linux-kernel - Re: [PATCH v1 25/26] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

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Message-ID: <a7e55445-20ee-4133-8455-b6c5f7a45ff3@huaweicloud.com>
Date: Thu, 20 Nov 2025 19:56:10 +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 25/26] mm: memcontrol: eliminate the problem of dying
 memory cgroup for LRU folios



On 2025/10/28 21:58, Qi Zheng wrote:
>  static void reparent_locks(struct mem_cgroup *src, struct mem_cgroup *dst)
>  {
> +	int nid, nest = 0;
> +
>  	spin_lock_irq(&objcg_lock);
> +	for_each_node(nid) {
> +		spin_lock_nested(&mem_cgroup_lruvec(src,
> +				 NODE_DATA(nid))->lru_lock, nest++);
> +		spin_lock_nested(&mem_cgroup_lruvec(dst,
> +				 NODE_DATA(nid))->lru_lock, nest++);
> +	}
>  }
>  
>  static void reparent_unlocks(struct mem_cgroup *src, struct mem_cgroup *dst)
>  {
> +	int nid;
> +
> +	for_each_node(nid) {
> +		spin_unlock(&mem_cgroup_lruvec(dst, NODE_DATA(nid))->lru_lock);
> +		spin_unlock(&mem_cgroup_lruvec(src, NODE_DATA(nid))->lru_lock);
> +	}
>  	spin_unlock_irq(&objcg_lock);
>  }
>  

The lock order follows S0→D0→S1→D1→…, and the correct unlock sequence should be Dn→Sn→…→D1→S0

However, the current unlock implementation uses D0→S0→D1→S1→…

I’m not certain whether this unlock order will cause any issues—could this lead to potential
problems like deadlocks or lock state inconsistencies?

-- 
Best regards,
Ridong