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

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Date:	Tue, 9 Jun 2009 00:44:06 +0200
From:	Jan Rekorajski <baggins@...h.mimuw.edu.pl>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	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 Mon, 08 Jun 2009, Valerie Aurora wrote:

> 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.

As I don't need rw NFS, i didn't even try that :)

> > 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.

Thanks for clarification.

> 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.

I did just that, --union didn't change standard NFS behaviour.

mount -t nfs server:/export/system /mnt
mount --union -t nfs server:/export/profile /mnt
mount.nfs: /mnt is busy or already mounted

I did an experiment by using different IP of the server (same machine)
when mounting the second fs, mount worked then, but 'ls -1 /mnt' oopsed.
I can reproduce this and send you the oops next week.

-- 
Jan Rekorajski            |  ALL SUSPECTS ARE GUILTY. PERIOD!
baggins<at>mimuw.edu.pl   |  OTHERWISE THEY WOULDN'T BE SUSPECTS, WOULD THEY?
BOFH, MANIAC              |                   -- TROOPS by Kevin Rubio
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