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Date:   Tue, 23 Feb 2021 07:35:49 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Muchun Song <songmuchun@bytedance.com>
CC:     <hannes@cmpxchg.org>, <mhocko@kernel.org>,
        <vdavydov.dev@gmail.com>, <akpm@linux-foundation.org>,
        <shakeelb@google.com>, <cgroups@vger.kernel.org>,
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Subject: Re: [PATCH] mm: memcontrol: fix get_active_memcg return value
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On Tue, Feb 23, 2021 at 05:11:01PM +0800, Muchun Song wrote:
> We use a global percpu int_active_memcg variable to store the remote
> memcg when we are in the interrupt context. But get_active_memcg always
> return the current->active_memcg or root_mem_cgroup. The remote memcg
> (set in the interrupt context) is ignored. This is not what we want.
> So fix it.
> 
> Fixes: 37d5985c003d ("mm: kmem: prepare remote memcg charging infra for interrupt contexts")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/memcontrol.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index be6bc5044150..bbe25655f7eb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1061,13 +1061,9 @@ static __always_inline struct mem_cgroup *get_active_memcg(void)
>  
>  	rcu_read_lock();
>  	memcg = active_memcg();
> -	if (memcg) {
> -		/* current->active_memcg must hold a ref. */
> -		if (WARN_ON_ONCE(!css_tryget(&memcg->css)))
> -			memcg = root_mem_cgroup;
> -		else
> -			memcg = current->active_memcg;
> -	}
> +	/* remote memcg must hold a ref. */
> +	if (memcg && WARN_ON_ONCE(!css_tryget(&memcg->css)))
> +		memcg = root_mem_cgroup;

Ouch, this is a good one!

Reviewed-by: Roman Gushchin <guro@fb.com>

Thank you!