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How does remote NFS open keep local SB in memory after umount?

neilb@xxxxxxxxxxxxxxx wrote:

 mount the filesystem over nfs
 open a file several directories deep. e.g.
     exec 3< /mnt/cramfs/my/long/path/name/file
 unmount/remount the file system on the server:
      exportfs -avu
      umount /test
      mount /test
      exportfs -av

try to read from the file 

And my question is this: what keeps the superblock of the filesystem
on the server from going away when unmounted (assuming it was the last
instance). I have been unable to locate in the code where any reference
counts pertaining to the superblock are incremented as a result of
opening a file.

Obviously however, something must keep the superblock from going away or
the final read would fail.  Even if by some means the inode was preserved
across mounts the field inode->i_sb would become invalid if the original
SB was destroyed.

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