linux-kernel - Re: [RFC][PATCH 10/14] In-kernel file copy between union mounted filesystems

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Message-ID: <20070523104122.GF3338@in.ibm.com>
Date:	Wed, 23 May 2007 16:11:22 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Shaya Potter <spotter@...columbia.edu>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Jan Blunck <j.blunck@...harburg.de>
Subject: Re: [RFC][PATCH 10/14] In-kernel file copy between union mounted filesystems

On Tue, May 22, 2007 at 08:35:17AM -0400, Shaya Potter wrote:
> Bharata B Rao wrote:
> >
> >In case of regular files, when we copyup a file, we are actually preventing
> >any writes to the lower layers (which we have designated as read only).
> >
> >Applying the same logic to devices, what do we achieve by copying them up ?
> >How does it matter if we write to the device through a node in the upper
> >layer or in the lower layer. Both the writes eventually do the same thing.
> 
> What happens if the lower layer is on a read only medium.  But the top 
> layer is RW.  Why can't one change permissions?  In your model, one can't.
> 
> What happens if one wants to share a lower layer read-only (I'm doing 
> this with my research into uses of union file systems), one doesn't want 
> permission change in one use of the lower layer to affect any of the 
> other uses.

Ok, makes sense. Thanks for the clarification.

So looks like in addition to copyup on open (which is what we do currently)
there is a case for doing copyups for other situations also.

Regards,
Bharata.
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