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

Qi Zheng
2025-09-23T09:17:20Z

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

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));
+
+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);
wb_memcg_offline(memcg);
lru_gen_offline_memcg(memcg);
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 3db2dea7db4c5..cbda5c2ee3241 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1387,6 +1387,7 @@ static void pgdat_init_split_queue(struct pglist_data *pgdat)
spin_lock_init(&ds_queue->split_queue_lock);
INIT_LIST_HEAD(&ds_queue->split_queue);
ds_queue->split_queue_len = 0;
+ ds_queue->is_dying = false;
}
#else
static void pgdat_init_split_queue(struct pglist_data *pgdat) {}
--
2.20.1