[patch] mm: memcontrol: don't pass a NULL memcg to mem_cgroup_end_move()

[patch] mm: memcontrol: don't pass a NULL memcg to mem_cgroup_end_move()

[Johannes Weiner]

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@cmpxchg.org>
Acked-by: Vladimir Davydov <vdavydov@parallels.com>
---
 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

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Re: [patch] mm: memcontrol: don't pass a NULL memcg to mem_cgroup_end_move()

[Michal Hocko]

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@cmpxchg.org>
> Acked-by: Vladimir Davydov <vdavydov@parallels.com>

Acked-by: Michal Hocko <mhocko@suse.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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