disk space not freed when deleting files in null_mounted filesystem

[richard bader]

Hello,

i've a filesystem mounted with mount_nullfs.
When i delete a large file in this filesystem, the space on the disk is 
not freed.
The space becomes freed when i unmount the filesystem.


to reproduce this:

root at main2:/tmp # mkdir lnk src
root at main2:/tmp # mount_nullfs src lnk
root at main2:/tmp # mount | grep lnk
/tmp/src on /tmp/lnk (nullfs, local)
root at main2:/tmp # df -h .
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ada0p2     58G     40G     13G    75%    /
root at main2:/tmp # dd if=/dev/zero of=lnk/foo bs=1G count=3
3+0 records in
3+0 records out
3221225472 bytes transferred in 7.856793 secs (409992411 bytes/sec)
root at main2:/tmp # df -h .
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ada0p2     58G     43G     10G    80%    /
root at main2:/tmp # rm lnk/foo
root at main2:/tmp # df -h .
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ada0p2     58G     43G     10G    80%    /
root at main2:/tmp # umount /tmp/lnk
root at main2:/tmp # df -h .
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ada0p2     58G     40G     13G    75%    /


my system:

root at main2:/tmp # uname -a
FreeBSD main2.bader.loc 10.2-RELEASE-p18 FreeBSD 10.2-RELEASE-p18 #8 
r301112: Wed Jun  1 11:08:24 CEST 2016 
root at main2.bader.loc:/usr/obj/usr/src/sys/MYKERNEL  amd64

are the any options for mount_nullfs ?

best regards

richard