linux-kernel - Re: [PATCH v6 20/29] memcg: skip memcg kmem allocations in specified code regions

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Date:	Mon, 5 Nov 2012 16:33:13 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v6 20/29] memcg: skip memcg kmem allocations in
 specified code regions

On Thu,  1 Nov 2012 16:07:36 +0400
Glauber Costa <glommer@...allels.com> wrote:

> This patch creates a mechanism that skip memcg allocations during
> certain pieces of our core code. It basically works in the same way
> as preempt_disable()/preempt_enable(): By marking a region under
> which all allocations will be accounted to the root memcg.
> 
> We need this to prevent races in early cache creation, when we
> allocate data using caches that are not necessarily created already.
> 
> ...
>
> +static inline void memcg_stop_kmem_account(void)
> +{
> +	if (!current->mm)
> +		return;

It is utterly unobvious to this reader why the code tests ->mm in this
fashion.  So we need either smarter readers or a code comment.

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