Excerpts from netbsd: 18-Jun-97 Re: vm_fault on i386... Jason
Thorpe@nas.nasa.go (1240)

>  > root@wraith# vm_fault(0xf880d100,0,1,0) -> 5
>  > kernal: page fault trap, code = 0
>  > stopped at _pmap_changebit + 0x83: movl 0(%edx),%eax

I've seen similar errors.
The reason was that pde's disappeared magically.
This seemed to be triggered by a combination of heavy DMA
activity (locked pages + tsleep()) and process creation /
termination.
Nasty race condition -  couldn't track it down at this time.

best regards
Matthias