[PATCH RFC 21/28] mm: zswap: prevent lruvec release in zswap_folio_swapin()

[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 zswap_folio_swapin().

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/zswap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index 204fb59da33c..4a41c2371f3d 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -752,8 +752,10 @@ void zswap_folio_swapin(struct folio *folio)
 	struct lruvec *lruvec;
 
 	if (folio) {
+		rcu_read_lock();
 		lruvec = folio_lruvec(folio);
 		atomic_long_inc(&lruvec->zswap_lruvec_state.nr_disk_swapins);
+		rcu_read_unlock();
 	}
 }
 
-- 
2.20.1