From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E9509ECAAA1 for ; Tue, 30 Aug 2022 11:21:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229752AbiH3LVB (ORCPT ); Tue, 30 Aug 2022 07:21:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43830 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229550AbiH3LVA (ORCPT ); Tue, 30 Aug 2022 07:21:00 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 78C5B642C7; Tue, 30 Aug 2022 04:20:57 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id A4735B816D8; Tue, 30 Aug 2022 11:20:55 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 39F55C433C1; Tue, 30 Aug 2022 11:20:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1661858454; bh=alCF5GpH5u+gpvCxHLRbmwtthtKRe9pS/J4xxwf7TlA=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=a7j+WOMxJ9NMH5hK1XagXQm5MHEZxrPMD+WSnBfFpTC3FVBQxNASK0R5/e3ZgYkTB RFnZLpxr+y1wsO6XVnF48yLtobWjttaRB/tDYfX8vUAvWp9kSRAxOd02h0qQ4myPys FJjFtGXofZyh60kfbMHBtmLl9jzMrURzy5I5ZefnmoLjaQqVr9gLDfMTViAHVrYYHe ixNouIUFv2Va5wVzA2qOdclJmWu66YI/1y0ZIxWWN/2MH7eTacr4XOkqA9hc48vB/Z N+CV8GXDJoVKxdF5RsbVQNIYvtmbeMxu2lHS/WrnRHpxT08PEeYW1j93IQSQ2mSrdG DEQb0qnOPHiiQ== Message-ID: Subject: Re: [PATCH v3 4/7] xfs: don't bump the i_version on an atime update in xfs_vn_update_time From: Jeff Layton To: Dave Chinner Cc: Amir Goldstein , Trond Myklebust , "djwong@kernel.org" , "zohar@linux.ibm.com" , "brauner@kernel.org" , "xiubli@redhat.com" , "neilb@suse.de" , "linux-api@vger.kernel.org" , "linux-xfs@vger.kernel.org" , "dwysocha@redhat.com" , "linux-kernel@vger.kernel.org" , "chuck.lever@oracle.com" , "linux-nfs@vger.kernel.org" , "tytso@mit.edu" , "viro@zeniv.linux.org.uk" , "jack@suse.cz" , "linux-ext4@vger.kernel.org" , "linux-btrfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "lczerner@redhat.com" , "adilger.kernel@dilger.ca" , "ceph-devel@vger.kernel.org" Date: Tue, 30 Aug 2022 07:20:50 -0400 In-Reply-To: <20220830000851.GV3600936@dread.disaster.area> References: <35d31d0a5c6c9a20c58f55ef62355ff39a3f18c6.camel@kernel.org> <079df2134120f847e8237675a8cc227d6354a153.camel@hammerspace.com> <732164ffb95468992035a6f597dc26e3ce39316d.camel@kernel.org> <20220829054848.GR3600936@dread.disaster.area> <8510ff07fdba7dd4c59a14e2f202ff38b83a9ef1.camel@kernel.org> <20220830000851.GV3600936@dread.disaster.area> Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.4 (3.44.4-1.fc36) MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Tue, 2022-08-30 at 10:08 +1000, Dave Chinner wrote: > On Mon, Aug 29, 2022 at 06:33:48AM -0400, Jeff Layton wrote: > > On Mon, 2022-08-29 at 15:48 +1000, Dave Chinner wrote: > > > >=20 > > > > The race window ought to be relatively small, and this wouldn't res= ult > > > > in incorrect behavior that you'd notice (other than loss of > > > > performance), but it's not ideal. We're doing more on-the-wire read= s > > > > than are necessary in this case. > > > >=20 > > > > It would be nice to have it not do that. If we end up taking this p= atch > > > > to make it elide the i_version bumps on atime updates, we may be ab= le to > > > > set the the NOIVER flag in other cases as well, and avoid some of t= hese > > > > extra bumps. > > >=20 > > >=20 > > > > > >=20 > > > Please don't make me repeat myself for the third time. > > >=20 > > > Once we have decided on a solid, unchanging definition for the > > > *statx user API variable*, we'll implement a new on-disk field that > > > provides this information. We will document it in the on-disk > > > specification as "this is how di_iversion behaves" so that it is > > > clear to everyone parsing the on-disk format or writing their own > > > XFS driver how to implement it and when to expect it to > > > change. > > >=20 > > > Then we can add a filesystem and inode feature flags that say "inode > > > has new iversion" and we use that to populate the kernel iversion > > > instead of di_changecount. We keep di_changecount exactly the way it > > > is now for the applications and use cases we already have for that > > > specific behaviour. If the kernel and/or filesystem don't support > > > the new di_iversion field, then we'll use di_changecount as it > > > currently exists for the kernel iversion code. > > >=20 > >=20 > > Aside from NFS and IMA, what applications are dependent on the current > > definition and how do they rely on i_version today? >=20 > I've answered this multiple times already: the di_changecount > behaviour is defined in the on-disk specification and hence we > *cannot change the behaviour* without changing the on-disk format > specification. >=20 > Apart from the forensics aspect of the change counter (which nobody > but us XFS developers seem to understand just how damn important > this is), there are *many* third party applications that parse the > XFS on-disk format directly. This: >=20 > https://codesearch.debian.net/search?q=3DXFS_SB_VERSION_DIRV2&literal=3D1 >=20 > Shows grub2, libparted, syslinux, partclone and fsarchiver as > knowing about XFS on-disk superblock flags that tell them what > format the directory structure is in. That alone is enough to > indicate they parse on-disk inodes directly, and hence may expect > di_changecount to have specific meaning and use it to detect > unexpected changes to files/directories they care about. >=20 > If I go looking for XFS_SB_MAGIC, I find things like libblkid, > klibc, qemu, Xen, testdisk, gpart, and virtualbox all parse the > on-disk superblocks directly from the block device, too. They also > rely directly on XFS developers ensuring there are no silent > incomaptible changes to the on disk format. >=20 > I also know of many other utilities that people and companies have > written that parse the on disk format directly from userspace. The > functions these perform include low level storage management tools, > copying and managing disk images (e.g. offline configuration for > cluster deployments), data recovery tools that scrape all the data > out of broken filesystems, etc. >=20 > These applications are reliant on the guarantee we provide that the > on-disk format will not silently change and that behaviour/structure > can always easily be discovered by feature flags in the superblock > and/or inodes. >=20 > IOWs, just because there aren't obvious "traditional" application on > top of the kernel filesystem that consumes the in-memory kernel > iversion field, it does not mean that the defined behaviour of the > on-disk di_changecount field is not used or relied on by other tools > that work directly on the on-disk format. >=20 > You might be right that NFS doesn't care about this, but the point > remains that NFS does not control the XFS on-disk format, nor does > the fact that what NFS wants from the change attribute has changed > over time override the fact that maintaining XFS on-disk format > compatibility is the responsibility of XFS developers. We're willing > to change the on-disk format to support whatever the new definition > of the statx change attribute ends up being, and that should be the > end of the discussion. >=20 Thanks for spelling this out in more detail. > > > Keep in mind that we've been doing dynamic inode format updates in > > > XFS for a couple of decades - users don't even have to be aware that > > > they need to perform format upgrades because often they just happen > > > whenever an inode is accessed. IOWs, just because we have to change > > > the on-disk format to support this new iversion definition, it > > > doesn't mean users have to reformat filesystems before the new > > > feature can be used. > > >=20 > > > Hence, over time, as distros update kernels, the XFS iversion > > > behaviour will change automagically as we update inodes in existing > > > filesystems as they are accessed to add and then use the new > > > di_iversion field for the VFS change attribute field instead of the > > > di_changecount field... > > >=20 > >=20 > > If you want to create a whole new on-disk field for this, then that's > > your prerogative, but before you do that, I'd like to better understand > > why and how the constraints on this field changed. > >=20 > > The original log message from the commit that added a change counter > > (below) stated that you were adding it for network filesystems like NFS= . > > When did this change and why? >=20 > It never changed. I'll repeat what I've already explained twice > before: >=20 > https://lore.kernel.org/linux-xfs/20220818030048.GE3600936@dread.disaster= .area/ > https://lore.kernel.org/linux-xfs/20220818033731.GF3600936@dread.disaster= .area/ >=20 > tl; dr: NFS requirements were just one of *many* we had at the time > for an atomic persistent change counter. >=20 > The fact is that NFS users are just going to have to put up with > random cache invalidations on XFS for a while longer. Nobody noticed > this and/or cared about this enough to raise it as an issue for the > past decade, so waiting another few months for upstream XFS to > change to a different on-disk format for the NFS/statx change > attribute isn't a big deal. >=20 Fair enough. I'll plan to drop this patch from the series for now, with the expectation that you guys will add a new i_version counter that better conforms to what NFS and IMA need. Thanks, --=20 Jeff Layton