[PATCH] mm: reuse the unshared swapcache page in do_wp_page

[Liang Zhang]

In current implementation, process's read requestions will fault in pages
with WP flags in PTEs. Next, if process emit a write requestion will go
into do_wp_page() and copy data to a new allocated page from the old one
due to refcount > 1 (page table mapped and swapcache), which could be
result in performance degradation. In fact, this page is exclusively owned
by this process and the duplication from old to a new allocated page is
really unnecessary.

So In this situation, these unshared pages can be reused by its process.

Signed-off-by: Liang Zhang <zhangliang5@xxxxxxxxxx>
---
This patch has been tested with redis benchmark. Here is the test
result.

Hardware
========
Memory (GB): 512G
CPU (total #): 88
NVMe SSD (GB): 1024

OS
==
kernel 5.10.0

Testcase
========
step 1:
  Run 16 VMs (4U8G), each running with redis-server, in a cgroup 
  limiting memory.limit_in_bytes to 100G. 
step 2:
  Run memtier_bemchmark in host with params "--threads=1 --clients=1 \
--pipeline=256 --data-size=2048 --requests=allkeys --key-minimum=1 \
--key-maximum=30000000 --key-prefix=memtier-benchmark-prefix-redistests"
  to test every VM concurrently.

Workset size
============
cat memory.memsw.usage_in_bytes
125403303936

Result
======
Comparing with Baseline, this patch can achieved 41% more Ops/sec, 
41% more Hits/sec, 41% more Misses/sec, 30% less Latency and 
41% more KB/sec. 

  Index(average)        Baseline kernel        Patched kernel
  Ops/sec               109497                 155428
  Hits/sec              8653                   12283
  Misses/sec            90889                  129014
  Latency               2.297                  1.603
  KB/sec                44569                  63186


 mm/memory.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 23f2f1300d42..fd4d868b1c2d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3291,10 +3291,16 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		struct page *page = vmf->page;
 
 		/* PageKsm() doesn't necessarily raise the page refcount */
-		if (PageKsm(page) || page_count(page) != 1)
+		if (PageKsm(page))
 			goto copy;
 		if (!trylock_page(page))
 			goto copy;
+
+		/* reuse the unshared swapcache page */
+		if (PageSwapCache(page) && reuse_swap_page(page, NULL)) {
+			goto reuse;
+		}
+
 		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
 			unlock_page(page);
 			goto copy;
@@ -3304,6 +3310,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		 * page count reference, and the page is locked,
 		 * it's dark out, and we're wearing sunglasses. Hit it.
 		 */
+reuse:
 		unlock_page(page);
 		wp_page_reuse(vmf);
 		return VM_FAULT_WRITE;
-- 
2.30.0