linux-kernel - Re: [PATCH -v2 3/6] memcg: mm == NULL is not allowed for mem_cgroup_try_charge

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Date:	Tue, 4 Feb 2014 11:05:58 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH -v2 3/6] memcg: mm == NULL is not allowed for
 mem_cgroup_try_charge_mm

On Tue, Feb 04, 2014 at 02:28:57PM +0100, Michal Hocko wrote:
> An ancient comment tries to explain that a given mm might be NULL when a
> task is migrated. It has been introduced by 8a9f3ccd (Memory controller:
> memory accounting) along with other bigger changes so it is not much
> more specific about the conditions.
> 
> Anyway, Even if the task is migrated to another memcg there is no way we
> can see NULL mm struct. So either this was not correct from the very
> beginning or it is not true anymore.
> The only remaining case would be seeing charges after exit_mm but that
> would be a bug on its own as the task doesn't have an address space
> anymore.
> 
> This patch replaces the check by VM_BUG_ON to make it obvious that we
> really expect non-NULL mm_struct.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Acked-by: Johannes Weiner <hannes@...xchg.org>
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