Unmounting by filesystem ID

[Kris Kennaway]

On Sat, Jun 28, 2003 at 10:54:04PM +0100, Ian Dowse wrote:
> 
> The patch below adds a new mount flag MNT_BYFSID that can be used
> to unmount a filesystem by specifying its filesystem ID instead of
> a path. The umount utility is changed to use this mechanism by
> default. This approach has a number of advantages:
> 
> - It avoids any lookup operations that could potentially block
>   forever, so filesystems such as NFS can be reliably unmounted
>   even if the server is not responding but looking up the root node
>   would require an RPC (maybe to an underlying filesystem).
> - The filesystem specification is unambiguous, so umount(8) can
>   be sure that it is unmounting the correct filesystem (more
>   work in umount(8) may be required here).
> - Detached filesystems can be unmounted. If a filesystem becomes
>   detached from the filesystem hierarchy because the underlying
>   filesystem got unmounted, it does not require a reboot to unmount
>   it.
> 
> Since unmounting by a path name is now only required for compatibility,
> in that case unmount() now just does a string comparison to find
> the correct filesystem. Also, this patch only affects unmounting;
> a similar approach could be applied to MNT_UPDATE mount operations.
> 
> I would like to commit this during the next few days. Any comments
> or suggestions?

The approach sounds great to me; these are long-standing annoyances in
FreeBSD.

Kris
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