memory hotplug: migrate swap cache page

In test, some pages in swap-cache can't be migrated, as they aren't rmap.

unmap_and_move() ignores swap-cache page which is just read in and hasn't
rmap (see the comments in the code), but swap_aops provides .migratepage.
Better to migrate such pages instead of ignore them.
Signed-off-by: NShaohua Li <shaohua.li@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Yakui Zhao <yakui.zhao@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -147,7 +147,7 @@ static void remove_migration_pte(struct vm_area_struct *vma,
 static void remove_file_migration_ptes(struct page *old, struct page *new)
 {
 	struct vm_area_struct *vma;
-	struct address_space *mapping = page_mapping(new);
+	struct address_space *mapping = new->mapping;
 	struct prio_tree_iter iter;
 	pgoff_t pgoff = new->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
 
@@ -664,13 +664,15 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 			 *    needs to be effective.
 			 */
 			try_to_free_buffers(page);
-		}
 			goto rcu_unlock;
 		}
+		goto skip_unmap;
+	}
 
 	/* Establish migration ptes or remove ptes */
 	try_to_unmap(page, 1);
 
+skip_unmap:
 	if (!page_mapped(page))
 		rc = move_to_new_page(newpage, page);