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 69178CE79CE for ; Wed, 20 Sep 2023 13:03:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236387AbjITNDz (ORCPT ); Wed, 20 Sep 2023 09:03:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47292 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235288AbjITNDx (ORCPT ); Wed, 20 Sep 2023 09:03:53 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58056AB; Wed, 20 Sep 2023 06:03:46 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 9027421BE4; Wed, 20 Sep 2023 13:03:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1695215024; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3YH4aY4cbPoScM3IUucMKFT40epHCwOjmoTvaSKlk4Y=; b=otKGaXz2/XrzfZupt6ktvrQuKInIDzegtQmfaPoLxY4VnNRmhq8GHrCWQI+1osXbmJXzc8 hsqSIqgaDK56ti5FatEpfKHR+a56lMxOto1VcAq5D4tteZwrApJ+/tkTZloeNfEYZLFbM/ mYxTcP/RIUrXsopZPm765X2swd+GJzw= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1695215024; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3YH4aY4cbPoScM3IUucMKFT40epHCwOjmoTvaSKlk4Y=; b=l1S9AoaClTRNwFpmRqvsn+Gd9YnYp+Fj4zpQGbOPeBxV22YGtYbvWxhVageo026HMgq9Vl V59SyZSRECSeZaCA== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 752C213A64; Wed, 20 Sep 2023 13:03:44 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id Eg6MHLDtCmX6bwAAMHmgww (envelope-from ); Wed, 20 Sep 2023 13:03:44 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id F3DF5A077D; Wed, 20 Sep 2023 15:03:43 +0200 (CEST) Date: Wed, 20 Sep 2023 15:03:43 +0200 From: Jan Kara To: Christian Brauner Cc: Jan Kara , Jeff Layton , Bruno Haible , Xi Ruoyao , bug-gnulib@gnu.org, Alexander Viro , Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , David Howells , Marc Dionne , Chris Mason , Josef Bacik , David Sterba , Xiubo Li , Ilya Dryomov , Jan Harkes , coda@cs.cmu.edu, Tyler Hicks , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Namjae Jeon , Sungjong Seo , Jan Kara , Theodore Ts'o , Andreas Dilger , Jaegeuk Kim , OGAWA Hirofumi , Miklos Szeredi , Bo b Peterson , Andreas Gruenbacher , Greg Kroah-Hartman , Tejun Heo , Trond Myklebust , Anna Schumaker , Konstantin Komarov , Mark Fasheh , Joel Becker , Joseph Qi , Mike Marshall , Martin Brandenburg , Luis Chamberlain , Kees Cook , Iurii Zaikin , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Tom Talpey , Sergey Senozhatsky , Richard Weinberger , Hans de Goede , Hugh Dickins , Andrew Morton , Amir Goldstein , "Darrick J. Wong" , Benjamin Coddington , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@coda.cs.cmu.edu, ecryptfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, devel@lists.orangefs.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-mtd@lists.infradead.org, linux-mm@kvack.org, linux-unionfs@vger.kernel.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps Message-ID: <20230920130343.qs2kuzngoomy4s3r@quack3> References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org> <20230919110457.7fnmzo4nqsi43yqq@quack3> <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> <4511209.uG2h0Jr0uP@nimes> <08b5c6fd3b08b87fa564bb562d89381dd4e05b6a.camel@kernel.org> <20230920-leerung-krokodil-52ec6cb44707@brauner> <20230920101731.ym6pahcvkl57guto@quack3> <20230920-kaulquappen-computer-0a4a0e4c3c71@brauner> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230920-kaulquappen-computer-0a4a0e4c3c71@brauner> Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Wed 20-09-23 12:30:52, Christian Brauner wrote: > On Wed, Sep 20, 2023 at 12:17:31PM +0200, Jan Kara wrote: > > On Wed 20-09-23 10:41:30, Christian Brauner wrote: > > > > > f1 was last written to *after* f2 was last written to. If the timestamp of f1 > > > > > is then lower than the timestamp of f2, timestamps are fundamentally broken. > > > > > > > > > > Many things in user-space depend on timestamps, such as build system > > > > > centered around 'make', but also 'find ... -newer ...'. > > > > > > > > > > > > > > > > > What does breakage with make look like in this situation? The "fuzz" > > > > here is going to be on the order of a jiffy. The typical case for make > > > > timestamp comparisons is comparing source files vs. a build target. If > > > > those are being written nearly simultaneously, then that could be an > > > > issue, but is that a typical behavior? It seems like it would be hard to > > > > rely on that anyway, esp. given filesystems like NFS that can do lazy > > > > writeback. > > > > > > > > One of the operating principles with this series is that timestamps can > > > > be of varying granularity between different files. Note that Linux > > > > already violates this assumption when you're working across filesystems > > > > of different types. > > > > > > > > As to potential fixes if this is a real problem: > > > > > > > > I don't really want to put this behind a mount or mkfs option (a'la > > > > relatime, etc.), but that is one possibility. > > > > > > > > I wonder if it would be feasible to just advance the coarse-grained > > > > current_time whenever we end up updating a ctime with a fine-grained > > > > timestamp? It might produce some inode write amplification. Files that > > > > > > Less than ideal imho. > > > > > > If this risks breaking existing workloads by enabling it unconditionally > > > and there isn't a clear way to detect and handle these situations > > > without risk of regression then we should move this behind a mount > > > option. > > > > > > So how about the following: > > > > > > From cb14add421967f6e374eb77c36cc4a0526b10d17 Mon Sep 17 00:00:00 2001 > > > From: Christian Brauner > > > Date: Wed, 20 Sep 2023 10:00:08 +0200 > > > Subject: [PATCH] vfs: move multi-grain timestamps behind a mount option > > > > > > While we initially thought we can do this unconditionally it turns out > > > that this might break existing workloads that rely on timestamps in very > > > specific ways and we always knew this was a possibility. Move > > > multi-grain timestamps behind a vfs mount option. > > > > > > Signed-off-by: Christian Brauner > > > > Surely this is a safe choice as it moves the responsibility to the sysadmin > > and the cases where finegrained timestamps are required. But I kind of > > wonder how is the sysadmin going to decide whether mgtime is safe for his > > system or not? Because the possible breakage needn't be obvious at the > > first sight... If I were a sysadmin, I'd rather opt for something like > > I think you'll basically enable this because you want to export a > filesystem via NFS. OK, that's what I thought but then you have to make a tough choice between: 1) Possibly inconsistent NFS caches on frequent changes. 2) Possibly broken builds on NFS. Pick your poison ;) > > finegrained timestamps + lazytime (if I needed the finegrained timestamps > > functionality). That should avoid the IO overhead of finegrained timestamps > > That would work with this patch, no? Or are you saying it would need > something else? Sorry, I was not really precise here. What I meant was that instead of having multigrain timestamps, I (as a sysadmin) would want the filesystem to set sb->s_time_gran to 1 ns and use lazytime to remove the IO overhead of the frequent timestamp updates. But that is just me brainstorming possible solutions of the original NFS problem. > > as well and I'd know I can have problems with timestamps only after a > > system crash. > > > > I've just got another idea how we could solve the problem: Couldn't we > > always just report coarsegrained timestamp to userspace and provide access > > to finegrained value only to NFS which should know what it's doing? > > What would changes would be involved for that? See my other email. It should be fairly small... > If this is invasive work and we decide this is something that we want to > do then we should remove FS_MGTIME from btrfs, xfs, ext4, and tmpfs for > v6.6. .. but let's see what Jeff thinks. I can miss some problem with the solution. Honza -- Jan Kara SUSE Labs, CR