Re: [PATCH] UBIFS: fill f_fsid

[David Woodhouse <dwmw2@infradead.org>]

On Tue, 2008-09-02 at 17:48 -0400, Christoph Hellwig wrote:
> On Tue, Sep 02, 2008 at 10:26:09PM +0100, David Woodhouse wrote:
> > > We also called the filesystem part of the NFS filehandle in a few
> > > places, and for those it's correct that it should be stable.  Currently
> > > the fsid is either created from the dev_t in kernelspace or from
> > > uuids extracted through libuuid in userspace.
> > 
> > Or from the f_fsid returned in statfs(), as of a week or so ago.
> 
> I think that's a rather bad idea.  As mentioned before f_fsid is
> basically random.

It's not that bad. Of the file systems which actually fill it in...

For NTFS it's the volume serial number.

For ext[234] it's the UUID xor-folded into 64 bits.
For btrfs the same, but with the root object ID xor'd in too.

For bfs and xfs it's the block device -- which isn't ideal, as Andreas
points out, but is what we'd fall back to anyway.

BFS seems to have a s_volume field which perhaps we could use, but we
should check for it being all spaces and leave f_fsid as zero in that
case. Presumably XFS could be using sb_uuid?

For efs, it'll be either EFS_MAGIC or EFS_NEWMAGIC, which is probably a
bad thing if you want to export multiple EFS file systems. I'll send a
patch to fix that (by leaving it zero) shortly.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation