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Date:	Thu, 23 Oct 2014 17:13:18 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: memcontrol: don't pass a NULL memcg to
 mem_cgroup_end_move()

On Thu 23-10-14 10:33:50, Johannes Weiner wrote:
> mem_cgroup_end_move() checks if the passed memcg is NULL, along with a
> lengthy comment to explain why this seemingly non-sensical situation
> is even possible.
> 
> Check in cancel_attach() itself whether can_attach() set up the move
> context or not, it's a lot more obvious from there.  Then remove the
> check and comment in mem_cgroup_end_move().
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Vladimir Davydov <vdavydov@...allels.com>

Acked-by: Michal Hocko <mhocko@...e.cz>

> ---
>  mm/memcontrol.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a5c9aa4688e8..3cd4f1e0bfb3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1469,12 +1469,7 @@ static void mem_cgroup_start_move(struct mem_cgroup *memcg)
>  
>  static void mem_cgroup_end_move(struct mem_cgroup *memcg)
>  {
> -	/*
> -	 * Now, mem_cgroup_clear_mc() may call this function with NULL.
> -	 * We check NULL in callee rather than caller.
> -	 */
> -	if (memcg)
> -		atomic_dec(&memcg->moving_account);
> +	atomic_dec(&memcg->moving_account);
>  }
>  
>  /*
> @@ -5489,7 +5484,8 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
>  static void mem_cgroup_cancel_attach(struct cgroup_subsys_state *css,
>  				     struct cgroup_taskset *tset)
>  {
> -	mem_cgroup_clear_mc();
> +	if (mc.to)
> +		mem_cgroup_clear_mc();
>  }
>  
>  static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
> -- 
> 2.1.2
> 

-- 
Michal Hocko
SUSE Labs
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