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Date:   Tue, 13 Apr 2021 10:47:12 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Muchun Song <songmuchun@bytedance.com>
CC:     <hannes@cmpxchg.org>, <mhocko@kernel.org>,
        <akpm@linux-foundation.org>, <shakeelb@google.com>,
        <vdavydov.dev@gmail.com>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>, <duanxiongchun@bytedance.com>,
        <fam.zheng@bytedance.com>
Subject: Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in
 get_mem_cgroup_from_mm
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References: <20210413065153.63431-1-songmuchun@bytedance.com>
 <20210413065153.63431-3-songmuchun@bytedance.com>
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On Tue, Apr 13, 2021 at 02:51:48PM +0800, Muchun Song wrote:
> When mm is NULL, we do not need to hold rcu lock and call css_tryget for
> the root memcg. And we also do not need to check !mm in every loop of
> while. So bail out early when !mm.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

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

Nice!

> ---
>  mm/memcontrol.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f229de925aa5..9cbfff59b171 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -901,20 +901,19 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>  	if (mem_cgroup_disabled())
>  		return NULL;
>  
> +	/*
> +	 * Page cache insertions can happen without an
> +	 * actual mm context, e.g. during disk probing
> +	 * on boot, loopback IO, acct() writes etc.
> +	 */
> +	if (unlikely(!mm))
> +		return root_mem_cgroup;
> +
>  	rcu_read_lock();
>  	do {
> -		/*
> -		 * Page cache insertions can happen without an
> -		 * actual mm context, e.g. during disk probing
> -		 * on boot, loopback IO, acct() writes etc.
> -		 */
> -		if (unlikely(!mm))
> +		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> +		if (unlikely(!memcg))
>  			memcg = root_mem_cgroup;
> -		else {
> -			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> -			if (unlikely(!memcg))
> -				memcg = root_mem_cgroup;
> -		}
>  	} while (!css_tryget(&memcg->css));
>  	rcu_read_unlock();
>  	return memcg;
> -- 
> 2.11.0
>