FreeBSD crashed - trying to find out why

[Rakhesh Sasidharan]

Hello,

I have a FreeBSD 8.1-RELEASE machine running inside a KVM VPS. 

I installed tmux a few days back, and today I was trying to update all
my ports via portupgrade. Everything was going fine, so after a while I
detached from the tmux session and disconnected from the machine (was
connected via SSH). When I returned a few hours later, I see that the
machine had rebooted. It appears that the machine rebooted due to a
kernel fault. From the timestamps I see that the machine rebooted some
10-15 minutes before I returned, so it looks like portupgrade and tmux
etc were working fine until then. 

Here's the messages from /var/log/messages - 

-8<---
kernel: Fatal trap 12: page fault while in kernel mode
kernel: cpuid = 0; apic id = 00
kernel: fault virtual address   = 0x250
kernel: fault code              = supervisor read data, page not present
kernel: instruction pointer     = 0x20:0xffffffff8052e574
kernel: stack pointer           = 0x28:0xffffff800019c8a0
kernel: frame pointer           = 0x28:0xffffff800019c8c0
kernel: code segment            = base 0x0, limit 0xfffff, type 0x1b
kernel: = DPL 0, pres 1, long 1, def32 0, gran 1
kernel: processor eflags        = interrupt enabled, resume, IOPL = 0
kernel: current process         = 50496 (tmux)
kernel: trap number             = 12
kernel: panic: page fault
kernel: cpuid = 0
kernel: Uptime: 3d22h30m42s
kernel: Cannot dump. Device not defined or unavailable.
kernel: Automatic reboot in 15 seconds - press a key on the console to
abort
-8<---

I went into all the background about tmux above since the logs highlight
tmux as the currently running process. 

Any idea why kernel crashed, or what I can do to prevent this in future?
I understand from the forums that this could be due to bad memory or bad
hard disk, but I was wondering whether this could also be due to any
incompatibilities with KVM (triggered by tmux perhaps). 

Thanks,
Rakhesh