why there is no "s_export_op" in nfs_super_block?

["lioupayphone" <lioupayphone@gmail.com>]

Hi, linux-nfs
	
	i have a question about NFS. 
	say, i have three different machines, Machine-1 is a NFS Server who exported its directory "/home/pub" to Machine-2, which mounted "mount -t nfs Machine-1:/home/pub /mnt/pub/".

	i want Machine-2 to be a NFS Server which exports its directory "/mnt/pub" to Machine-3, permission denied when i execute "mount -t nfs Machine-2:/mnt/pub/ /mnt/" on Machine-3.
	( btw : all the three machines are of linux 2.6.17-1, a little old kernel :-) )

	i browsed the code of 2.6.17-1 and found check_export() in fs/nfsd/exports.c , who told me there are two reasons for that :
	
	/* There are two requirements on a filesystem to be exportable.
	 * 1:  We must be able to identify the filesystem from a number.
	 *       either a device number (so FS_REQUIRES_DEV needed)
	 *       or an FSID number (so NFSEXP_FSID needed).
	 * 2:  We must be able to find an inode from a filehandle.
	 *       This means that s_export_op must be set.
	 */

	i think if i set "/home/pub Machin-2(sync,root_squash,fsid=1)" and "/mnt/pub Machine-2(sync,root_squash,fsid=2)" individually in "/etc/exports" of Machine-1 and Machine-2, the requirement-1 will be meet.
	but nfs_fill_super() failed to register its s_export_op. why? i am puzzled. thank you for your suggestions and explanations. :-)

 				
Best Regards!
lioupayphone