mm: swap: add back full cluster when no entry is reclaimed

If no swap cache is reclaimed, cluster taken off from full_clusters list
will not be put in any list and we can't reclaime HAS_CACHE slots
efficiently.  Do relocate_cluster for such cluster to avoid inefficiency.

Link: https://lkml.kernel.org/r/20250224113910.522439-1-shikemeng@huaweicloud.com
Fixes: 3b64477 ("mm, swap: reduce contention on device lock")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kemeng Shi

mm/swapfile.c

@@ -858,6 +858,10 @@ static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
 			offset++;
 		}
 
+		/* in case no swap cache is reclaimed */
+		if (ci->flags == CLUSTER_FLAG_NONE)
+			relocate_cluster(si, ci);
+
 		unlock_cluster(ci);
 		if (to_scan <= 0)
 			break;