Index: head/sys/amd64/amd64/mem.c
===================================================================
--- head/sys/amd64/amd64/mem.c
+++ head/sys/amd64/amd64/mem.c
@@ -140,7 +140,7 @@
 				error = uiomove((void *)vd, c, uio);
 				break;
 			}
-			if (v >= (1ULL << cpu_maxphyaddr)) {
+			if (v > cpu_getmaxphyaddr()) {
 				error = EFAULT;
 				break;
 			}
@@ -169,7 +169,7 @@
     int prot __unused, vm_memattr_t *memattr __unused)
 {
 	if (dev2unit(dev) == CDEV_MINOR_MEM) {
-		if (offset >= (1ULL << cpu_maxphyaddr))
+		if (offset > cpu_getmaxphyaddr())
 			return (-1);
 		*paddr = offset;
 		return (0);
Index: head/sys/i386/i386/mem.c
===================================================================
--- head/sys/i386/i386/mem.c
+++ head/sys/i386/i386/mem.c
@@ -108,8 +108,11 @@
 			continue;
 		}
 		if (dev2unit(dev) == CDEV_MINOR_MEM) {
-			pa = uio->uio_offset;
-			pa &= ~PAGE_MASK;
+			if (uio->uio_offset > cpu_getmaxphyaddr()) {
+				error = EFAULT;
+				break;
+			}
+			pa = trunc_page(uio->uio_offset);
 		} else {
 			/*
 			 * Extract the physical page since the mapping may
@@ -162,6 +165,8 @@
     int prot __unused, vm_memattr_t *memattr __unused)
 {
 	if (dev2unit(dev) == CDEV_MINOR_MEM) {
+		if (offset > cpu_getmaxphyaddr())
+			return (-1);
 		*paddr = offset;
 		return (0);
 	}
Index: head/sys/x86/include/x86_var.h
===================================================================
--- head/sys/x86/include/x86_var.h
+++ head/sys/x86/include/x86_var.h
@@ -94,6 +94,20 @@
  */
 typedef void alias_for_inthand_t(void);
 
+/*
+ * Returns the maximum physical address that can be used with the
+ * current system.
+ */
+static __inline vm_paddr_t
+cpu_getmaxphyaddr(void)
+{
+#if defined(__i386__) && !defined(PAE)
+	return (0xffffffff);
+#else
+	return ((1ULL << cpu_maxphyaddr) - 1);
+#endif
+}
+
 void	*alloc_fpusave(int flags);
 void	busdma_swi(void);
 bool	cpu_mwait_usable(void);