Re: [PATCH 4/4] oom: don't ignore rss in nascent mm

[Oleg Nesterov <oleg@redhat.com>]

On 09/16, KOSAKI Motohiro wrote:
>
> ChangeLog
>  o since v1
>    - Always use thread group leader's ->in_exec_mm.

Confused ;)

> +static unsigned long oom_rss_swap_usage(struct task_struct *p)
> +{
> +	struct task_struct *t = p;
> +	struct task_struct *leader = p->group_leader;
> +	unsigned long points = 0;
> +
> +	do {
> +		task_lock(t);
> +		if (t->mm) {
> +			points += get_mm_rss(t->mm);
> +			points += get_mm_counter(t->mm, MM_SWAPENTS);
> +			task_unlock(t);
> +			break;
> +		}
> +		task_unlock(t);
> +	} while_each_thread(p, t);
> +
> +	/*
> +	 * If the process is in execve() processing, we have to concern
> +	 * about both old and new mm.
> +	 */
> +	task_lock(leader);
> +	if (leader->in_exec_mm) {
> +		points += get_mm_rss(leader->in_exec_mm);
> +		points += get_mm_counter(leader->in_exec_mm, MM_SWAPENTS);
> +	}
> +	task_unlock(leader);
> +
> +	return points;
> +}

This patch relies on fact that we can't race with de_thread() (and btw
the change in de_thread() looks bogus). Then why ->in_exec_mm lives in
task_struct ?

To me, this looks a bit strange. I think we should either do not use
->group_leader to hold ->in_exec_mm like your previous patch did, or
move ->in_exec_mm into signal_struct. The previous 3/4 ensures that
only one thread can set ->in_exec_mm.

And I don't think oom_rss_swap_usage() should replace find_lock_task_mm()
in oom_badness(), I mean something like this:

	static unsigned long oom_rss_swap_usage(struct mm_struct *mm)
	{
		return get_mm_rss(mm) + get_mm_counter(mm, MM_SWAPENTS);
	}

	unsigned int oom_badness(struct task_struct *p, ...)
	{
		int points = 0;

		if (unlikely(p->signal->in_exec_mm)) {
			task_lock(p->group_leader);
			if (p->signal->in_exec_mm)
				points = oom_rss_swap_usage(p->signal->in_exec_mm);
			task_unlock(p->group_leader);
		}

		p = find_lock_task_mm(p);
		if (!p)
			return points;

		...
	}

but this is the matter of taste.

What do you think?

Oleg.