From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bharata B Rao Subject: Re: [RFC][PATCH 10/14] In-kernel file copy between union mounted filesystems Date: Wed, 23 May 2007 16:11:22 +0530 Message-ID: <20070523104122.GF3338@in.ibm.com> References: <20070514093722.GB4139@in.ibm.com> <20070514094329.GL4139@in.ibm.com> <20070518111042.GC4869@in.ibm.com> <464DAE73.7000302@cs.columbia.edu> <20070522031327.GA4728@in.ibm.com> <20070522083826.GD4728@in.ibm.com> <4652E385.30803@cs.columbia.edu> Reply-To: bharata@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Engelhardt , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Jan Blunck To: Shaya Potter Return-path: Received: from e32.co.us.ibm.com ([32.97.110.150]:54038 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758263AbXEWKeJ (ORCPT ); Wed, 23 May 2007 06:34:09 -0400 Content-Disposition: inline In-Reply-To: <4652E385.30803@cs.columbia.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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.