linux-kernel - Re: [PATCH 00/32] VFS based Union Mount (V3)

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Message-ID: <20090608195752.GD4363@fsbox>
Date:	Mon, 8 Jun 2009 12:57:52 -0700
From:	Valerie Aurora <vaurora@...hat.com>
To:	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	Jan Blunck <jblunck@...e.de>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	bharata@...ibm.com, dwmw2@...radead.org, mszeredi@...e.cz
Subject: Re: [PATCH 00/32] VFS based Union Mount (V3)

On Thu, May 21, 2009 at 02:54:19PM +0200, Jan Rekorajski wrote:
> On Mon, 18 May 2009, Jan Blunck wrote:
> 
> > Here is another post of the VFS based union mount implementation.
> 
> Is there any chance this will support NFS? I can union-mount tmpfs over

NFS as the read-only layer ought to work.  NFS as the read-write layer
is still up in the air.

> nfs mounted fs, but if I try to mount --union two NFS filesystems I
> always get -EBUSY on second mount on the same mountpoint.
> 
> Something along these lines:
> 
> doesn't matter if I use --union on first mount, the result is always the
> same.
> 
> mount <--union> -t nfs server:/export/system /mnt
> OK
> mount --union -t nfs server:/export/profile /mnt
> mount.nfs: /mnt is busy or already mounted
> 
> I patched mount.nfs so it knows about MS_UNION, and strace shows me that
> it passes that flag to kernel.

FYI, using --union on the first mount will make it union with the
local directory below it.  The --union option is not needed when you
mount the lower read-only layer.

You'll get -EBUSY on the second mount of any NFS file system over
another - try it again with the --union flag.  Support for NFS on NFS
union mount would have to change this.

-VAL
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