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: <cf600d82-82f5-414c-b880-71133379d5d4@linux.dev>
Date: Thu, 20 Nov 2025 21:45:19 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Chen Ridong <chenridong@...weicloud.com>, 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 11/20/25 7:56 PM, Chen Ridong wrote:
> 
> 
> 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?

As long as the order in which the locks are held is consistent, there 
should be no deadlock problem?

>