mm/swap: take folio refcount after testing the LRU flag

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]
Link: https://lkml.kernel.org/r/1723542743-32179-1-git-send-email-yangge1116@126.comSigned-off-by: yangge <yangge1116@126.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swap.c

@@ -226,12 +226,10 @@ 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;
-	}
+
+	folio_get(folio);
 
 	if (disable_irq)
 		local_lock_irqsave(&cpu_fbatches.lock_irq, flags);