linux-kernel - Re: can't oom-kill zap the victim's memory?

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Message-ID: <20150919151006.GC31952@redhat.com>
Date:	Sat, 19 Sep 2015 17:10:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kyle Walker <kwalker@...hat.com>, Christoph Lameter <cl@...ux.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, hannes@...xchg.org,
	vdavydov@...allels.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: Re: can't oom-kill zap the victim's memory?

(off-topic)

On 09/19, Oleg Nesterov wrote:
>
> @@ -570,8 +590,8 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  		victim = p;
>  	}
>
> -	/* mm cannot safely be dereferenced after task_unlock(victim) */
>  	mm = victim->mm;
> +	atomic_inc(&mm->mm_count);

Btw, I think we need this change anyway. This is pure theoretical, but
otherwise this task can exit and free its mm_struct right after task_unlock(),
then this mm_struct can be reallocated and used by another task, so we
can't trust the "p->mm == mm" check below.

Oleg.

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