mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio()

Patch series "mm: drop redundant lru_add_drain in anon folio reuse paths",
v3.

We are doing a large number of redundant lru_add_drain() calls in both
wp_can_reuse_anon_folio() and do_swap_page(), leading to LRU lock
contention and unnecessary overhead.

In wp_can_reuse_anon_folio(), we can check the refcount against the
lru_cache before deciding to drain.  In do_swap_page(), the drain is now
entirely redundant after Kairui's work to route SYNC I/O through the
swapcache in the same way as ASYNC I/O.

Build the kernel within a 1 GB memcg using 20 threads with zRAM swap.  The
number of lru_add_drain() calls is reduced from 276,278 to 226,318, a
reduction of about 18%.

Build the kernel within an 800 MB memcg using 20 threads with zRAM swap. 
The number of lru_add_drain() calls is reduced from 778,950 to 541,149, a
reduction of 30.5%.


This patch (of 4):

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`.

[baohua@kernel.org: rename in_lru_cache to maybe_in_lru_cache]
  Link: https://lore.kernel.org/20260708145718.82690-1-baohua@kernel.org
Link: https://lore.kernel.org/20260701235955.36126-1-baohua@kernel.org
Link: https://lore.kernel.org/20260701235955.36126-2-baohua@kernel.org
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 6637c5b13c9b..c10b8a166828 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4174,6 +4174,9 @@ 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 maybe_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);
 
@@ -4184,15 +4187,16 @@ 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 + maybe_in_lru_cache + in_swapcache)
 		return false;
-	if (!folio_test_lru(folio))
+	if (maybe_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;