linux-kernel - Re: [PATCH 2/2] mm, oom_reaper: do not mmput synchronously from the oom reaper context

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Date:	Fri, 20 May 2016 10:30:53 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Michal Hocko <mhocko@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/2] mm, oom_reaper: do not mmput synchronously from the
 oom reaper context

Forking new thread because my comment is not related to this patch's
purpose but found a thing during reading this patch.

On Tue, Apr 26, 2016 at 04:04:30PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Tetsuo has properly noted that mmput slow path might get blocked waiting
> for another party (e.g. exit_aio waits for an IO). If that happens the
> oom_reaper would be put out of the way and will not be able to process
> next oom victim. We should strive for making this context as reliable
> and independent on other subsystems as much as possible.
> 
> Introduce mmput_async which will perform the slow path from an async
> (WQ) context. This will delay the operation but that shouldn't be a
> problem because the oom_reaper has reclaimed the victim's address space
> for most cases as much as possible and the remaining context shouldn't
> bind too much memory anymore. The only exception is when mmap_sem
> trylock has failed which shouldn't happen too often.
> 
> The issue is only theoretical but not impossible.

The mmput_async is used for only OOM reaper which is enabled on CONFIG_MMU.
So until someone who want to use mmput_async in !CONFIG_MMU come out,
we could save sizeof(struct work_struct) per mm in !CONFIG_MMU.