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Tue, 19 Sep 2023 16:52:43 +0200 (CEST) From: Bruno Haible To: Jan Kara , Xi Ruoyao , bug-gnulib@gnu.org Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps Date: Tue, 19 Sep 2023 16:52:43 +0200 Message-ID: <4511209.uG2h0Jr0uP@nimes> In-Reply-To: <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org> <20230919110457.7fnmzo4nqsi43yqq@quack3> <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-BeenThere: linux-erofs@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development of Linux EROFS file system List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Latchesar Ionkov , Martin Brandenburg , Konstantin Komarov , linux-xfs@vger.kernel.org, "Darrick J. Wong" , Dominique Martinet , Christian Schoenebeck , ecryptfs@vger.kernel.org, linux-unionfs@vger.kernel.org, David Howells , Chris Mason , Andreas Dilger , Hans de Goede , Marc Dionne , codalist@coda.cs.cmu.edu, linux-afs@lists.infradead.org, linux-mtd@lists.infradead.org, Mike Marshall , Paulo Alcantara , linux-cifs@vger.kernel.org, Eric Van Hensbergen , bug-gnulib@gnu.org, Andreas Gruenbacher , Miklos Szeredi , Richard Weinberger , Mark Fasheh , Hugh Dickins , Benjamin Coddington , Tyler Hicks , cluster-devel@redhat.com, coda@cs.cmu.edu, linux-mm@kvack.org, Jeff Layton , Ilya Dryomov , Iurii Zaikin , Namjae Jeon , Trond Myklebust , Shyam Prasad N , Amir Goldstein , Kees Cook , ocfs2-devel@lists.linux.dev, Josef Bacik , Tom Talpey , Tejun Heo , Yue Hu , Alexander Viro , Ronnie Sahlberg , David Sterba , Jaegeuk Kim , ceph-devel@vger.kernel.org, Xiubo Li , OGAWA Hirofumi , Jan Harkes , Christian Brauner , linux-ext4@vger.kernel.org, Theodore Ts'o , Joseph Qi , Greg Kroah-Hartman , v9fs@lists.linux.dev, ntfs3@lists.linux.dev, samba-technical@lists.samba.org, linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, Steve French , Sergey Senozhatsky , Luis Chamberlain , devel@lists.orangefs.org, Anna Schumaker , Jan Kara , Bob Peterson , linux-fsdevel@vger.kernel.org, Andrew Morton , Sungjong Seo , linux-erofs@lists.ozlabs.org, linux-nfs@vger.kernel.org, linux-btrfs@vger.kernel.org, Joel Becker Errors-To: linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org Sender: "Linux-erofs" Jeff Layton wrote: > I'm not sure what we can do for this test. The nap() function is making > an assumption that the timestamp granularity will be constant, and that > isn't necessarily the case now. This is only of secondary importance, because the scenario by Jan Kara shows a much more fundamental breakage: > > The ultimate problem is that a sequence like: > > > > write(f1) > > stat(f2) > > write(f2) > > stat(f2) > > write(f1) > > stat(f1) > > > > can result in f1 timestamp to be (slightly) lower than the final f2 > > timestamp because the second write to f1 didn't bother updating the > > timestamp. That can indeed be a bit confusing to programs if they compare > > timestamps between two files. Jeff? > > > > Basically yes. 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 ...'. Bruno