kernel backtrace

[Vitaly Liaschuk]

Hi, list!
	I have a server with mpd and freeradius. Sometimes this server
	crashing down. 
# uname -a
FreeBSD billing.com 8.1-RELEASE-p2 FreeBSD 8.1-RELEASE-p2 #0: Mon Feb
	21 10:31:53 EET 2011local at localhost.com:/usr/src/sys/i386/compile/GENERIC  i386
# 
	Here is a crash log.

Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x28
fault code              = supervisor write, page not present
instruction pointer     = 0x20:0xc08c6960
stack pointer           = 0x28:0xc6bbbbbc
frame pointer           = 0x28:0xc6bbbc18
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 12 (swi4: clock)
trap number             = 12
panic: page fault
cpuid = 0
Uptime: 4h55m10s
Physical memory: 3315 MB
Dumping 228 MB: 213 197 181 165 149 133 117 101 85 69 53 37 21 5



I want to see the backtrace, but I can not do that.

# kgdb kernel.debug /var/crash/vmcore.4
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...
Cannot access memory at address 0x0
(kgdb) bt
#0  0x00000000 in ?? ()
(kgdb) 


So, my question is: Why I can not see a backtrace?