Hi. I get this on x86_64 master. I don't know how to reproduce, but it happened on my box just now three times within one hour.
DragonFly zoidberg.luxh.dyndns.info 2.11-DEVELOPMENT DragonFly v2.11.0.281.g34c13-DEVELOPMENT #13: Sat May 28 03:25:11 CEST 2011 root@zoidberg.luxh.dyndns.info:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64
panic: vm_fault: fault on stack guard, addr: 0xffffffe01dbea000
cpuid = 0
Trace beginning at frame 0xffffffe01dbed778
panic() at panic+0x1ed
panic() at panic+0x1ed
vm_fault() at vm_fault+0x1ed
trap_pfault() at trap_pfault+0x103
trap() at trap+0x43e
calltrap() at calltrap+0x8
--- trap 000000000000000c, rip = ffffffff80712e86, rsp = ffffffe01dbedab0, rbp = ffffffe01dbedb30 ---
ovbcopy() at ovbcopy+0x16
rl_rxeof() at rl_rxeof+0x10b
rl_intr() at rl_intr+0x7e
lwkt_serialize_handler_call() at lwkt_serialize_handler_call+0x80
ithread_handler() at ithread_handler+0x1ac
Debugger("panic")
(kgdb) bt
#0 _get_mycpu (di=<value optimized out>) at ./machine/thread.h:69
#1 md_dumpsys (di=<value optimized out>) at /usr/src/sys/platform/pc64/x86_64/dump_machdep.c:263
#2 0xffffffff8047ba45 in dumpsys () at /usr/src/sys/kern/kern_shutdown.c:927
#3 0xffffffff8047c0ab in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:389
#4 0xffffffff8047c354 in panic (fmt=0xffffffff80764604 "from debugger") at /usr/src/sys/kern/kern_shutdown.c:833
#5 0xffffffff80296eb5 in db_panic (addr=<value optimized out>, have_addr=0, count=0, modif=0x0) at /usr/src/sys/ddb/db_command.c:448
#6 0xffffffff8029757f in db_command () at /usr/src/sys/ddb/db_command.c:344
#7 db_command_loop () at /usr/src/sys/ddb/db_command.c:470
#8 0xffffffff8029a411 in db_trap (type=<value optimized out>, code=<value optimized out>) at /usr/src/sys/ddb/db_trap.c:71
#9 0xffffffff80713ff8 in kdb_trap (type=<value optimized out>, code=<value optimized out>, regs=<value optimized out>) at /usr/src/sys/platform/pc64/x86_64/db_interface.c:176
#10 0xffffffff80719b06 in trap (frame=0xffffffe01dbed6a8) at /usr/src/sys/platform/pc64/x86_64/trap.c:701
#11 0xffffffff807125ae in calltrap () at /usr/src/sys/platform/pc64/x86_64/exception.S:187
#12 0xffffffff80713e8f in breakpoint (msg=<value optimized out>) at ./cpu/cpufunc.h:73
#13 Debugger (msg=<value optimized out>) at /usr/src/sys/platform/pc64/x86_64/db_interface.c:361
#14 0xffffffff8047c334 in panic (fmt=0xffffffff807f1c88 "vm_fault: fault on stack guard, addr: %p") at /usr/src/sys/kern/kern_shutdown.c:824
#15 0xffffffff8067f8b3 in vm_fault (map=0xffffffff80c933a0, vaddr=18446743936769630208, fault_type=<value optimized out>, fault_flags=<value optimized out>)
at /usr/src/sys/vm/vm_fault.c:334
#16 0xffffffff8071950f in trap_pfault (frame=0xffffffe01dbed9f8, usermode=<value optimized out>) at /usr/src/sys/platform/pc64/x86_64/trap.c:864
#17 0xffffffff80719a52 in trap (frame=0xffffffe01dbed9f8) at /usr/src/sys/platform/pc64/x86_64/trap.c:591
#18 0xffffffff807125ae in calltrap () at /usr/src/sys/platform/pc64/x86_64/exception.S:187
#19 0xffffffff80712e86 in ovbcopy () at /usr/src/sys/platform/pc64/x86_64/support.s:113
#20 0xffffffff804c1128 in m_devget (buf=0xffffffe01dbe9fda "", len=1073, offset=<value optimized out>, ifp=<value optimized out>, copy=<value optimized out>)
at /usr/src/sys/kern/uipc_mbuf.c:1858
#21 0xffffffff805e4e8f in rl_rxeof (sc=0xffffffe0191b39f0) at /usr/src/sys/dev/netif/rl/if_rl.c:1056
#22 0xffffffff805e5303 in rl_intr (arg=<value optimized out>) at /usr/src/sys/dev/netif/rl/if_rl.c:1233
#23 0xffffffff8048c51a in lwkt_serialize_handler_call (s=0xffffffe0191b3d90, func=0xffffffff805e5285 <rl_intr>, arg=0xffffffe0191b39f0, frame=0x0)
at /usr/src/sys/kern/lwkt_serialize.c:229
#24 0xffffffff80453cc4 in ithread_handler (arg=<value optimized out>) at /usr/src/sys/kern/kern_intr.c:824
#25 0xffffffff80486bcd in lwkt_deschedule_self (td=0xffffffe042322020) at /usr/src/sys/kern/lwkt_thread.c:283
Backtrace stopped: previous frame inner to this frame (corrupt stack?)
-Max