[PATCH v6 14/33] mm: mglru: prevent memory cgroup release in mglru

[Qi Zheng <qi.zheng@linux.dev>]

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>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/vmscan.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 61303ec85d587..024ff870b1a03 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3443,8 +3443,10 @@ 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;
 }
@@ -4202,12 +4204,12 @@ 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);
@@ -4242,6 +4244,12 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 		}
 	}
 
+	memcg = get_mem_cgroup_from_folio(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);
+
 	lazy_mmu_mode_enable();
 
 	pte -= (addr - start) / PAGE_SIZE;
@@ -4282,6 +4290,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	if (mm_state && suitable_to_scan(i, young))
 		update_bloom_filter(mm_state, max_seq, pvmw->pmd);
 
+	mem_cgroup_put(memcg);
+
 	return true;
 }
 
-- 
2.20.1