mm, swap: prefer nonfull over free clusters

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 higher
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: 6176.34s  64kB/swpout: 1659757  64kB/swpout_fallback: 139503
After:  sys time: 6194.11s  64kB/swpout: 1689470  64kB/swpout_fallback: 56147

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

After:  sys time: 5531.49s  64kB/swpout: 1791142  64kB/swpout_fallback: 17676
After:  sys time: 5587.53s  64kB/swpout: 1811598  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:451599 swpout_fallback:54525
After:  swpout:502710 swpout_fallback:870

256kB swap failure rate:
Before: swpout:63652  swpout_fallback:2708
After:  swpout:65913  swpout_fallback:20

512kB swap failure rate:
Before: swpout:11663  swpout_fallback:1767
After:  swpout:14480  swpout_fallback:6

2M swap failure rate:
Before: swpout:24     swpout_fallback:1442
After:  swpout:1329   swpout_fallback:7

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]
Link: https://lkml.kernel.org/r/20250806161748.76651-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files 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 @@ new_cluster:
 			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