2.6.38.3 leaking disk space of files created via NFS

It seems that 2.6.38.3 is still having pretty severe problems with NFS. Files that were created/written (or only read?) via NFS can be deleted from the server (after unmounting the client), but the disk space is not freed. More precisely it's Gentoo's /usr/portage/distfiles on ext4 and clients save downloaded package tarballs there.

There are no stale .nfsXX files or open file handles; the disk space is returned after a reboot of the server, which is obviously not really an option. I do get a message about deleted orphan inodes on reboot.

Is this a known problem? Sorry if this is a bit vague but I can't constantly reboot the server for experiments. Anyone else who can observe this problem?

thanks,
Holger