[PATCH RFC 22/28] mm: swap: prevent lruvec release in swap module

[Muchun Song]

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@xxxxxxxxxxxxx>
---
 mm/swap.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index ee19e171857d..fbf887578dbe 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -291,8 +291,10 @@ void lru_note_cost(struct lruvec *lruvec, bool file,
 
 void lru_note_cost_refault(struct folio *folio)
 {
+	rcu_read_lock();
 	lru_note_cost(folio_lruvec(folio), folio_is_file_lru(folio),
 		      folio_nr_pages(folio), 0);
+	rcu_read_unlock();
 }
 
 static void lru_activate(struct lruvec *lruvec, struct folio *folio)
@@ -406,18 +408,20 @@ 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, 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