Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756324AbZJATen (ORCPT ); Thu, 1 Oct 2009 15:34:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755554AbZJATem (ORCPT ); Thu, 1 Oct 2009 15:34:42 -0400 Received: from [76.245.85.235] ([76.245.85.235]:58143 "EHLO cynthia.pants.nu" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1755546AbZJATel (ORCPT ); Thu, 1 Oct 2009 15:34:41 -0400 X-Greylist: delayed 2653 seconds by postgrey-1.27 at vger.kernel.org; Thu, 01 Oct 2009 15:34:41 EDT Date: Thu, 1 Oct 2009 11:49:35 -0700 From: Brad Boyer To: Valerie Aurora Cc: Jan Blunck , Alexander Viro , Christoph Hellwig , Andy Whitcroft , Scott James Remnant , Sandu Popa Marius , Jan Rekorajski , "J. R. Okajima" , Arnd Bergmann , Vladimir Dronnikov , Felix Fietkau , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] Union mounts/writable overlays design Message-ID: <20091001184935.GA27667@cynthia.pants.nu> References: <20091001145547.GA29152@shell> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20091001145547.GA29152@shell> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1375 Lines: 30 On Thu, Oct 01, 2009 at 10:55:48AM -0400, Valerie Aurora wrote: > We need to guarantee that a file system will be read-only for as long > as it is the bottom layer of a writable overlay. To do this, we track > the number of "read-only users" of a file system in its VFS superblock > structure. When we mount a writable overlay over a file system, we > increment its read-only user count. The file system can only be > mounted read-write if its read-only users count is zero. > > Todo: > > - Support really really read-only NFS mounts. See discussion here: > > http://markmail.org/message/3mkgnvo4pswxd7lp Is there any way for a file system driver to just come out and say "I can't guarantee that this mount is really read-only"? I can imagine this might be an issue for things other than NFS. I think it would be worthwhile to have a flag maybe on a per sb level that says that even if it is mounted with the "ro" option that it might not really be stable. I don't think this is essential, but it would be a good feature as long as it doesn't damage the design or performance too much. Brad Boyer flar@allandria.com -- 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/