[PATCH v1 19/26] mm: swap: prevent lruvec release in swap module

Qi Zheng
2025-10-28T14:04:24Z

From: Muchun Song <songmuchun@bytedance.com>

In the near future, a folio will no longer pin its corresponding
memory cgroup. So an lruvec returned by folio_lruvec() could be
released without the rcu read lock or a reference to its memory
cgroup.

In the current patch, the rcu read lock is employed to safeguard
against the release of the lruvec in lru_note_cost_refault() and
lru_activate().

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/swap.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index ec0c654e128dc..0606795f3ccf3 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -412,18 +412,20 @@ static void lru_gen_inc_refs(struct folio *folio)

static bool lru_gen_clear_refs(struct folio *folio)
{
- struct lru_gen_folio *lrugen;
int gen = folio_lru_gen(folio);
int type = folio_is_file_lru(folio);
+ unsigned long seq;

if (gen < 0)
return true;

set_mask_bits(&folio->flags.f, LRU_REFS_FLAGS | BIT(PG_workingset), 0);

- lrugen = &folio_lruvec(folio)->lrugen;
+ rcu_read_lock();
+ seq = READ_ONCE(folio_lruvec(folio)->lrugen.min_seq[type]);
+ rcu_read_unlock();
/* whether can do without shuffling under the LRU lock */
- return gen == lru_gen_from_seq(READ_ONCE(lrugen->min_seq[type]));
+ return gen == lru_gen_from_seq(seq);
}

#else /* !CONFIG_LRU_GEN */
--
2.20.1