Build a kernel with DDB and DEBUG. Add the following two patches to
trap.c:
1. In the initial test "if (trapdebug)", add a delay(5000000)
or so. (Also turn on trapdebug.)
2. In the "case T_PAGEFLT|T_USER:", after "vm" is set but
before it is referenced add:
if ((int)vm == 0)
goto we_re_toast;