linux-kernel - Re: [PATCH v2 15/28] mm: memcontrol: prevent memory cgroup release in mem_cgroup_swap

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Message-ID: <59867ffa-7e5d-442f-8e8e-79d5512e03c1@linux.dev>
Date: Mon, 22 Dec 2025 12:02:11 +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,
 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 15/28] mm: memcontrol: prevent memory cgroup release in
 mem_cgroup_swap_full()



On 12/20/25 9:05 AM, Shakeel Butt wrote:
> On Wed, Dec 17, 2025 at 03:27:39PM +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 mem_cgroup_swap_full().
>>
>> 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 | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 131f940c03fa0..f2c891c1f49d5 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5267,17 +5267,21 @@ bool mem_cgroup_swap_full(struct folio *folio)
>>   	if (do_memsw_account())
>>   		return false;
>>   
>> -	memcg = folio_memcg(folio);
>> -	if (!memcg)
>> +	if (!folio_memcg_charged(folio))
>>   		return false;
>>   
>> +	rcu_read_lock();
>> +	memcg = folio_memcg(folio);
>>   	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
>>   		unsigned long usage = page_counter_read(&memcg->swap);
>>   
>>   		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
>> -		    usage * 2 >= READ_ONCE(memcg->swap.max))
>> +		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
>> +			rcu_read_unlock();
>>   			return true;
>> +		}
>>   	}
>> +	rcu_read_unlock();
>>   
>>   	return false;
>>   }
> 
> How about the following?
> 
> 
>   bool mem_cgroup_swap_full(struct folio *folio)
>   {
>   	struct mem_cgroup *memcg;
> +	bool ret = false;
>   
>   	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>   
>   	if (vm_swap_full())
>   		return true;
> -	if (do_memsw_account())
> -		return false;
>   
> -	if (!folio_memcg_charged(folio))
> -		return false;
> +	if (do_memsw_account() || !folio_memcg_charged(folio))
> +		return ret;
>   
>   	rcu_read_lock();
>   	memcg = folio_memcg(folio);
> @@ -5277,13 +5276,13 @@ bool mem_cgroup_swap_full(struct folio *folio)
>   
>   		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
>   		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
> -			rcu_read_unlock();
> -			return true;
> +			ret = true;
> +			break;
>   		}
>   	}
>   	rcu_read_unlock();
>   
> -	return false;
> +	return ret;
>   }

LGTM, will do.

>   
> 
> Anyways LGTM.
> 
> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>

Thanks!