> > When I try to unmount /flam/stuffy I get:
> > 
> > # umount -fv /flam/stuffy
> > /stuffy: unmount from /flam/stuffy
> > umount: /flam/stuffy: Invalid argument
> > 
> > # mount -uv /flam/stuffy 
> > exec: mount_nfs -o update flam:/stuffy /flam/stuffy
> > mount_nfs: flam:/stuffy on /flam/stuffy: Invalid argument
> > 
> > Despite the fact that /flam/stuffy appears empty I have
> > open session of xpdf reading book residing on /flam/stuffy,
> > and I still can read it.
> 
> Did  this filesystem get changed in some way on the server ? It looks like
> the filehandle the client has for the mount point isn't valid any more,

filesystem /stuffy was mounted read-only, should be no changes.

> which can happen if e.g. the directory was renamed, or moved.

probably it is the point.  Server was rebooted because of system update,
and my procedure for system updating includes executing some scripts.
Directory /stuffy was removed and then recreated again.

So, the reason why /flam and /flam/volatile remain mounted and
/flam/stuffy doesn't is clear.

But anyway it seems not very beatiful that client can not umount
this filesystem.

I tried just mount it again instead of umount or mount -u:

# mount /flam/stuffy

It works, but now df lists two filesystems on /flam/stuffy:

flam:/            290799    171644    104616    62%    /flam
flam:/volatile  19689614   8750118   9955016    46%    /flam/volatile
flam:/stuffy   111928190  51431130  54900652    48%    /flam/stuffy
flam:/stuffy   111928190  51431130  54900652    48%    /flam/stuffy