[PATCH v2 4/4] mm: try to free swapcache for non-LRU folios

[Barry Song (Xiaomi)]

Originally, we unconditionally called lru_add_drain() for write
swap-in page faults. This might drop the reference held by the per-CPU
LRU cache if the folio happened to reside there. However, there was no
guarantee that the folio was actually cached on the current CPU.

Now that lru_add_drain() has been removed, we have lost one
opportunity to drop a reference held by the LRU cache. We could
instead incorporate that possibility into the condition evaluated by
should_try_to_free_swap().

Suggested-by: Kairui Song <ryncsn@xxxxxxxxx>
Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/memory.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2983a6baf474..14577c67c61a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5087,8 +5087,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	 * Remove the swap entry and conditionally try to free up the swapcache.
 	 * Do it after mapping, so raced page faults will likely see the folio
 	 * in swap cache and wait on the folio lock.
+	 * Assume non-LRU folios may be queued in the LRU cache, which contributes
+	 * an additional reference to the folio.
 	 */
-	if (should_try_to_free_swap(si, folio, vma, nr_pages, vmf->flags))
+	if (should_try_to_free_swap(si, folio, vma, nr_pages +
+			!folio_test_lru(folio), vmf->flags))
 		folio_free_swap(folio);
 
 	folio_unlock(folio);
-- 
2.39.3 (Apple Git-146)