[PATCH v1 15/26] mm: workingset: prevent memory cgroup release in lru_gen_eviction()

Qi Zheng
2025-10-28T14:03:54Z

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 lru_gen_eviction().

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/workingset.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 8cad8ee6dec6a..c4d21c15bad51 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -241,11 +241,14 @@ static void *lru_gen_eviction(struct folio *folio)
int refs = folio_lru_refs(folio);
bool workingset = folio_test_workingset(folio);
int tier = lru_tier_from_refs(refs, workingset);
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg;
struct pglist_data *pgdat = folio_pgdat(folio);
+ unsigned short memcg_id;

BUILD_BUG_ON(LRU_GEN_WIDTH + LRU_REFS_WIDTH > BITS_PER_LONG - EVICTION_SHIFT);

+ rcu_read_lock();
+ memcg = folio_memcg(folio);
lruvec = mem_cgroup_lruvec(memcg, pgdat);
lrugen = &lruvec->lrugen;
min_seq = READ_ONCE(lrugen->min_seq[type]);
@@ -253,8 +256,10 @@ static void *lru_gen_eviction(struct folio *folio)

hist = lru_hist_from_seq(min_seq);
atomic_long_add(delta, &lrugen->evicted[hist][type][tier]);
+ memcg_id = mem_cgroup_id(memcg);
+ rcu_read_unlock();

- return pack_shadow(mem_cgroup_id(memcg), pgdat, token, workingset);
+ return pack_shadow(memcg_id, pgdat, token, workingset);
}

/*
--
2.20.1