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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 Sqx7GLDtCmX3bwAAMHmgww (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> X-Stat-Signature: d895qxg5ze4x5b3u8hzdjpnyafdcjifs X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C36CFA007E X-Rspam-User: X-HE-Tag: 1695215026-41253 X-HE-Meta: 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 vf79WqvI FUCvhGyCpTZc4OvZqB8nLg/z1IFxJDvnZjYkN1PJqkVIktYffjwNBHl2CUgyFV1wL240oDTSpKU55YCtnFHPdMWtl03Gi78kpSkSHJ0XsfAlFPNL5blV4hy6yllMiHtPTI4pnemfh3HL7f0fO5hfFTD170vAKe45YRROYb8nqV3s3oR/CAFaRTcPmnVIC05KoYOIIpCnKUbcKNqVIGodMIKJKXPlXEGhxtI68rqyVcpkLQ7oAIVydgYa93TmyEAf6H3x5PWIqw/x5RRcivRFGv+jTak1YHQXkxwnMf3fl0+EJIqFTrUczSTacDQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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