[issue513] crash on HEAD while building world inside VKERNEL

[Matthew Dillon]

::Hi,
::
::I got this crash while make buildworld was running inside a vkernel and xor=
::g was
::compiling in the real host.
::
::I'm attaching screenshots of the crash and trace.
::
::Regards,
::Nuno
:
:    That is one I haven't seen before.  I will attempt to reproduce it.
:
:    A kernel crash dump would be nice, it shouldn't be possible for it
:    to get a null-pointer derefence there.  Hmmm.
:
:						-Matt

    I think I know what is going on here.  There are a couple of places
    in the pmap code where the PV list is being scanned and in the
    particular place shown in the snapshot you took, it is possible that
    a page with active I/O on it (i.e. when the system is under a heavy
    paging load) can cause the calling process to block.  This can cause
    the list scan to go stale and blow up in our faces.

    I will commit a workaround.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>