linux-kernel - Re: [PATCH v4 30/31] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

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Message-ID: <37b79f65-7de3-483a-a675-75eab94d2776@linux.dev>
Date: Tue, 10 Feb 2026 11:11:47 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
 roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...nel.org,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com,
 yosry.ahmed@...ux.dev, imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
 axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 chenridong@...weicloud.com, mkoutny@...e.com, akpm@...ux-foundation.org,
 hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com,
 lance.yang@...ux.dev, bhe@...hat.com, 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 v4 30/31] mm: memcontrol: eliminate the problem of dying
 memory cgroup for LRU folios



On 2/10/26 1:53 AM, Shakeel Butt wrote:
> On Mon, Feb 09, 2026 at 11:49:43AM +0800, Qi Zheng wrote:
>>
>>
>> On 2/8/26 6:25 AM, Shakeel Butt wrote:
>>> On Thu, Feb 05, 2026 at 05:01:49PM +0800, Qi Zheng wrote:
>>>> From: Muchun Song <songmuchun@...edance.com>
>>>>
>>>> Now that everything is set up, switch folio->memcg_data pointers to
>>>> objcgs, update the accessors, and execute reparenting on cgroup death.
>>>>
>>>> Finally, folio->memcg_data of LRU folios and kmem folios will always
>>>> point to an object cgroup pointer. The folio->memcg_data of slab
>>>> folios will point to an vector of object cgroups.
>>>>
>>>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>>>>    /*
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index e7d4e4ff411b6..0e0efaa511d3d 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -247,11 +247,25 @@ static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgr
>>>>    static inline void reparent_locks(struct mem_cgroup *memcg, struct mem_cgroup *parent)
>>>>    {
>>>> +	int nid, nest = 0;
>>>> +
>>>>    	spin_lock_irq(&objcg_lock);
>>>> +	for_each_node(nid) {
>>>> +		spin_lock_nested(&mem_cgroup_lruvec(memcg,
>>>> +				 NODE_DATA(nid))->lru_lock, nest++);
>>>> +		spin_lock_nested(&mem_cgroup_lruvec(parent,
>>>> +				 NODE_DATA(nid))->lru_lock, nest++);
>>>
>>> Is there a reason to acquire locks for all the node together? Why not do
>>> the for_each_node(nid) in memcg_reparent_objcgs() and then reparent the
>>> LRUs for each node one by one and taking and releasing lock
>>> individually. Though the lock for the offlining memcg might not be
>>
>> To do this, we first need to convert objcg from per-memcg to per-memcg
>> per-node. In this way, we can hold the lru lock and objcg lock for
>> each node to reparent the folio and the corresponding objcg together.
> 
> Oh we want reparenting of both objcg and folio atomic. Let's add a

Right.

> comment here with the explanation.

OK, will do this refactoring and send v5.

Thanks,
Qi

>