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/
Link: https://lkml.kernel.org/r/1723542743-32179-1-git-send-email-yangge1116@126.com
Signed-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>

diff --git a/mm/swap.c b/mm/swap.c
index 67a246772811bb0120927ec8919d6789bc3aa311..6b838986d2940fcafccef4dcb833abd23d87b838 100644 (file)
--- a/mm/swap.c
+++ b/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);