powerpc64/pmap.c trouble report

The openbsd-ppc64-n2vi Go builder machine is converting over to LUCI
build infrastructure and the new workload may have stepped on a
pagedaemon corner case. While running 7.5-stable I reproducibly get
kernel panics "pmap_enter: failed to allocate pted". I saw recent
powerpc64/pmap.c changes from gkoehler@ and kettenis@, so updated the
machine to 7.5-snapshot and now see "trap type 300" from pmap_remove.
In an effort to reproduce this with a more familiar workload, I tried
"/usr/src$ make -j32 build" to pound on the hardware with a similar
load average and temperature, but that runs without crashing. I'd
welcome suggestions on anything I can do to reduce this to a useful
bug report.

https://n2vi.com/t.dmesg   latest dmesg
https://n2vi.com/t.crash1   ddb serial console from the 7.5-stable panics
https://n2vi.com/t.crash2   ddb serial console from the 7.5-snapshot trap

Joel has an ssh login on this machine; if power64/pmap.c developers
need that as well, I'm happy to accommodate.

p.s. Theo, thanks for adding em(4) to the default build; it makes all
this much easier.