[RFC PATCH] mm: Avoiding split large folios if swap has no space

[Barry Song (Xiaomi)]

When swap is disabled or exhausted, swap slot allocation
may fail during swapout, causing large folios to be split
into small folios. The splitting is reasonable when we
truly fail to obtain contiguous swap slots, but it is
pointless in the no-space case.

A simple way to reproduce this is to invoke MADV_PAGEOUT on
a system with mTHP enabled but without swap configured.

 #define SIZE (16 * 1024 * 1024)
 int main(void)
 {
         char *buf = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
         memset(buf, 1, SIZE);
         madvise(buf, SIZE, MADV_PAGEOUT);
         munmap(buf, SIZE);
         return 0;
 }

With 16KB mTHP enabled, we observe:
~ # cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/split
1024

This patch checks swap space before splitting. If there is
no available space, it skips splitting. After the patch, we
observe:
~ # cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/split
0

Reported-by: Nanzhe Zhao <zhaonanzhe@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
Cc: Nico Pache <npache@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Lance Yang <lance.yang@xxxxxxxxx>
Cc: Kairui Song <kasong@xxxxxxxxxxx>
Cc: Qi Zheng <qi.zheng@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Yuanchu Xie <yuanchu@xxxxxxxxxx>
Cc: Wei Xu <weixugc@xxxxxxxxxx>
Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/vmscan.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 299b5d9e8836..33f84a5fe7ee 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -339,8 +339,7 @@ static bool can_demote(int nid, struct scan_control *sc,
 	return !nodes_empty(allowed_mask);
 }
 
-static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
-					  int nid,
+static inline bool __can_reclaim_anon_pages(struct mem_cgroup *memcg,
 					  struct scan_control *sc)
 {
 	if (memcg == NULL) {
@@ -356,6 +355,16 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
 			return true;
 	}
 
+	return false;
+}
+
+static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
+					  int nid,
+					  struct scan_control *sc)
+{
+	if (__can_reclaim_anon_pages(memcg, sc))
+		return true;
+
 	/*
 	 * The page can not be swapped.
 	 *
@@ -1280,6 +1289,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 
 				if (!folio_test_large(folio))
 					goto activate_locked_split;
+				if (!__can_reclaim_anon_pages(memcg, sc))
+					goto activate_locked_split;
 				/* Fallback to swap normal pages */
 				if (split_folio_to_list(folio, folio_list))
 					goto activate_locked;
-- 
2.39.3 (Apple Git-146)