Index: include/bitstring.h
===================================================================
--- include/bitstring.h
+++ include/bitstring.h
@@ -31,6 +31,7 @@
 
 #include <stdlib.h>
 #include <strings.h>
+#include <sys/param.h>
 #include <sys/bitstring.h>
 
 #endif /* _BITSTRING_H_ */
Index: sys/sys/bitstring.h
===================================================================
--- sys/sys/bitstring.h
+++ sys/sys/bitstring.h
@@ -65,6 +65,7 @@
 #ifdef _KERNEL
 #include <sys/libkern.h>
 #include <sys/malloc.h>
+#include <sys/param.h>
 #endif
 
 #include <sys/types.h>
@@ -105,8 +106,7 @@
 
 /*----------------------------- Public Interface -----------------------------*/
 /* Number of bytes consumed by a bit string of nbits bits */
-#define	bitstr_size(_nbits) \
-	(((_nbits) + _BITSTR_BITS - 1) / 8)
+#define	bitstr_size(_nbits) (howmany(_nbits, 8))
 
 /* Allocate a bit string initialized with no bits set. */
 #ifdef _KERNEL
@@ -125,7 +125,7 @@
 
 /* Allocate a bit string on the stack with no bits set. */
 #define	bit_decl(name, nbits) \
-	((name)[bitstr_size(nbits) / sizeof(bitstr_t)])
+	((name)[howmany(bitstr_size(nbits), sizeof(bitstr_t))])
 
 /* Is bit N of bit string set? */
 static inline int