Re: [PATCH v5 4/4] mm: thp: reparent the split queue during memcg offline

[Qi Zheng <qi.zheng@linux.dev>]

On 10/21/25 2:09 PM, Harry Yoo wrote:
> On Wed, Oct 15, 2025 at 02:35:33PM +0800, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> Similar to list_lru, the split queue is relatively independent and does
>> not need to be reparented along with objcg and LRU folios (holding
>> objcg lock and lru lock). So let's apply the similar mechanism as list_lru
>> to reparent the split queue separately when memcg is offine.
>>
>> This is also a preparation for reparenting LRU folios.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Muchun Song <muchun.song@linux.dev>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>> ---
> 
> Looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

Thanks.

> 
> with a question:
> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index e850bc10da3e2..9323039418201 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1117,8 +1117,19 @@ static struct deferred_split *split_queue_lock(int nid, struct mem_cgroup *memcg
>>   {
>>   	struct deferred_split *queue;
>>   
>> +retry:
>>   	queue = memcg_split_queue(nid, memcg);
>>   	spin_lock(&queue->split_queue_lock);
>> +	/*
>> +	 * There is a period between setting memcg to dying and reparenting
>> +	 * deferred split queue, and during this period the THPs in the deferred
>> +	 * split queue will be hidden from the shrinker side.
>> +	 */
> 
> You mean it will be hidden if the shrinker bit is not set for the node
> in the parent memcg, right?

Look at the following situation:

CPU 0                   CPU 1
-----                   -----

set CSS_DYING
                         deferred_split_scan
                             /*
                              * See CSS_DYING, and return the parent
                              * memcg's ds_queue. But the pages on the
                              * child memcg's ds_queue has not yet been
                              * reparented to the parent memcg, that is,
                              * it is hidden.
                              */
                         --> ds_queue = split_queue_lock_irqsave()

reparent_deferred_split_queue

Thanks,
Qi

>