[PATCH V2] mm/swap: take folio refcount after testing the LRU flag

[yangge1116@126.com]

From: yangge <yangge1116@126.com>

Whoever passes a folio to __folio_batch_add_and_move() must hold
a reference, otherwise something else would already be messed up.
If the folio is referenced, it will not be freed elsewhere, so we
can safely clear the folio's lru flag. As discussed with David
in [1], we should take the reference after testing the LRU flag,
not before.

Link: https://lore.kernel.org/lkml/d41865b4-d6fa-49ba-890a-921eefad27dd@redhat.com/ [1]
Signed-off-by: yangge <yangge1116@126.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/swap.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

V2:
   Add sanity check suggested by David

diff --git a/mm/swap.c b/mm/swap.c
index 67a2467..c048659 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -226,12 +226,11 @@ static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
 {
 	unsigned long flags;
 
-	folio_get(folio);
-
-	if (on_lru && !folio_test_clear_lru(folio)) {
-		folio_put(folio);
+	if (on_lru && !folio_test_clear_lru(folio))
 		return;
-	}
+
+	VM_WARN_ON_ONCE(!folio_ref_count(folio));
+	folio_get(folio);
 
 	if (disable_irq)
 		local_lock_irqsave(&cpu_fbatches.lock_irq, flags);
-- 
2.7.4