linux-kernel - Re: [PATCH 5/5] union: hybrid union filesystem prototype

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Message-Id: <E1Or5xN-0001uq-Ob@pomaz-ex.szeredi.hu>
Date:	Thu, 02 Sep 2010 11:19:41 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, neilb@...e.de,
	viro@...iv.linux.org.uk, jblunck@...e.de, hch@...radead.org
Subject: Re: [PATCH 5/5] union: hybrid union filesystem prototype

On Wed, 1 Sep 2010, Valerie Aurora wrote:
> > +
> > +		err = vfs_create(upperdir, newdentry, attr->ia_mode, NULL);
> 
> Passing a NULL namiedata pointer to vfs_create() is a convenient
> temporary hack, but unfortunately NFS, ceph, etc. still use the
> nameidata passed to vfs_create() and other ops.
> 
> The way union mounts gets a valid nameidata is by doing the create in
> the VFS before calling file system ops that may trigger a copyup,
> while we still have the original nameidata.  This is one of the major
> reasons union mounts lives in the VFS.

Not a big deal, just set up nd as if this was a single component
lookup.  The previous version did it like this:

+       struct nameidata nd = {
+               .last_type = LAST_NORM,
+               .last = *name,
+       };
+
+       nd.path = pue->upperpath;
+       path_get(&nd.path);
+
+       newdentry = lookup_create(&nd, S_ISDIR(attr->ia_mode));

But that's not a solution to the NFS suckage, it's just a workaround.

"Fortunately" NFS isn't good for a writable layer of a union for other
reasons, so this isn't a big concern at the moment.

Thanks,
Miklos
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