mm/swapfile.c: open code cluster_alloc_swap()

It's only called in scan_swap_map_slots().

And also remove the stale code comment in scan_swap_map_slots() because
it's not fit for the current cluster allocation mechanism.

Link: https://lkml.kernel.org/r/20250205092721.9395-13-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Baoquan He

mm/swapfile.c

@@ -1163,39 +1163,13 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	swap_usage_sub(si, nr_entries);
 }
 
-static int cluster_alloc_swap(struct swap_info_struct *si,
-			     unsigned char usage, int nr,
-			     swp_entry_t slots[], int order)
-{
-	int n_ret = 0;
-
-	while (n_ret < nr) {
-		unsigned long offset = cluster_alloc_swap_entry(si, order, usage);
-
-		if (!offset)
-			break;
-		slots[n_ret++] = swp_entry(si->type, offset);
-	}
-
-	return n_ret;
-}
-
 static int scan_swap_map_slots(struct swap_info_struct *si,
 			       unsigned char usage, int nr,
 			       swp_entry_t slots[], int order)
 {
 	unsigned int nr_pages = 1 << order;
+	int n_ret = 0;
 
-	/*
-	 * We try to cluster swap pages by allocating them sequentially
-	 * in swap.  Once we've allocated SWAPFILE_CLUSTER pages this
-	 * way, however, we resort to first-free allocation, starting
-	 * a new cluster.  This prevents us from scattering swap pages
-	 * all over the entire swap partition, so that we reduce
-	 * overall disk seek times between swap pages.  -- sct
-	 * But we do now try to find an empty cluster.  -Andrea
-	 * And we let swap pages go all over an SSD partition.  Hugh
-	 */
 	if (order > 0) {
 		/*
 		 * Should not even be attempting large allocations when huge
@@ -1215,7 +1189,15 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 			return 0;
 	}
 
-	return cluster_alloc_swap(si, usage, nr, slots, order);
+	while (n_ret < nr) {
+		unsigned long offset = cluster_alloc_swap_entry(si, order, usage);
+
+		if (!offset)
+			break;
+		slots[n_ret++] = swp_entry(si->type, offset);
+	}
+
+	return n_ret;
 }
 
 static bool get_swap_device_info(struct swap_info_struct *si)