Re: [PATCH v2 15/28] mm: memcontrol: prevent memory cgroup release in mem_cgroup_swap_full()

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/12/20 9:05, Shakeel Butt wrote:
> On Wed, Dec 17, 2025 at 03:27:39PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@bytedance.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@bytedance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Reviewed-by: Harry Yoo <harry.yoo@oracle.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;
>  }
>  
> 
> Anyways LGTM.
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

More compact.

LGTM.

-- 
Best regards,
Ridong