mm: migrate: one less atomic operation

migrate_page_move_mapping() drops a reference from the old page after
unfreezing its counter.  Both operations can be merged into a single
atomic operation by directly unfreezing to one less reference.

The same applies to migrate_huge_page_move_mapping().
Signed-off-by: NJacobo Giralt <jacobo.giralt@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -269,12 +269,12 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 
 	radix_tree_replace_slot(pslot, newpage);
 
-	page_unfreeze_refs(page, expected_count);
 	/*
-	 * Drop cache reference from old page.
+	 * Drop cache reference from old page by unfreezing
+	 * to one less reference.
 	 * We know this isn't the last reference.
 	 */
-	__put_page(page);
+	page_unfreeze_refs(page, expected_count - 1);
 
 	/*
 	 * If moved to a different zone then also account
@@ -334,9 +334,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
 
 	radix_tree_replace_slot(pslot, newpage);
 
-	page_unfreeze_refs(page, expected_count);
-
-	__put_page(page);
+	page_unfreeze_refs(page, expected_count - 1);
 
 	spin_unlock_irq(&mapping->tree_lock);
 	return 0;