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

Roman Gushchin
2025-09-24T09:24:02Z

Qi Zheng <zhengqi.arch@bytedance.com> writes:

> 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.

Nit: commit logs should really focus on the actual change, not the future
plans.

>
> 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
>
> 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)) {
> + 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)) {
> + spin_unlock_irqrestore(&queue->split_queue_lock, *flags);
> + memcg = parent_mem_cgroup(memcg);
> + goto retry;
> + }
>
> return queue;
> }
> @@ -4284,6 +4296,33 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> return split;
> }
>
> +void reparent_deferred_split_queue(struct mem_cgroup *memcg)
> +{
> + struct mem_cgroup *parent = parent_mem_cgroup(memcg);
> + struct deferred_split *ds_queue = &memcg->deferred_split_queue;
> + struct deferred_split *parent_ds_queue = &parent->deferred_split_queue;
> + int nid;
> +
> + spin_lock_irq(&ds_queue->split_queue_lock);
> + spin_lock_nested(&parent_ds_queue->split_queue_lock, SINGLE_DEPTH_NESTING);
> +
> + if (!ds_queue->split_queue_len)
> + goto unlock;
> +
> + list_splice_tail_init(&ds_queue->split_queue, &parent_ds_queue->split_queue);
> + parent_ds_queue->split_queue_len += ds_queue->split_queue_len;
> + ds_queue->split_queue_len = 0;
> + /* Mark the ds_queue dead */
> + ds_queue->is_dying = true;
> +
> + for_each_node(nid)
> + set_shrinker_bit(parent, nid, shrinker_id(deferred_split_shrinker));

Does this loop need to be under locks?

> +
> +unlock:
> + spin_unlock(&parent_ds_queue->split_queue_lock);
> + spin_unlock_irq(&ds_queue->split_queue_lock);
> +}
> +
> #ifdef CONFIG_DEBUG_FS
> static void split_huge_pages_all(void)
> {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e090f29eb03bd..d03da72e7585d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3887,6 +3887,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
> zswap_memcg_offline_cleanup(memcg);
>
> memcg_offline_kmem(memcg);
> + reparent_deferred_split_queue(memcg);
> reparent_shrinker_deferred(memcg);

I guess the naming can be a bit more consistent here :)

Thanks!