diff --git a/sys/compat/linuxkpi/common/include/linux/kernel.h b/sys/compat/linuxkpi/common/include/linux/kernel.h
--- a/sys/compat/linuxkpi/common/include/linux/kernel.h
+++ b/sys/compat/linuxkpi/common/include/linux/kernel.h
@@ -150,6 +150,7 @@
 
 #undef	ALIGN
 #define	ALIGN(x, y)		roundup2((x), (y))
+#define	ALIGN_DOWN(x, y)	rounddown2(x, y)
 #undef PTR_ALIGN
 #define	PTR_ALIGN(p, a)		((__typeof(p))ALIGN((uintptr_t)(p), (a)))
 #define	IS_ALIGNED(x, a)	(((x) & ((__typeof(x))(a) - 1)) == 0)