[PATCH v1 13/26] mm: mglru: prevent memory cgroup release in mglru

Qi Zheng
2025-10-28T14:03:43Z

From: Muchun Song <songmuchun@bytedance.com>

In the near future, a folio will no longer pin its corresponding
memory cgroup. To ensure safety, it will only be appropriate to
hold the rcu read lock or acquire a reference to the memory cgroup
returned by folio_memcg(), thereby preventing it from being released.

In the current patch, the rcu read lock is employed to safeguard
against the release of the memory cgroup in mglru.

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
mm/vmscan.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 660cd40cfddd4..676e6270e5b45 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3445,8 +3445,10 @@ static struct folio *get_pfn_folio(unsigned long pfn, struct mem_cgroup *memcg,
if (folio_nid(folio) != pgdat->node_id)
return NULL;

+ rcu_read_lock();
if (folio_memcg(folio) != memcg)
- return NULL;
+ folio = NULL;
+ rcu_read_unlock();

return folio;
}
@@ -4203,12 +4205,12 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
unsigned long addr = pvmw->address;
struct vm_area_struct *vma = pvmw->vma;
struct folio *folio = pfn_folio(pvmw->pfn);
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg;
struct pglist_data *pgdat = folio_pgdat(folio);
- struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
- struct lru_gen_mm_state *mm_state = get_mm_state(lruvec);
- DEFINE_MAX_SEQ(lruvec);
- int gen = lru_gen_from_seq(max_seq);
+ struct lruvec *lruvec;
+ struct lru_gen_mm_state *mm_state;
+ unsigned long max_seq;
+ int gen;

lockdep_assert_held(pvmw->ptl);
VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);
@@ -4243,6 +4245,13 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
}
}

+ rcu_read_lock();
+ memcg = folio_memcg(folio);
+ lruvec = mem_cgroup_lruvec(memcg, pgdat);
+ max_seq = READ_ONCE((lruvec)->lrugen.max_seq);
+ gen = lru_gen_from_seq(max_seq);
+ mm_state = get_mm_state(lruvec);
+
arch_enter_lazy_mmu_mode();

pte -= (addr - start) / PAGE_SIZE;
@@ -4279,6 +4288,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)

arch_leave_lazy_mmu_mode();

+ rcu_read_unlock();
+
/* feedback from rmap walkers to page table walkers */
if (mm_state && suitable_to_scan(i, young))
update_bloom_filter(mm_state, max_seq, pvmw->pmd);
--
2.20.1