port-evbarm/55639: Assertion "anon != NULL && anon->an_ref != 0" fails on evbarm-earmv7hf

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55639
>Category:       port-evbarm
>Synopsis:       Assertion "anon != NULL && anon->an_ref != 0" fails on evbarm-earmv7hf
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-evbarm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Sep 03 06:55:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.08.14.09.06.15
>Organization:
>Environment:
System: NetBSD
Architecture: arm
Machine: evbarm
>Description:

On the TNF evbarm-earmv7hf testbed, most test runs have failed to
complete since the middle of August:

  http://releng.netbsd.org/b5reports/evbarm-earmv7hf/commits-2020.08.html#2020.08.14.09.06.15

The system panics with the message:

  panic: kernel diagnostic assertion "uvm_pagelookup(uobj, offset) == NULL || ((a->ar_flags & UVM_PAGE_ARRAY_FILL_DIRTY) != 0 && !uvm_obj_page_dirty_p(pg))" failed: file "/tmp/bracket/build/2020.08.14.09.06.15-evbarm-earmv7hf/src/sys/uvm/uvm_vnode.c", line 321

The first failure happened after this commit:

  2020.08.14.09.06.14 chs src/sys/miscfs/genfs/genfs_io.c 1.100
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_extern.h 1.231
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_object.c 1.24
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_object.h 1.39
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_page.c 1.245
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_page_status.c 1.6
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_pager.c 1.129
  2020.08.14.09.06.15 chs src/sys/uvm/uvm_vnode.c 1.116

Since there have been some random successful runs, it is not 100%
certain that this commit is the cause, but given that both the commit
and the panic are uvm related, it does seems likely.

>How-To-Repeat:

>Fix: