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

From: Valerie Aurora
Date: Mon Jun 08 2009 - 15:59:24 EST

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.

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