From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Whitcroft Subject: Re: How to cope with two incompatible overlayfs formats out in the wild Date: Tue, 18 Nov 2014 17:07:18 +0000 Message-ID: <20141118170718.GH8154@bark> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-wg0-f46.google.com ([74.125.82.46]:42779 "EHLO mail-wg0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754497AbaKRRGz (ORCPT ); Tue, 18 Nov 2014 12:06:55 -0500 Received: by mail-wg0-f46.google.com with SMTP id x12so403282wgg.5 for ; Tue, 18 Nov 2014 09:06:54 -0800 (PST) Content-Disposition: inline In-Reply-To: Sender: linux-unionfs-owner@vger.kernel.org List-Id: linux-unionfs@vger.kernel.org To: Miklos Szeredi Cc: Serge Hallyn , Neil Brown , linux-unionfs@vger.kernel.org, Kernel Mailing List , Linus Torvalds , Al Viro On Tue, Nov 18, 2014 at 03:28:03PM +0100, Miklos Szeredi wrote: > [CC-ing mailing lists, Al and Linus for wider exposure] > > This issue is this: Ubuntu and SUSE carry an "old" format of overlayfs > while mainline has a "new" format. The differences are: > > - whiteouts are represented differently (symlink + xattr in the old > format, chardev in the new) > > - new one needs a "workdir" mount option, which points to a directory > on the same filesystem as upperdir. If upperdir was the root of the > filesystem then it needs to be moved into a subdir to make space for > the work directory. > > Migrating from old to new is not a big issue, but Ubuntu people have > expressed concerns about systems with mixed kernel versions and want > to support the old format alongside the new. > > This can all be done with out-of-tree code. > > So from mainline we need two things: > > - when mounting distinguish between old and new format. > > - userspace can detect which formats are supported by the kernel. > > If we'd have a different filesystem type for the old and new formats, > then that would solve both (checking /proc/filesystems would indicate > which one is supported). > > Unfortunately that would mean having to change "overlayfs" type to > something else in 3.18. Question is, is there some sane name which > would fit? "overlayfs2" is perhaps the best, but I'm not overly > enthusiastic about it. > > Any other ideas? ext4 makes use of feature flags in /sys/fs/ext4/features. Perhaps we could make use of this, say /sys/fs/overlayfs/features/{workdir,whiteout-chrdev}, or a even some kind of version in /sys/fs/overlayfs/version. The presence of /sys/fs/overlayfs itself might be enought to assume the presence of support for the new format. -apw