mm/migrate: add comment about permanent failure path

Let's add a comment about where the failed page goes to, which makes
code more readable.

Signed-off-by: Naoya Horiguchi <[email protected]>
Signed-off-by: Joonsoo Kim <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Reviewed-by: Wanpeng Li <[email protected]>
Acked-by: Rafael Aquini <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zhang Yanfei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/migrate.c

@@ -1125,7 +1125,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 				nr_succeeded++;
 				break;
 			default:
-				/* Permanent failure */
+				/*
+				 * Permanent failure (-EBUSY, -ENOSYS, etc.):
+				 * unlike -EAGAIN case, the failed page is
+				 * removed from migration page list and not
+				 * retried in the next outer loop.
+				 */
 				nr_failed++;
 				break;
 			}