On Wed, 14 Nov 2012, Randy Dunlap wrote:

From: Randy Dunlap <redacted>

Fix build when CONFIG_BUG is not enabled by adding header file
<asm-generic/bug.h>:

mm/balloon_compaction.c: In function 'balloon_page_putback':
mm/balloon_compaction.c:243:3: error: implicit declaration of function '__WARN'

This is fixed by 
mm-introduce-a-common-interface-for-balloon-pages-mobility-fix-fix-fix.patch 
in -mm which converts it to WARN_ON(1) which is the generic way to trigger 
a warning.