linux-kernel - Re: [PATCH v2 08/28] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from

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Message-ID: <f59a7a6d-2685-4c90-a02b-33dce49898cf@linux.dev>
Date: Thu, 18 Dec 2025 14:31:51 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev, muchun.song@...ux.dev, david@...nel.org,
 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,
 chenridong@...weicloud.com, mkoutny@...e.com, akpm@...ux-foundation.org,
 hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com,
 lance.yang@...ux.dev, 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 v2 08/28] mm: memcontrol: prevent memory cgroup release in
 get_mem_cgroup_from_folio()



On 12/18/25 5:45 AM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:32PM +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>
>> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
>> ---
>>   mm/memcontrol.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 21b5aad34cae7..431b3154c70c5 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)))
>> -		memcg = root_mem_cgroup;
>> +retry:
>> +	memcg = folio_memcg(folio);
>> +	if (unlikely(!css_tryget(&memcg->css)))
>> +		goto retry;
> 
> So starting in patch 27, the tryget can fail if the memcg is offlined,
> and the folio's objcg is reparented concurrently. We'll retry until we
> find a memcg that isn't dead yet. There's always root_mem_cgroup.
> 
> It makes sense, but a loop like this begs the question of how it is
> bounded. I pieced it together looking ahead. Since this is a small
> diff, it would be nicer to fold it into 27. I didn't see anything in
> between depending on it, but correct me if I'm wrong.

Right, will fold it into #27 in the next version.

> 
> Minor style preference:
> 
> 	/* Comment explaining the above */
> 	do {
> 		memcg = folio_memcg(folio);
> 	} while (!css_tryget(&memcg->css));

OK, will do.

Thanks,
Qi