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

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Message-ID: <20100513002038.GE13617@dastard>
Date:	Thu, 13 May 2010 10:20:38 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andreas Dilger <andreas.dilger@...cle.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.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 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?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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