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

[Qi Zheng]

Hi David,

On 9/24/25 8:38 PM, David Hildenbrand wrote:
> 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@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;
>
> 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 

There is indeed a CSS_DYING flag. But we must modify 'is_dying' under
the protection of the split_queue_lock, otherwise the folio may be added
back to the deferred_split of child memcg.

> the pglist_data part where it might not make sense at all?).

Maybe:

#ifdef CONFIG_MEMCG
    bool is_dying;
#endif

> >   };
> >   #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))

Will do.

> > +        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))

Will do.

> > +        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 :)

Thanks,
Qi

>