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 59E49C54EE9 for ; Thu, 8 Sep 2022 11:35:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231448AbiIHLfI (ORCPT ); Thu, 8 Sep 2022 07:35:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38558 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231379AbiIHLfD (ORCPT ); Thu, 8 Sep 2022 07:35:03 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C16D43120A; Thu, 8 Sep 2022 04:35:00 -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 dfw.source.kernel.org (Postfix) with ESMTPS id 348FE61C99; Thu, 8 Sep 2022 11:34:59 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 853D8C433C1; Thu, 8 Sep 2022 11:34:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1662636898; bh=wNz25kbhUUH7fFXO2OLBZ10iI63lbEA4XVm9xrD79g4=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=YCxwNLWJTuv2/+xkGeyE+DVN7imflpeXqLBjPZ5XuwsY7Ib7GsF1/8O+dgv5McqEW 8PQhNwat2z4BVVYnVgklGLdWYdtVYHLF/JYs8ku++tMGQ5aE7Hwp/EiV1chQj+yyMh B5HGp6XuMb66zmsQgxrv1I5imYBVmOYCDggOoWNBazfcdwE+idGv3lGtMMbP0C/SJF bI3zMtyTUIT0H7k6Xx3BfpyUhkvHNvLUaeUyiM2ySI1UlCzz5Nqn02yB8Bw/mv4STL K2hKF9N84ZY1xNoAE4h/jrBWZ0TI6q15QmXeSQufbp5VIHy0k7mB+xYsRltWX4K1hK Fp8bDMuvjBhxA== Message-ID: Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field From: Jeff Layton To: NeilBrown Cc: Trond Myklebust , "bfields@fieldses.org" , "zohar@linux.ibm.com" , "djwong@kernel.org" , "xiubli@redhat.com" , "brauner@kernel.org" , "linux-api@vger.kernel.org" , "linux-xfs@vger.kernel.org" , "david@fromorbit.com" , "fweimer@redhat.com" , "linux-kernel@vger.kernel.org" , "chuck.lever@oracle.com" , "linux-man@vger.kernel.org" , "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: Thu, 08 Sep 2022 07:34:55 -0400 In-Reply-To: <166259764365.30452.5588074352157110414@noble.neil.brown.name> References: <20220907111606.18831-1-jlayton@kernel.org> , <166255065346.30452.6121947305075322036@noble.neil.brown.name> , <79aaf122743a295ddab9525d9847ac767a3942aa.camel@kernel.org> , <20220907125211.GB17729@fieldses.org> , <771650a814ab1ff4dc5473d679936b747d9b6cf5.camel@kernel.org> , <8a71986b4fb61cd9b4adc8b4250118cbb19eec58.camel@hammerspace.com> , <166259764365.30452.5588074352157110414@noble.neil.brown.name> 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 Thu, 2022-09-08 at 10:40 +1000, NeilBrown wrote: > On Thu, 08 Sep 2022, Jeff Layton wrote: > > On Wed, 2022-09-07 at 13:55 +0000, Trond Myklebust wrote: > > > On Wed, 2022-09-07 at 09:12 -0400, Jeff Layton wrote: > > > > On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote: > > > > > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote: > > > > > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote: > > > > > > > On Wed, 07 Sep 2022, Jeff Layton wrote: > > > > > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic wi= th > > > > > > > > respect to the > > > > > > > > +other changes in the inode. On a write, for instance, the > > > > > > > > i_version it usually > > > > > > > > +incremented before the data is copied into the pagecache. > > > > > > > > Therefore it is > > > > > > > > +possible to see a new i_version value while a read still > > > > > > > > shows the old data. > > > > > > >=20 > > > > > > > Doesn't that make the value useless? > > > > > > >=20 > > > > > >=20 > > > > > > No, I don't think so. It's only really useful for comparing to = an > > > > > > older > > > > > > sample anyway. If you do "statx; read; statx" and the value > > > > > > hasn't > > > > > > changed, then you know that things are stable.=20 > > > > >=20 > > > > > I don't see how that helps.=A0 It's still possible to get: > > > > >=20 > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0reader=A0=A0=A0= =A0=A0=A0=A0=A0=A0=A0writer > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0------=A0=A0=A0= =A0=A0=A0=A0=A0=A0=A0------ > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0= =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0i_version++ > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0statx > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0read > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0statx > > > > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0= =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0update page cache > > > > >=20 > > > > > right? > > > > >=20 > > > >=20 > > > > Yeah, I suppose so -- the statx wouldn't necessitate any locking. I= n > > > > that case, maybe this is useless then other than for testing purpos= es > > > > and userland NFS servers. > > > >=20 > > > > Would it be better to not consume a statx field with this if so? Wh= at > > > > could we use as an alternate interface? ioctl? Some sort of global > > > > virtual xattr? It does need to be something per-inode. > > >=20 > > > I don't see how a non-atomic change attribute is remotely useful even > > > for NFS. > > >=20 > > > The main problem is not so much the above (although NFS clients are > > > vulnerable to that too) but the behaviour w.r.t. directory changes. > > >=20 > > > If the server can't guarantee that file/directory/... creation and > > > unlink are atomically recorded with change attribute updates, then th= e > > > client has to always assume that the server is lying, and that it has > > > to revalidate all its caches anyway. Cue endless readdir/lookup/getat= tr > > > requests after each and every directory modification in order to chec= k > > > that some other client didn't also sneak in a change of their own. > > >=20 > >=20 > > We generally hold the parent dir's inode->i_rwsem exclusively over most > > important directory changes, and the times/i_version are also updated > > while holding it. What we don't do is serialize reads of this value vs. > > the i_rwsem, so you could see new directory contents alongside an old > > i_version. Maybe we should be taking it for read when we query it on a > > directory? >=20 > We do hold i_rwsem today. I'm working on changing that. Preserving > atomic directory changeinfo will be a challenge. The only mechanism I > can think if is to pass a "u64*" to all the directory modification ops, > and they fill in the version number at the point where it is incremented > (inode_maybe_inc_iversion_return()). The (nfsd) caller assumes that > "before" was one less than "after". If you don't want to internally > require single increments, then you would need to pass a 'u64 [2]' to > get two iversions back. >=20 That's a major redesign of what the i_version counter is today. It may very well end up being needed, but that's going to touch a lot of stuff in the VFS. Are you planning to do that as a part of your locking changes? > >=20 > > Achieving atomicity with file writes though is another matter entirely. > > I'm not sure that's even doable or how to approach it if so. > > Suggestions? >=20 > Call inode_maybe_inc_version(page->host) in __folio_mark_dirty() ?? >=20 Writes can cover multiple folios so we'd be doing several increments per write. Maybe that's ok? Should we also be updating the ctime at that point as well? Fetching the i_version under the i_rwsem is probably sufficient to fix this though. Most of the write_iter ops already bump the i_version while holding that lock, so this wouldn't add any extra locking to the write codepaths. --=20 Jeff Layton