linux-kernel - Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm checks in oom_kill

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Date:	Thu, 1 Oct 2015 14:56:48 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm
 checks in oom_kill_process()

On Wed 30-09-15 20:24:11, Oleg Nesterov wrote:
> Both "child->mm == mm" and "p->mm != mm" checks in oom_kill_process()
> are wrong. task->mm can be NULL if the task is the exited group leader.
> This means in particular that "kill sharing same memory" loop can miss
> a process with a zombie leader which uses the same ->mm.
> 
> Note: the process_has_mm(child, p->mm) check is still not 100% correct,
> p->mm can be NULL too. This is minor, but probably deserves a fix or a
> comment anyway.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/oom_kill.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index c189ee5..034d219 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -483,6 +483,18 @@ void oom_killer_enable(void)
>  	oom_killer_disabled = false;
>  }
>  
> +static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
> +{
> +	struct task_struct *t;
> +
> +	for_each_thread(p, t) {
> +		struct mm_struct *t_mm = READ_ONCE(t->mm);
> +		if (t_mm)
> +			return t_mm == mm;
> +	}
> +	return false;
> +}
> +
>  #define K(x) ((x) << (PAGE_SHIFT-10))
>  /*
>   * Must be called while holding a reference to p, which will be released upon
> @@ -530,7 +542,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  		list_for_each_entry(child, &t->children, sibling) {
>  			unsigned int child_points;
>  
> -			if (child->mm == p->mm)
> +			if (process_shares_mm(child, p->mm))
>  				continue;
>  			/*
>  			 * oom_badness() returns 0 if the thread is unkillable
> @@ -584,7 +596,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  	 */
>  	rcu_read_lock();
>  	for_each_process(p) {
> -		if (p->mm != mm)
> +		if (!process_shares_mm(p, mm))
>  			continue;
>  		if (same_thread_group(p, victim))
>  			continue;
> -- 
> 2.4.3

-- 
Michal Hocko
SUSE Labs
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