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

["Carl Spalletta" <cspalletta@gmail.com>]

neilb@cse.unsw.edu.au 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

So 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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