linux-kernel - Re: [PATCH 2/4] mm: thp: introduce folio_split_queue

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Message-ID: <28f36249-f064-4d32-9d8a-ba03db5b491c@bytedance.com>
Date: Mon, 22 Sep 2025 15:56:00 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Zi Yan <ziy@...dia.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, 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, Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH 2/4] mm: thp: introduce folio_split_queue_lock and its
 variants

Hi Zi,

On 9/19/25 11:39 PM, Zi Yan wrote:
> On 18 Sep 2025, at 23:46, Qi Zheng wrote:
> 
>> From: Muchun Song <songmuchun@...edance.com>
>>
>> In future memcg removal, the binding between a folio and a memcg may
>> change, making the split lock within the memcg unstable when held.
>>
>> A new approach is required to reparent the split queue to its parent. This
>> patch starts introducing a unified way to acquire the split lock for
>> future work.
>>
>> It's a code-only refactoring with no functional changes.
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> Acked-by: Johannes Weiner <hannes@...xchg.org>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   include/linux/memcontrol.h | 10 +++++
>>   mm/huge_memory.c           | 89 ++++++++++++++++++++++++++------------
>>   2 files changed, 71 insertions(+), 28 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 16fe0306e50ea..99876af13c315 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -1662,6 +1662,11 @@ int alloc_shrinker_info(struct mem_cgroup *memcg);
>>   void free_shrinker_info(struct mem_cgroup *memcg);
>>   void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id);
>>   void reparent_shrinker_deferred(struct mem_cgroup *memcg);
>> +
>> +static inline int shrinker_id(struct shrinker *shrinker)
>> +{
>> +	return shrinker->id;
>> +}
>>   #else
>>   #define mem_cgroup_sockets_enabled 0
>>
>> @@ -1693,6 +1698,11 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg,
>>   				    int nid, int shrinker_id)
>>   {
>>   }
>> +
>> +static inline int shrinker_id(struct shrinker *shrinker)
>> +{
>> +	return -1;
>> +}
>>   #endif
>>
>>   #ifdef CONFIG_MEMCG
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 582628ddf3f33..d34516a22f5bb 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1078,26 +1078,62 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
>>
>>   #ifdef CONFIG_MEMCG
>>   static inline
>> -struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> +struct mem_cgroup *folio_split_queue_memcg(struct folio *folio,
>> +					   struct deferred_split *queue)
>>   {
>> -	struct mem_cgroup *memcg = folio_memcg(folio);
>> -	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> -
>> -	if (memcg)
>> -		return &memcg->deferred_split_queue;
>> -	else
>> -		return &pgdat->deferred_split_queue;
>> +	if (mem_cgroup_disabled())
>> +		return NULL;
>> +	if (&NODE_DATA(folio_nid(folio))->deferred_split_queue == queue)
>> +		return NULL;
>> +	return container_of(queue, struct mem_cgroup, deferred_split_queue);
>>   }
>>   #else
>>   static inline
>> -struct deferred_split *get_deferred_split_queue(struct folio *folio)
>> +struct mem_cgroup *folio_split_queue_memcg(struct folio *folio,
>> +					   struct deferred_split *queue)
>>   {
>> -	struct pglist_data *pgdat = NODE_DATA(folio_nid(folio));
>> -
>> -	return &pgdat->deferred_split_queue;
>> +	return NULL;
>>   }
>>   #endif
>>
>> +static struct deferred_split *folio_split_queue_lock(struct folio *folio)
>> +{
>> +	struct mem_cgroup *memcg;
>> +	struct deferred_split *queue;
>> +
>> +	memcg = folio_memcg(folio);
>> +	queue = memcg ? &memcg->deferred_split_queue :
>> +			&NODE_DATA(folio_nid(folio))->deferred_split_queue;
>> +	spin_lock(&queue->split_queue_lock);
>> +
>> +	return queue;
>> +}
>> +
>> +static struct deferred_split *
>> +folio_split_queue_lock_irqsave(struct folio *folio, unsigned long *flags)
>> +{
>> +	struct mem_cgroup *memcg;
>> +	struct deferred_split *queue;
>> +
>> +	memcg = folio_memcg(folio);
>> +	queue = memcg ? &memcg->deferred_split_queue :
>> +			&NODE_DATA(folio_nid(folio))->deferred_split_queue;
>> +	spin_lock_irqsave(&queue->split_queue_lock, *flags);
>> +
>> +	return queue;
>> +}
> 
> A helper function to get queue from a folio would get rid of duplicated
> code in the two functions above. Hmm, that is the deleted
> get_deferred_split_queue(). So probably retain it.

After PATCH #4, we may retry after getting parent memcg in the above
functions. so we may not need to retrieve get_deferred_split_queue().

> 
> Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@...dia.com>

Thanks!

> 
> Best Regards,
> Yan, Zi