mm/migrate.c: unify "not queued for migration" handling in do_pages_move()

It can currently happen that we store the status of a page twice:
* Once we detect that it is already on the target node
* Once we moved a bunch of pages, and a page that's already on the
  target node is contained in the current interval.

Let's simplify the code and always call do_move_pages_to_node() in case we
did not queue a page for migration.  Note that pages that are already on
the target node are not added to the pagelist and are, therefore, ignored
by do_move_pages_to_node() - there is no functional change.

The status of such a page is now only stored once.

[david@redhat.com rephrase changelog]
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: http://lkml.kernel.org/r/20200214003017.25558-5-richardw.yang@linux.intel.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -1683,18 +1683,16 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 		err = add_page_for_migration(mm, addr, current_node,
 				&pagelist, flags & MPOL_MF_MOVE_ALL);
 
-		if (!err) {
-			/* The page is already on the target node */
-			err = store_status(status, i, current_node, 1);
-			if (err)
-				goto out_flush;
-			continue;
-		} else if (err > 0) {
+		if (err > 0) {
 			/* The page is successfully queued for migration */
 			continue;
 		}
 
-		err = store_status(status, i, err, 1);
+		/*
+		 * If the page is already on the target node (!err), store the
+		 * node, otherwise, store the err.
+		 */
+		err = store_status(status, i, err ? : current_node, 1);
 		if (err)
 			goto out_flush;