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

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 9/23/25 11:44 PM, Zi Yan wrote:
> On 23 Sep 2025, at 5:16, 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@bytedance.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
>>
>> 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;
>>
>>   	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;
>>   }
>> @@ -1108,9 +1114,15 @@ folio_split_queue_lock_irqsave(struct folio *folio, unsigned long *flags)
>>   	struct deferred_split *queue;
>>
>>   	memcg = folio_memcg(folio);
>> +retry:
>>   	queue = memcg ? &memcg->deferred_split_queue :
>>   			&NODE_DATA(folio_nid(folio))->deferred_split_queue;
>>   	spin_lock_irqsave(&queue->split_queue_lock, *flags);
>> +	if (unlikely(queue->is_dying == true)) {
>> +		spin_unlock_irqrestore(&queue->split_queue_lock, *flags);
>> +		memcg = parent_mem_cgroup(memcg);
>> +		goto retry;
>> +	}
>>
>>   	return queue;
>>   }
>> @@ -4284,6 +4296,33 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>   	return split;
>>   }
>>
>> +void reparent_deferred_split_queue(struct mem_cgroup *memcg)
>> +{
>> +	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
>> +	struct deferred_split *ds_queue = &memcg->deferred_split_queue;
>> +	struct deferred_split *parent_ds_queue = &parent->deferred_split_queue;
>> +	int nid;
>> +
>> +	spin_lock_irq(&ds_queue->split_queue_lock);
>> +	spin_lock_nested(&parent_ds_queue->split_queue_lock, SINGLE_DEPTH_NESTING);
>> +
>> +	if (!ds_queue->split_queue_len)
>> +		goto unlock;
> 
> Should ds_queue still be marked as dying even if it is empty?
> Otherwise, new folios still can be added to it, based on my
> understanding of the changes to folio_split_queue_lock*().

I think you are right, will do in the next version.

Thanks,
Qi

> 
>> +
>> +	list_splice_tail_init(&ds_queue->split_queue, &parent_ds_queue->split_queue);
>> +	parent_ds_queue->split_queue_len += ds_queue->split_queue_len;
>> +	ds_queue->split_queue_len = 0;
>> +	/* Mark the ds_queue dead */
>> +	ds_queue->is_dying = true;
>> +
>> +	for_each_node(nid)
>> +		set_shrinker_bit(parent, nid, shrinker_id(deferred_split_shrinker));
>> +
>> +unlock:
>> +	spin_unlock(&parent_ds_queue->split_queue_lock);
>> +	spin_unlock_irq(&ds_queue->split_queue_lock);
>> +}
>> +
>>   #ifdef CONFIG_DEBUG_FS
>>   static void split_huge_pages_all(void)
>>   {
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index e090f29eb03bd..d03da72e7585d 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3887,6 +3887,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
>>   	zswap_memcg_offline_cleanup(memcg);
>>
>>   	memcg_offline_kmem(memcg);
>> +	reparent_deferred_split_queue(memcg);
>>   	reparent_shrinker_deferred(memcg);
>>   	wb_memcg_offline(memcg);
>>   	lru_gen_offline_memcg(memcg);
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 3db2dea7db4c5..cbda5c2ee3241 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1387,6 +1387,7 @@ static void pgdat_init_split_queue(struct pglist_data *pgdat)
>>   	spin_lock_init(&ds_queue->split_queue_lock);
>>   	INIT_LIST_HEAD(&ds_queue->split_queue);
>>   	ds_queue->split_queue_len = 0;
>> +	ds_queue->is_dying = false;
>>   }
>>   #else
>>   static void pgdat_init_split_queue(struct pglist_data *pgdat) {}
>> -- 
>> 2.20.1
> 
> 
> Best Regards,
> Yan, Zi