linux-kernel - Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from

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Message-ID: <6d372282-d443-4fe4-942b-998aa742eb0a@linux.dev>
Date: Thu, 20 Nov 2025 21:32:09 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: 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,
 imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
 axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 akpm@...ux-foundation.org, 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 07/26] mm: memcontrol: prevent memory cgroup release in
 get_mem_cgroup_from_folio()



On 11/19/25 4:06 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:20PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@...edance.com>
>>
>> In the near future, a folio will no longer pin its corresponding
>> memory cgroup. To ensure safety, it will only be appropriate to
>> hold the rcu read lock or acquire a reference to the memory cgroup
>> returned by folio_memcg(), thereby preventing it from being released.
>>
>> In the current patch, the rcu read lock is employed to safeguard
>> against the release of the memory cgroup in get_mem_cgroup_from_folio().
>>
>> This serves as a preparatory measure for the reparenting of the
>> LRU pages.
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
> 
> This patch looks correct to me, so:
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> 
> with a few side question on current implementation rather than
> this change...
> 
>>   mm/memcontrol.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index d484b632c790f..1da3ad77054d3 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
>>    */
>>   struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
>>   {
>> -	struct mem_cgroup *memcg = folio_memcg(folio);
>> +	struct mem_cgroup *memcg;
>>   
>>   	if (mem_cgroup_disabled())
>>   		return NULL;
>>   
>> +	if (!folio_memcg_charged(folio))
>> +		return root_mem_cgroup;
>> +
>>   	rcu_read_lock();
>> -	if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
> 
> I have no idea why we're already calling rcu_read_lock() when a folio pins
> the memcg :)

+1, this was introduced by 
https://github.com/torvalds/linux/commit/f77f0c7514789577125c1b2df145703161736359,
may be some defensive programming.

> 
>> -		memcg = root_mem_cgroup;
>> +retry:
>> +	memcg = folio_memcg(folio);
>> +	if (unlikely(!css_tryget(&memcg->css)))
>> +		goto retry;
>>   	rcu_read_unlock();
>>   	return memcg;
>>   }
>> -- 
>> 2.20.1
>