On Thu, May 13, 2010 at 11:53:33AM +0530, Aneesh Kumar K. V wrote:
> On Thu, 13 May 2010 10:20:38 +1000, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > On Wed, May 12, 2010 at 03:49:49PM -0600, Andreas Dilger wrote:
> > > On 2010-05-12, at 09:50, Aneesh Kumar K.V wrote:
> > > > +static long do_sys_name_to_handle(struct path *path,
> > > > + struct file_handle __user *ufh)
> > > > +{
> > > > + if (handle_size <= f_handle.handle_size) {
> > > > + /* get the uuid */
> > > > + retval = sb->s_op->get_fsid(sb, &this_fs_id);
> > > > + if (!retval) {
> > > > + /*
> > > > + * Now verify whether we get the same vfsmount
> > > > + * if we lookup with uuid. In case we end up having
> > > > + * same uuid for the multiple file systems. When doing
> > > > + * uuid based lookup we would return the first one.So
> > > > + * with name_to_handle if we don't find the same
> > > > + * vfsmount with lookup return EOPNOTSUPP
> > > > + */
> > > > + mnt = fs_get_vfsmount(current, &this_fs_id);
> > > > + if (mnt != path->mnt) {
> > > > + retval = -EOPNOTSUPP;
> > > > + mntput(mnt);
> > > > + goto err_free_out;
> > > > + }
> > >
> > > I don't see that this does anything for us except add overhead.
> > > This is no protection against mounting a second filesystem with
> > > the same UUID after the handle is returned, since there is no
> > > expiration for file handles.
> > >
> > > At best I think we could start by changing the list-based UUID
> > > lookup with a hash-based one, and when adding a duplicate UUID at
> > > mount time start by printing out an error message to the console
> > > in case of duplicated UUIDs, and maybe at some point in the future
> > > this might cause the mount to fail (though I don't think we can
> > > make that decision lightly or quickly).
> > >
> > > That moves the overhead to mount time instead of for each
> > > name_to_handle() call (which would be brutal for a system with
> > > many filesystems mounted).
> >
> > That will pretty much match exactly what XFS already does. Can we
> > start by moving the XFS functionality (xfs_uuid_mount(), "nouuid"
> > mount option, etc) to the VFS level and then optimise from there?
> >
>
> I will do this. But should the uuid be unique in a system wide manner or
> should it be unique for a mount namespace ? With containers isn't it
> valid for the second container to mount a file system with same uuid of
> a file system in the first container, but uuid itself is unique in the
> second container ?
I don't know how containers and mount namespaces interact, so I
can't really comment with any authority. However, two different
filesystems with the same UUID means that someone or something
doesn't understand what unique means and that, I think, makes the
container issue moot.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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