[PATCH v3 1/4] mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio()

["Barry Song (Xiaomi)" <baohua@kernel.org>]

There is a case where `folio_ref_count(folio) == 3` and
`!folio_test_swapcache(folio)`. In that case, both
`folio_ref_count(folio) > 3` and
`folio_ref_count(folio) > 1 + folio_test_swapcache(folio)` evaluate
false, causing an unnecessary local LRU drain.

During an Ubuntu boot, I observed over 5,000 redundant local LRU
drains. For a kernel build with a minimal configuration, I observed
more than 20,000 redundant drains.

Fix this by checking against: `1 + in_swapcache + in_lrucache`
instead of hardcoding `folio_ref_count(folio) > 3`.

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/memory.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ff338c2abe92..87da78eb1abd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4181,6 +4181,9 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
 static bool wp_can_reuse_anon_folio(struct folio *folio,
 				    struct vm_area_struct *vma)
 {
+	const bool in_lru_cache = !folio_test_lru(folio);
+	const bool in_swapcache = folio_test_swapcache(folio);
+
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && folio_test_large(folio))
 		return __wp_can_reuse_large_anon_folio(folio, vma);
 
@@ -4191,15 +4194,16 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
 	 *
 	 * KSM doesn't necessarily raise the folio refcount.
 	 */
-	if (folio_test_ksm(folio) || folio_ref_count(folio) > 3)
+	if (folio_test_ksm(folio) ||
+	    folio_ref_count(folio) > 1 + in_lru_cache + in_swapcache)
 		return false;
-	if (!folio_test_lru(folio))
+	if (in_lru_cache)
 		/*
 		 * We cannot easily detect+handle references from
 		 * remote LRU caches or references to LRU folios.
 		 */
 		lru_add_drain();
-	if (folio_ref_count(folio) > 1 + folio_test_swapcache(folio))
+	if (folio_ref_count(folio) > 1 + in_swapcache)
 		return false;
 	if (!folio_trylock(folio))
 		return false;
-- 
2.39.3 (Apple Git-146)