kern/51447: nullfs seems to not delete files properly

[coypu%sdf.org@localhost]

>Number:         51447
>Category:       kern
>Synopsis:       nullfs seems to not delete files properly
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 27 11:30:02 +0000 2016
>Originator:     coypu
>Release:        NetBSD-7.99.36
>Organization:
>Environment:
NetBSD  7.99.36 NetBSD 7.99.36 (GENERIC.201608232230Z) amd64
>Description:
My system:
/ is FFSv2 (no special mount options)
/usr/pkgsrc is mounted as nullfs at /netbsd6/usr/pkgsrc.

I ran:
# rm -rf /usr/pkgsrc/*/*/work

It appears some of my work directories were not deleted properly:
# cd /usr/pkgsrc/x11/qt5-qttools
# mkdir work
mkdir: work: No such file or directory

Running fsck -f /dev/rwd0a I see, among others:

UNALLOCATED  I=8075587  OWNER=0 MODE=0
SIZE=0 MTIME=Jan  1 00:00 1970  
NAME=/usr/pkgsrc/x11/qt5-qttools/work

REMOVE? [yn] 

I hae also seen errors of link count wrong and unref files.
finally there was (maybe because it was fsck on a live system):

FREE BLK COUNT(S) WRONG IN SUPERBLK
SALVAGE? yes

SUMMARY INFORMATION BAD
SALVAGE? yes

BLK(S) MISSING IN BIT MAPS
SALVAGE? yes
>How-To-Repeat:
Most likely, it can be repeated by running any heavy filesystem load
in a similar setup. after it, fsck should be clean, but it is not.
>Fix: