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

Qi Zheng
2025-09-25T06:29:45Z

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@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
>
> 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;
>> }
>