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

[Qi Zheng]

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@xxxxxxxxxxxxx>
> > ---
> >   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;
> >   }