linux-kernel - Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from

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Date:   Wed, 14 Apr 2021 11:24:34 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     guro@...com, hannes@...xchg.org, akpm@...ux-foundation.org,
        shakeelb@...gle.com, vdavydov.dev@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, fam.zheng@...edance.com
Subject: Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in
 get_mem_cgroup_from_mm

On Tue 13-04-21 14:51:48, 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.

mem_cgroup_charge and other callers unconditionally drop the reference
so how come this does not underflow reference count?
 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> ---
>  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

-- 
Michal Hocko
SUSE Labs