linux-kernel - Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero

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Message-ID: <YCpBUm2N4Bqm5PM5@dhcp22.suse.cz>
Date:   Mon, 15 Feb 2021 10:39:30 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     hannes@...xchg.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero

On Sat 13-02-21 01:01:58, Muchun Song wrote:
> The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id,
> so idr_find() is pointless and wastes CPU cycles.

Is this possible at all to happen? If not why should we add a test for
_all_ invocations?

> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/memcontrol.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a3f26522765a..68ed4b297c13 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
>  struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
>  {
>  	WARN_ON_ONCE(!rcu_read_lock_held());
> +	/* The memcg ID cannot be zero. */
> +	if (id == 0)
> +		return NULL;
>  	return idr_find(&mem_cgroup_idr, id);
>  }
>  
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs