Index: sys/sys/param.h
===================================================================
RCS file: /cvsroot/src/sys/sys/param.h,v
retrieving revision 1.424
diff -p -u -r1.424 param.h
--- sys/sys/param.h	20 Dec 2012 08:03:44 -0000	1.424
+++ sys/sys/param.h	21 Mar 2013 15:43:11 -0000
@@ -370,7 +370,14 @@
 #endif
 #define	roundup(x, y)	((((x)+((y)-1))/(y))*(y))
 #define	rounddown(x,y)	(((x)/(y))*(y))
-#define	roundup2(x, m)	(((x) + (m) - 1) & ~((m) - 1))
+/*
+ * The x - x term forces m - 1 to be converted to the width of x before
+ * taking the complement, so that if m is unsigned int and x is larger,
+ * then m will be extended before the complement -- and, critically,
+ * not *zero*-extended after the complement as it would be without the
+ * correction because it is unsigned.
+ */
+#define	roundup2(x, m)	(((x) + (m) - 1) & ~(((m) - 1) + ((x) - (x))))
 #define	powerof2(x)	((((x)-1)&(x))==0)
 
 /* Macros for min/max. */