linux-kernel - Re: [PATCH v2 3/3] mm: memcontrol: add sanity checks for memcg->id.ref on get/put

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Date:	Tue, 2 Aug 2016 13:27:36 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] mm: memcontrol: add sanity checks for
 memcg->id.ref on get/put

On Tue, Aug 02, 2016 at 06:00:50PM +0300, Vladimir Davydov wrote:
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>
> ---
>  mm/memcontrol.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 67109d556a4a..32b2f33865f9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4033,6 +4033,7 @@ static DEFINE_IDR(mem_cgroup_idr);
>  
>  static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
>  {
> +	VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
>  	atomic_add(n, &memcg->id.ref);
>  }
>  
> @@ -4056,6 +4057,7 @@ static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg)
>  
>  static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
>  {
> +	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
>  	if (atomic_sub_and_test(n, &memcg->id.ref)) {
>  		idr_remove(&mem_cgroup_idr, memcg->id.id);
>  		memcg->id.id = 0;
> @@ -4176,6 +4178,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  	INIT_LIST_HEAD(&memcg->cgwb_list);
>  #endif
>  	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> +	atomic_set(&memcg->id.ref, 1);
>  	return memcg;
>  fail:
>  	if (memcg->id.id > 0)
> @@ -4245,7 +4248,6 @@ fail:
>  static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>  {
>  	/* Online state pins memcg ID, memcg ID pins CSS */
> -	mem_cgroup_id_get(mem_cgroup_from_css(css));
>  	css_get(css);
>  	return 0;

This comment and code is no very, very confusing. Can you move the
atomic_set(&memcg->id.ref, 1) down here instead?