[PATCH RFC 19/28] mm: workingset: prevent memory cgroup release in lru_gen_eviction()

[Muchun Song]

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

diff --git a/mm/workingset.c b/mm/workingset.c
index ebafc0eaafba..e14b9e33f161 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