[PATCH v2] mm/swapfile.c: introduce function alloc_swap_scan_list()

[Chris Li <chrisl@kernel.org>]

This the alloc_swap_scan_list() will scan the whole list or the first
cluster.

This reduces the repeat patterns of isolating a cluster then scanning
that cluster. As a result, cluster_alloc_swap_entry() is shorter and shallower.

No functional change.

Signed-off-by: Chris Li <chrisl@kernel.org>
---
This patch goes on top of Kairui's swap improve cluster scan series:
https://lore.kernel.org/linux-mm/20250806161748.76651-1-ryncsn@gmail.com/
---
Changes in v2:
- Adjust change base on Andrew's feedback on int type and break out of
  loop.
- Link to v1: https://lore.kernel.org/r/20250806-swap-scan-list-v1-1-a5fe2d9340a2@kernel.org
---
 mm/swapfile.c | 86 ++++++++++++++++++++++++++++++++---------------------------
 1 file changed, 47 insertions(+), 39 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4a0cf4fb348d..f26678d68874 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -820,6 +820,29 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
 	return found;
 }
 
+static unsigned int alloc_swap_scan_list(struct swap_info_struct *si,
+					 struct list_head *list,
+					 unsigned int order,
+					 unsigned char usage,
+					 bool scan_all)
+{
+	unsigned int found = SWAP_ENTRY_INVALID;
+
+	do {
+		struct swap_cluster_info *ci = isolate_lock_cluster(si, list);
+		unsigned long offset;
+
+		if (!ci)
+			break;
+		offset = cluster_offset(si, ci);
+		found = alloc_swap_scan_cluster(si, ci, offset, order, usage);
+		if (found)
+			break;
+	} while (scan_all);
+
+	return found;
+}
+
 static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
 {
 	long to_scan = 1;
@@ -913,32 +936,24 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 	 * 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;
-		}
+		found = alloc_swap_scan_list(si, &si->free_clusters, order, usage,
+					     false);
+		if (found)
+			goto done;
 	}
 
 	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),
-							order, usage);
-			if (found)
-				goto done;
-		}
+		found = alloc_swap_scan_list(si, &si->nonfull_clusters[order],
+					     order, usage, 0);
+		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;
-		}
+		found = alloc_swap_scan_list(si, &si->free_clusters, order, usage,
+					     false);
+		if (found)
+			goto done;
 	}
 
 	/* Try reclaim full clusters if free and nonfull lists are drained */
@@ -952,13 +967,10 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 		 * failure is not critical. Scanning one cluster still
 		 * keeps the list rotated and reclaimed (for HAS_CACHE).
 		 */
-		ci = isolate_lock_cluster(si, &si->frag_clusters[order]);
-		if (ci) {
-			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
-							order, usage);
-			if (found)
-				goto done;
-		}
+		found = alloc_swap_scan_list(si, &si->frag_clusters[order], order,
+					     usage, true);
+		if (found)
+			goto done;
 	}
 
 	/*
@@ -977,19 +989,15 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si, int o
 		 * Clusters here have at least one usable slots and can't fail order 0
 		 * allocation, but reclaim may drop si->lock and race with another user.
 		 */
-		while ((ci = isolate_lock_cluster(si, &si->frag_clusters[o]))) {
-			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
-							0, usage);
-			if (found)
-				goto done;
-		}
+		found = alloc_swap_scan_list(si, &si->frag_clusters[o],
+					     0, usage, true);
+		if (found)
+			goto done;
 
-		while ((ci = isolate_lock_cluster(si, &si->nonfull_clusters[o]))) {
-			found = alloc_swap_scan_cluster(si, ci, cluster_offset(si, ci),
-							0, usage);
-			if (found)
-				goto done;
-		}
+		found = alloc_swap_scan_list(si, &si->nonfull_clusters[o],
+					     0, usage, true);
+		if (found)
+			goto done;
 	}
 done:
 	if (!(si->flags & SWP_SOLIDSTATE))

---
base-commit: f89484324d5876ee10765fa61da0332899fa1a6a
change-id: 20250806-swap-scan-list-2b89e3424b0a

Best regards,
-- 
Chris Li <chrisl@kernel.org>