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

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Message-Id: <201509301116.GDA41748.HFOOVFLtMQSFOJ@I-love.SAKURA.ne.jp>
Date:	Wed, 30 Sep 2015 11:16:05 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	oleg@...hat.com, akpm@...ux-foundation.org
Cc:	rientjes@...gle.com, kwalker@...hat.com, mhocko@...nel.org,
	skozina@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 3/3] mm/oom_kill: fix the wrong task->mm == mm checks in

Oleg Nesterov wrote:
> Both "child->mm == mm" and "p->mm != mm" checks in oom_kill_process()
> are wrong. ->mm can be if task is the exited group leader. This means

can be [missing word here?] if task



> +static bool process_has_mm(struct task_struct *p, struct mm_struct *mm)
> +{
> +	struct task_struct *t;
> +
> +	for_each_thread(p, t)
> +		if (t->mm)

Can t->mm change between pevious line and next line?

> +			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 +541,7 @@ 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_has_mm(child, p->mm))
>  				continue;

We hold read_lock(&tasklist_lock) but not rcu_read_lock().
Is for_each_thread() safe without rcu_read_lock()?

>  			/*
>  			 * oom_badness() returns 0 if the thread is unkillable
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