linux-kernel - Re: [PATCH 21/38] union-mount: Support for mounting union mount file systems

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Date:	Wed, 4 Aug 2010 17:59:43 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Ian Kent <raven@...maw.net>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	Jan Blunck <jblunck@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 21/38] union-mount: Support for mounting union mount file systems

On Tue, Jul 20, 2010 at 11:12:17AM +0800, Ian Kent wrote:
> On Fri, 2010-07-16 at 17:02 -0400, Valerie Aurora wrote:
> > On Tue, Jul 13, 2010 at 12:47:02PM +0800, Ian Kent wrote:
> > > On Tue, Jun 15, 2010 at 11:39:51AM -0700, Valerie Aurora wrote:
> > > > +
> > > > +static int
> > > > +check_mnt_union(struct path *mntpnt, struct vfsmount *topmost_mnt, int mnt_flags)
> > > > +{
> > > > +	struct vfsmount *lower_mnt = mntpnt->mnt;
> > > > +
> > > > +	if (!(mnt_flags & MNT_UNION))
> > > > +		return 0;
> > > > +
> > > > +#ifndef CONFIG_UNION_MOUNT
> > > > +	return -EINVAL;
> > > > +#endif
> > > > +	if (!(lower_mnt->mnt_sb->s_flags & MS_RDONLY))
> > > > +		return -EBUSY;
> > > > +
> > > > +	if (!list_empty(&lower_mnt->mnt_mounts))
> > > > +		return -EBUSY;
> > > > +
> > > > +	if (!IS_ROOT(mntpnt->dentry))
> > > > +		return -EINVAL;
> > > > +
> > > > +	if (mnt_flags & MNT_READONLY)
> > > > +		return -EROFS;
> > > > +
> > > > +	if (!(topmost_mnt->mnt_sb->s_flags & MS_WHITEOUT))
> > > > +		return -EINVAL;
> > > > +
> > > 
> > > Is there a need to check fallthru, umm ... that probably doesn't
> > > apply for the ROOT(), right?
> > 
> > Actually, that's on my todo list - right now I'm assuming MS_WHITEOUT
> > implies fallthru support as well.  But it doesn't.
> > 
> > We're a little short on MS_* flags.  I'm thinking of just checking
> > ->whiteout and ->fallthru for non-NULL on the root dir and getting rid
> > of MS_WHITEOUT entirely.  Thoughts?
> 
> Checking for the methods is a good idea I think, since they are assumed
> to be present by the code, at least in some places.
> 
> Although it shouldn't happen, it is possible for a file system to create
> the root dentry with these methods defined but other dentrys without
> them defined, so a file system implementation error could cause some
> unpleasant crashes. Maybe requiring the flags to indicate support would
> help avoid unpleasant implementation problems like this, not sure
> really. 
> 
> Also not sure if a method existence check should always be made prior to
> use, regardless.

I went for MS_WHITEOUT and MS_FALLTHRU, and added the checks for the
ops being non-null.

-VAL
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