[PATCH v2] mm/vmscan: don't try to reclaim freed folios

[Miaohe Lin <linmiaohe@huawei.com>]

If folios were freed from under us, there's no need to reclaim them. Skip
these folios to save lots of cpu cycles and avoid possible unnecessary
disk I/O.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
v2:
  use folio_ref_freeze to guard against race with GUP (fast). Many thanks
  Matthew for pointing this out.
---
 mm/vmscan.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 13d34d9593bb..547ae7ae6ab1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1610,13 +1610,19 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		folio = lru_to_folio(page_list);
 		list_del(&folio->lru);
 
+		nr_pages = folio_nr_pages(folio);
+
+		if (folio_ref_count(folio) == 1 &&
+		    folio_ref_freeze(folio, 1)) {
+			/* folio was freed from under us. So we are done. */
+			goto free_it;
+		}
+
 		if (!folio_trylock(folio))
 			goto keep;
 
 		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
 
-		nr_pages = folio_nr_pages(folio);
-
 		/* Account the number of base pages */
 		sc->nr_scanned += nr_pages;
 
-- 
2.23.0