Michal Hocko wrote:
> On Wed 17-02-16 19:33:07, Tetsuo Handa wrote:
> > >From 4924ca3031444bfb831b2d4f004e5a613ad48d68 Mon Sep 17 00:00:00 2001
> > From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > Date: Wed, 17 Feb 2016 16:35:12 +0900
> > Subject: [PATCH 4/6] mm,oom: exclude oom_task_origin processes if they are OOM-unkillable.
> > 
> > oom_scan_process_thread() returns OOM_SCAN_SELECT when there is a
> > thread which returns oom_task_origin() == true. But it is possible
> > that that thread is marked as OOM-unkillable.
> > 
> > This patch changes oom_scan_process_thread() not to select it
> > if it is marked as OOM-unkillable.
> 
> oom_task_origin is only swapoff and ksm_store right now. I seriously
> doubt anybody sane will run them as OOM disabled (directly or
> indirectly).

I think that the OOM reaper will update such task as OOM-unkillable
after reaping that task's memory. This patch is intended for not to
fall into infinite loop after the OOM reaper updated it.

> 
> But you have a point that returing anything but OOM_SCAN_CONTINUE for
> OOM_SCORE_ADJ_MIN from oom_scan_process_thread sounds suboptimal.
> Sure such a check would be racy but do we actually care about a OOM vs.
> oom_score_adj_write. I am dubious to say the least.
> 
> So wouldn't it make more sense to check for OOM_SCORE_ADJ_MIN at the
> very top of oom_scan_process_thread instead?

Are you suggesting something like below?
(OOM_SCORE_ADJ_MIN check needs to be done after TIF_MEMDIE check)

enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
					struct task_struct *task, unsigned long totalpages)
{
	if (oom_unkillable_task(task, NULL, oc->nodemask))
		return OOM_SCAN_CONTINUE;

	/*
	 * This task already has access to memory reserves and is being killed.
	 * Don't allow any other task to have access to the reserves.
	 */
	if (test_tsk_thread_flag(task, TIF_MEMDIE)) {
		if (!is_sysrq_oom(oc))
			return OOM_SCAN_ABORT;
	}
	if (!task->mm || task->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
		return OOM_SCAN_CONTINUE;
(...snipped...)
}