mm, swap: fix reclaim offset calculation error during allocation

There is a code error that will cause the swap entry allocator to reclaim
and check the whole cluster with an unexpected tail offset instead of the
part that needs to be reclaimed.  This may cause corruption of the swap
map, so fix it.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 3b64477 ("mm, swap: reduce contention on device lock")
Signed-off-by: Kairui Song <[email protected]>
Cc: Chris Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ryncsn

mm/swapfile.c

@@ -794,7 +794,7 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
 		if (!cluster_scan_range(si, ci, offset, nr_pages, &need_reclaim))
 			continue;
 		if (need_reclaim) {
-			ret = cluster_reclaim_range(si, ci, start, end);
+			ret = cluster_reclaim_range(si, ci, offset, offset + nr_pages);
 			/*
 			 * Reclaim drops ci->lock and cluster could be used
 			 * by another order. Not checking flag as off-list