linux-kernel - Re: [RFC PATCH 1/4] Union mount documentation.

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Message-ID: <20070620132542.GA6169@infradead.org>
Date:	Wed, 20 Jun 2007 14:25:42 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Blunck <jblunck@...e.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/4] Union mount documentation.

On Wed, Jun 20, 2007 at 12:43:56PM +0000, Jan Blunck wrote:
> On Wed, 20 Jun 2007 13:32:23 +0100, Christoph Hellwig wrote:
> 
> > On Wed, Jun 20, 2007 at 07:29:55AM +0000, Jan Blunck wrote:
> >> Mounting a file system twice is bad in the first place. This should be
> >> done by using bind mounts and bind a mounted file system into a union.
> >> After that the normal locking rules apply (and hopefully work ;).
> > 
> > From the kernel POV mounting a filesystem twice is the same as doing
> > a bind mount.
> 
> Somehow I thought about doing this:
> 
>  mount /dev/dasda1 /mnt/A
>  mount /dev/dasda1 /mnt/B
> 
> ... which doesn't result in a bind mount.

But the kernel internal effect is exactly the same.  One superblock instance,
two vfsmounts referring to it. 
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