linux-kernel - Re: [PATCH v1 01/26] mm: memcontrol: remove dead code of checking parent memory cgroup

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Message-ID: <0d78a4cf-0973-4874-ab6a-978621a17068@huaweicloud.com>
Date: Thu, 20 Nov 2025 17:07:47 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Qi Zheng <qi.zheng@...ux.dev>, hannes@...xchg.org, hughd@...gle.com,
 mhocko@...e.com, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
 muchun.song@...ux.dev, david@...hat.com, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, harry.yoo@...cle.com, imran.f.khan@...cle.com,
 kamalesh.babulal@...cle.com, axelrasmussen@...gle.com, yuanchu@...gle.com,
 weixugc@...gle.com, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, Muchun Song <songmuchun@...edance.com>,
 Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v1 01/26] mm: memcontrol: remove dead code of checking
 parent memory cgroup



On 2025/10/28 21:58, Qi Zheng wrote:
> From: Muchun Song <songmuchun@...edance.com>
> 
> Since the no-hierarchy mode has been deprecated after the commit:
> 
>   commit bef8620cd8e0 ("mm: memcg: deprecate the non-hierarchical mode").
> 
> As a result, parent_mem_cgroup() will not return NULL except when passing
> the root memcg, and the root memcg cannot be offline. Hence, it's safe to
> remove the check on the returned value of parent_mem_cgroup(). Remove the
> corresponding dead code.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>  mm/memcontrol.c | 5 -----
>  mm/shrinker.c   | 6 +-----
>  2 files changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 93f7c76f0ce96..d5257465c9d75 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3339,9 +3339,6 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
>  		return;
>  
>  	parent = parent_mem_cgroup(memcg);
> -	if (!parent)
> -		parent = root_mem_cgroup;
> -
>  	memcg_reparent_list_lrus(memcg, parent);
>  
>  	/*
> @@ -3632,8 +3629,6 @@ struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
>  			break;
>  		}
>  		memcg = parent_mem_cgroup(memcg);
> -		if (!memcg)
> -			memcg = root_mem_cgroup;
>  	}
>  	return memcg;
>  }
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index 4a93fd433689a..e8e092a2f7f41 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -286,14 +286,10 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>  {
>  	int nid, index, offset;
>  	long nr;
> -	struct mem_cgroup *parent;
> +	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
>  	struct shrinker_info *child_info, *parent_info;
>  	struct shrinker_info_unit *child_unit, *parent_unit;
>  
> -	parent = parent_mem_cgroup(memcg);
> -	if (!parent)
> -		parent = root_mem_cgroup;
> -
>  	/* Prevent from concurrent shrinker_info expand */
>  	mutex_lock(&shrinker_mutex);
>  	for_each_node(nid) {

LGTM.

Reviewed-by: Chen Ridong <chenridong@...wei.com>

-- 
Best regards,
Ridong