Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761425AbXEVIgL (ORCPT ); Tue, 22 May 2007 04:36:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756132AbXEVIf4 (ORCPT ); Tue, 22 May 2007 04:35:56 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:51961 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756034AbXEVIfy (ORCPT ); Tue, 22 May 2007 04:35:54 -0400 Date: Tue, 22 May 2007 14:08:26 +0530 From: Bharata B Rao To: Jan Engelhardt Cc: Shaya Potter , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Jan Blunck Subject: Re: [RFC][PATCH 10/14] In-kernel file copy between union mounted filesystems Message-ID: <20070522083826.GD4728@in.ibm.com> Reply-To: bharata@linux.vnet.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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1873 Lines: 45 On Tue, May 22, 2007 at 08:25:16AM +0200, Jan Engelhardt wrote: > > On May 22 2007 08:43, Bharata B Rao wrote: > >On Fri, May 18, 2007 at 09:47:31AM -0400, Shaya Potter wrote: > >> Bharata B Rao wrote: > >> > >> > > >> >Not really. This is called during copyup of a file residing in a lower > >> >layer. And that is done only for regular files. > >> > >> That is broken. > > > >But it only breaks the semantics (in other cases we allow writes only to the > >top layer files). So the question is why do we have to copy up the device > >node ? What difference it makes to writing to the device itself ? > > Because `chmod 666 blockdevnode` is not the same as writing > to the device itself? What if that chmod is applied on the lower level device node ? This is what we do currently, even for regular files. Copyup happens only when the file is opened for writing. Let me rephrase my earlier question: 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 I am trying to understand is, if the need for copyup is purely a matter of conforming to semantics (of not allowing writes to the lower layers in case of union mount) or do we achieve anything else by doing a device copyup ? Are there any cases where copying up of device nodes are absolutely essential for sane behaviour ? Regards, Bharata. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/