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

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Message-ID: <b041b58d-b0e4-4a01-a459-5449c232c437@redhat.com>
Date: Wed, 24 Sep 2025 14:38:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hannes@...xchg.org,
 hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev, muchun.song@...ux.dev, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, harry.yoo@...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
Cc: 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

On 23.09.25 11:16, 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@...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;

It's a bit weird to query whether the "struct deferred_split" is dying. 
Shouldn't this be a memcg property? (and in particular, not exist for 
the pglist_data part where it might not make sense at all?).

>   };
>   #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)) {

if (unlikely(queue->is_dying))

> +		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)) {

if (unlikely(queue->is_dying))

> +		spin_unlock_irqrestore(&queue->split_queue_lock, *flags);
> +		memcg = parent_mem_cgroup(memcg);
> +		goto retry;
> +	}
>   
>   	return queue;
>   }

Nothing else jumped at me, but I am not a memcg expert :)

-- 
Cheers

David / dhildenb