linux-kernel - Re: [PATCH 6/6] mm, oom: fortify task_will_free

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Date:	Thu, 26 May 2016 23:11:47 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.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 6/6] mm, oom: fortify task_will_free_mem

Michal Hocko wrote:
> +/*
> + * Checks whether the given task is dying or exiting and likely to
> + * release its address space. This means that all threads and processes
> + * sharing the same mm have to be killed or exiting.
> + */
> +static inline bool task_will_free_mem(struct task_struct *task)
> +{
> +	struct mm_struct *mm = NULL;
> +	struct task_struct *p;
> +
> +	/*
> +	 * If the process has passed exit_mm we have to skip it because
> +	 * we have lost a link to other tasks sharing this mm, we do not
> +	 * have anything to reap and the task might then get stuck waiting
> +	 * for parent as zombie and we do not want it to hold TIF_MEMDIE
> +	 */
> +	p = find_lock_task_mm(task);
> +	if (!p)
>  		return false;
>  
> +	if (!__task_will_free_mem(p)) {
> +		task_unlock(p);
> +		return false;
> +	}
> +
> +	/*
> +	 * Check whether there are other processes sharing the mm - they all have
> +	 * to be killed or exiting.
> +	 */
> +	if (atomic_read(&p->mm->mm_users) > get_nr_threads(p)) {
> +		mm = p->mm;
> +		/* pin the mm to not get freed and reused */
> +		atomic_inc(&mm->mm_count);
> +	}
> +	task_unlock(p);
> +
> +	if (mm) {
> +		rcu_read_lock();
> +		for_each_process(p) {
> +			bool vfork;
> +
> +			/*
> +			 * skip over vforked tasks because they are mostly
> +			 * independent and will drop the mm soon
> +			 */
> +			task_lock(p);
> +			vfork = p->vfork_done;
> +			task_unlock(p);
> +			if (vfork)
> +				continue;
> +
> +			if (!__task_will_free_mem(p))
> +				break;
> +		}
> +		rcu_read_unlock();
> +		mmdrop(mm);

Did you forget "if (something) return false;" here?

> +	}
> +

If task_will_free_mem() == true is always followed by mark_oom_victim()
and wake_oom_reaper(), can we call them from here?

>  	return true;
>  }