kern/47041: amd64 kernel core dumps are broken

[Taylor R Campbell <campbell+netbsd%mumble.net@localhost>]

>Number:         47041
>Category:       kern
>Synopsis:       amd64 kernel core dumps are broken
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 05 14:10:00 +0000 2012
>Originator:     Taylor R Campbell <campbell+netbsd%mumble.net@localhost>
>Release:        NetBSD 6.99.11
>Organization:
>Environment:
Architecture: amd64
Machine: amd64
>Description:

        After a panic in namei the other day, my system wrote a kernel
        core dump which savecore had trouble reading:

Oct  3 14:40:58 manticore savecore: reboot after panic: leaf `current.ro' 
should be empty
Oct  3 14:40:58 manticore savecore: system went down at Tue Oct  2 18:17:25 
2012 
Oct  3 14:40:58 manticore savecore: writing core to ./netbsd.0.core
Oct  3 18:06:41 manticore savecore: writing kernel to ./netbsd.0
Oct  3 18:06:41 manticore savecore: kvm_read: invalid translation (invalid 
level 4 PDE)
Oct  3 18:06:41 manticore savecore: (null): Bad address

        The `(null): Bad address' error I have been seeing for years on
        i386, but the `invalid level 3/4 PDE' messages I have not seen
        before.  Also, the three and a half hour delay in writing the
        core dump is...odd.  Most of the time, savecore -- and anything
        else trying to stat netbsd.0.core, such as `ls -l' -- was stuck
        waiting for a vnode lock, presumably of netbsd.0.core, but I
        don't know who held it (crash(8) is broken when it comes to
        vnodes), and nothing seemed to be spinning.

        Attempting to load the core in gdb didn't work either:

(gdb) target kvm netbsd.0.core
invalid translation (invalid level 3 PDE)
(gdb) bt
invalid translation (invalid level 3 PDE)
invalid translation (invalid level 3 PDE)
invalid translation (invalid level 3 PDE)
(gdb) bt
invalid translation (invalid level 3 PDE)
invalid translation (invalid level 3 PDE)
(gdb) bt
invalid translation (invalid level 3 PDE)
invalid translation (invalid level 3 PDE)
(gdb) 

>How-To-Repeat:

        Panic and then try to savecore.

>Fix:

        Yes, please!

        Let me know what other diagnostics to run to learn more about
        this...