linux-kernel - Re: [PATCH -V7 3/9] vfs: Add name to file handle conversion support

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Message-ID: <20100513073136.GH13617@dastard>
Date:	Thu, 13 May 2010 17:31:36 +1000
From:	Dave Chinner <david@...morbit.com>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Andreas Dilger <andreas.dilger@...cle.com>, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....COM, corbet@....net,
	serue@...ibm.com, neilb@...e.de, linux-fsdevel@...r.kernel.org,
	sfrench@...ibm.com, philippe.deniel@....FR,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V7 3/9] vfs: Add name to file handle conversion support

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@...morbit.com> 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@...morbit.com
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