linux-kernel - Re: [PATCH 3/2] oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

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Date:	Mon, 11 Jan 2016 11:52:14 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 3/2] oom: clear TIF_MEMDIE after oom_reaper managed to
 unmap the address space

This patch looks already good to me. I just have one question:

On Mon, Jan 11, 2016 at 01:42:00PM +0100, Michal Hocko wrote:
> @@ -463,60 +479,66 @@ static bool __oom_reap_vmas(struct mm_struct *mm)
>  	}
>  	tlb_finish_mmu(&tlb, 0, -1);
>  	up_read(&mm->mmap_sem);
> +
> +	/*
> +	 * Clear TIF_MEMDIE because the task shouldn't be sitting on a
> +	 * reasonably reclaimable memory anymore. OOM killer can continue
> +	 * by selecting other victim if unmapping hasn't led to any
> +	 * improvements. This also means that selecting this task doesn't
> +	 * make any sense.
> +	 */
> +	tsk->signal->oom_score_adj = OOM_SCORE_ADJ_MIN;
> +	exit_oom_victim(tsk);

When the OOM killer scans tasks and encounters a PF_EXITING one, it
force-selects that one regardless of the score. Is there a possibility
that the task might hang after it has set PF_EXITING? In that case the
OOM killer should be able to move on to the next task.

Frankly, I don't even know why we check for exiting tasks in the OOM
killer. We've tried direct reclaim at least 15 times by the time we
decide the system is OOM, there was plenty of time to exit and free
memory; and a task might exit voluntarily right after we issue a kill.
This is testing pure noise.

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index b8a4210..7dfb351 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -305,9 +305,6 @@ enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
 	if (oom_task_origin(task))
 		return OOM_SCAN_SELECT;
 
-	if (task_will_free_mem(task) && !is_sysrq_oom(oc))
-		return OOM_SCAN_ABORT;
-
 	return OOM_SCAN_OK;
 }