linux-kernel - Re: [PATCH v3 02/21] memcg: make mm_match

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Message-ID: <20120223180352.GA1701@cmpxchg.org>
Date:	Thu, 23 Feb 2012 19:03:52 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH v3 02/21] memcg: make mm_match_cgroup() hirarchical

On Thu, Feb 23, 2012 at 05:51:46PM +0400, Konstantin Khlebnikov wrote:
> Check mm-owner cgroup membership hierarchically.

I think this one cat just beat up this other cat in front of my
window, yelling something about money and missing product.  Anyway, I
already forgot why we want this patch.  Could you describe that in the
changelog, please?

> @@ -821,6 +821,26 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
>  				struct mem_cgroup, css);
>  }
>  
> +/**
> + * mm_match_cgroup - cgroup hierarchy mm membership test
> + * @mm		mm_struct to test
> + * @cgroup	target cgroup
> + *
> + * Returns true if mm belong this cgroup or any its child in hierarchy
> + */
> +int mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *cgroup)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	rcu_read_lock();
> +	memcg = mem_cgroup_from_task(rcu_dereference((mm)->owner));
> +	while (memcg != cgroup && memcg && memcg->use_hierarchy)
> +		memcg = parent_mem_cgroup(memcg);
> +	rcu_read_unlock();
> +
> +	return cgroup == memcg;
> +}

Please don't duplicate mem_cgroup_same_or_subtree()'s functionality in
a worse way.  The hierarchy information is kept in a stack such that
ancestry can be detected in linear time, check out css_is_ancestor().

If you don't want to nest rcu_read_lock(), you could push the
rcu_read_lock() from css_is_ancestor() into its sole user and provide
a __mem_cgroup_is_ancestor() that assumes rcu already read-locked.

No?
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