linux-kernel - [PATCH 2/2] mm, swap: prefer nonfull over free clusters

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Message-ID: <20250804172439.2331-3-ryncsn@gmail.com>
Date: Tue,  5 Aug 2025 01:24:39 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Kemeng Shi <shikemeng@...weicloud.com>,
	Chris Li <chrisl@...nel.org>,
	Nhat Pham <nphamcs@...il.com>,
	Baoquan He <bhe@...hat.com>,
	Barry Song <baohua@...nel.org>,
	"Huang, Ying" <ying.huang@...ux.alibaba.com>,
	linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>
Subject: [PATCH 2/2] mm, swap: prefer nonfull over free clusters

From: Kairui Song <kasong@...cent.com>

We prefer a free cluster over a nonfull cluster whenever a CPU local
cluster is drained to respect the SSD discard behavior [1]. It's not
a best practice for non-discarding devices. And this is causing a
chigher fragmentation rate.

So for a non-discarding device, prefer nonfull over free clusters. This
reduces the fragmentation issue by a lot.

Testing with make -j96, defconfig, using 64k mTHP, 8G ZRAM:

Before: sys time: 6121.0s  64kB/swpout: 1638155  64kB/swpout_fallback: 189562
After:  sys time: 6145.3s  64kB/swpout: 1761110  64kB/swpout_fallback: 66071

Testing with make -j96, defconfig, using 64k mTHP, 10G ZRAM:

Before: sys time 5527.9s  64kB/swpout: 1789358  64kB/swpout_fallback: 17813
After:  sys time 5538.3s  64kB/swpout: 1813133  64kB/swpout_fallback: 0

Performance is basically unchanged, and the large allocation failure rate
is lower. Enabling all mTHP sizes showed a more significant result:

Using the same test setup with 10G ZRAM and enabling all mTHP sizes:

128kB swap failure rate:
Before: swpout:449548 swpout_fallback:55894
After:  swpout:497519 swpout_fallback:3204

256kB swap failure rate:
Before: swpout:63938  swpout_fallback:2154
After:  swpout:65698  swpout_fallback:324

512kB swap failure rate:
Before: swpout:11971  swpout_fallback:2218
After:  swpout:14606  swpout_fallback:4

2M swap failure rate:
Before: swpout:12     swpout_fallback:1578
After:  swpout:1253   swpout_fallback:15

The success rate of large allocations is much higher.

Link: https://lore.kernel.org/linux-mm/87v8242vng.fsf@yhuang6-desk2.ccr.corp.intel.com/ [1]
Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swapfile.c | 38 ++++++++++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 10 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5fdb3cb2b8b7..4a0cf4fb348d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -908,18 +908,20 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 	}
 
 new_cluster:
-	ci = isolate_lock_cluster(si, &si->free_clusters);
-	if (ci) {
-		found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
-						order, usage);
-		if (found)
-			goto done;
+	/*
+	 * If the device need discard, prefer new cluster over nonfull
+	 * to spread out the writes.
+	 */
+	if (si->flags & SWP_PAGE_DISCARD) {
+		ci = isolate_lock_cluster(si, &si->free_clusters);
+		if (ci) {
+			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
+							order, usage);
+			if (found)
+				goto done;
+		}
 	}
 
-	/* Try reclaim from full clusters if free clusters list is drained */
-	if (vm_swap_full())
-		swap_reclaim_full_clusters(si, false);
-
 	if (order < PMD_ORDER) {
 		while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[order]))) {
 			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
@@ -927,7 +929,23 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 			if (found)
 				goto done;
 		}
+	}
 
+	if (!(si->flags & SWP_PAGE_DISCARD)) {
+		ci = isolate_lock_cluster(si, &si->free_clusters);
+		if (ci) {
+			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
+							order, usage);
+			if (found)
+				goto done;
+		}
+	}
+
+	/* Try reclaim full clusters if free and nonfull lists are drained */
+	if (vm_swap_full())
+		swap_reclaim_full_clusters(si, false);
+
+	if (order < PMD_ORDER) {
 		/*
 		 * Scan only one fragment cluster is good enough. Order 0
 		 * allocation will surely success, and large allocation
-- 
2.50.1