nullfs with double mount is broken in FreeBSD 10 Stable

[Andreas Longwitz]

The described problem does not exist in V8, but in V10 Stable r317936.

I use a device /dev/md2 defined in /etc/rc.conf:

mdconfig_md2="-t swap -s 384m"
mdconfig_md2_newfs="-n"
mdconfig_md2_owner="root:wheel"
mdconfig_md2_perms="750"

In /etc/fstab I have:

 /dev/md2   /tmp1       ufs     rw,async,noatime,noauto 0  0
 /tmp1      /home/tmp1  nullfs  rw,noauto               0  0
 /tmp1      /var/tmp1   nullfs  rw,noauto               0  0

Now I run:

mount /home/tmp1
mount /var/tmp1
while true; do
   rm -f /home/tmp1/*
   cpdup /boot/kernel /home/tmp1
   cp /var/tmp1/kernel /dev/null
   df -h /tmp1
done

The output is

Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/md2      372M     88M    254M    26%    /tmp1
Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/md2      372M    100M    242M    29%    /tmp1
Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/md2      372M    112M    229M    33%    /tmp1
Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/md2      372M    125M    217M    36%    /tmp1
.....

The "Used" space grows quickly until "No space available".
After umount /var/tmp1 the space is back again.

I see the same behaviour when md is replaced by tmpfs.

I have tried the patch from commit 317936 (V11) without success.
The problem exists on i386 and amd64. In production (V8) I use this
double mount construction for jail communication without any problems.

-- 
Andreas Longwitz