linux-kernel - Re: [PATCH 10/10] mm, oom: hide mm which is shared with kthread or global init

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Date:	Fri, 10 Jun 2016 00:15:18 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, linux-mm@...ck.org
Cc:	rientjes@...gle.com, oleg@...hat.com, vdavydov@...allels.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mhocko@...e.com
Subject: Re: [PATCH 10/10] mm, oom: hide mm which is shared with kthread or global init

Michal Hocko wrote:
> The only case where the oom_reaper is not triggered for the oom victim
> is when it shares the memory with a kernel thread (aka use_mm) or with
> the global init. After "mm, oom: skip vforked tasks from being selected"
> the victim cannot be a vforked task of the global init so we are left
> with clone(CLONE_VM) (without CLONE_SIGHAND). use_mm() users are quite
> rare as well.

CONFIG_MMU=n is the other case where the oom_reaper is not triggered for
the oom victim.

> 
> In order to guarantee a forward progress for the OOM killer make
> sure that this really rare cases will not get into the way and hide
> the mm from the oom killer by setting MMF_OOM_REAPED flag for it.
> oom_scan_process_thread will ignore any TIF_MEMDIE task if it has
> MMF_OOM_REAPED flag set to catch these oom victims.

Nobody will set MMF_OOM_REAPED flag if can_oom_reap == true on
CONFIG_MMU=n kernel. If a TIF_MEMDIE thread in CONFIG_MMU=n kernel
is blocked before exit_oom_victim() in exit_mm() from do_exit() is
called, the system will lock up. This is not handled in the patch
nor explained in the changelog.

> 
> After this patch we should guarantee a forward progress for the OOM
> killer even when the selected victim is sharing memory with a kernel
> thread or global init.