linux-kernel - Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg offline

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Message-ID: <77114896-5a7a-413d-afa1-7d0a17312c99@bytedance.com>
Date: Thu, 25 Sep 2025 14:29:32 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
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,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 lance.yang@...ux.dev, akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg
 offline

Hi Harry,

On 9/24/25 10:22 PM, Harry Yoo wrote:
> On Tue, Sep 23, 2025 at 05:16:25PM +0800, Qi Zheng wrote:
>> In the future, we will reparent LRU folios during memcg offline to
>> eliminate dying memory cgroups, which requires reparenting the split queue
>> to its parent.
>>
>> 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 same mechanism as list_lru
>> to reparent the split queue separately when memcg is offine.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   include/linux/huge_mm.h |  2 ++
>>   include/linux/mmzone.h  |  1 +
>>   mm/huge_memory.c        | 39 +++++++++++++++++++++++++++++++++++++++
>>   mm/memcontrol.c         |  1 +
>>   mm/mm_init.c            |  1 +
>>   5 files changed, 44 insertions(+)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index f327d62fc9852..a0d4b751974d2 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -417,6 +417,7 @@ static inline int split_huge_page(struct page *page)
>>   	return split_huge_page_to_list_to_order(page, NULL, ret);
>>   }
>>   void deferred_split_folio(struct folio *folio, bool partially_mapped);
>> +void reparent_deferred_split_queue(struct mem_cgroup *memcg);
>>   
>>   void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>   		unsigned long address, bool freeze);
>> @@ -611,6 +612,7 @@ static inline int try_folio_split(struct folio *folio, struct page *page,
>>   }
>>   
>>   static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
>> +static inline void reparent_deferred_split_queue(struct mem_cgroup *memcg) {}
>>   #define split_huge_pmd(__vma, __pmd, __address)	\
>>   	do { } while (0)
>>   
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 7fb7331c57250..f3eb81fee056a 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -1346,6 +1346,7 @@ struct deferred_split {
>>   	spinlock_t split_queue_lock;
>>   	struct list_head split_queue;
>>   	unsigned long split_queue_len;
>> +	bool is_dying;
>>   };
>>   #endif
> 
> The scheme in Muchun's version was:
> 
> retry:
> queue = folio_split_queue(folio);
> spin_lock(&queue->split_queue_lock);
> if (folio_memcg(folio) != folio_split_queue_memcg(folio, queue)) {
>      /* split queue was reparented, retry */
>      spin_unlock(&queue->split_queue_lock);
>      goto retry;
> }
> /* now we have a stable mapping between the folio and the split queue */
> spin_unlock(&queue->split_queue_lock);
> 
> Oh, I see. We can't use this scheme yet because we don't reparent LRU
> folios. (I was wondering why we're adding is_dying property)

Right. And reparenting THP split queue independently can avoid the
following situations:

```
acquire child and parent split_queue_lock
acquire child and parent objcg_lock
acquire child and parent lru lock

reparent THP split queue
reparent objcg
reparent LRU folios

release child and parent lru lock
release child and parent objcg_lock
release child and parent split_queue_lock
```

> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 48b51e6230a67..de7806f759cba 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1094,9 +1094,15 @@ static struct deferred_split *folio_split_queue_lock(struct folio *folio)
>>   	struct deferred_split *queue;
> 
> 
> For now it's safe to not call rcu_read_lock() here because memcgs won't
> disappear under us as long as there are folios to split (we don't reparent
> LRU folios), right?

Right. We will add rcu_read_lock() when reparenting LRU folios.

Thanks,
Qi

> 
>>   	memcg = folio_memcg(folio);
>> +retry:
>>   	queue = memcg ? &memcg->deferred_split_queue :
>>   			&NODE_DATA(folio_nid(folio))->deferred_split_queue;
>>   	spin_lock(&queue->split_queue_lock);
>> +	if (unlikely(queue->is_dying == true)) {
>> +		spin_unlock(&queue->split_queue_lock);
>> +		memcg = parent_mem_cgroup(memcg);
>> +		goto retry;
>> +	}
>>   	return queue;
>>   }
>