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Thu, 28 Sep 2023 20:50:24 -0700 (PDT) MIME-Version: 1.0 References: <20230928110554.34758-1-jlayton@kernel.org> <20230928110554.34758-2-jlayton@kernel.org> <6020d6e7-b187-4abb-bf38-dc09d8bd0f6d@app.fastmail.com> <20230928171943.GK11439@frogsfrogsfrogs> <6a6f37d16b55a3003af3f3dbb7778a367f68cd8d.camel@kernel.org> <20230928212656.GC189345@mit.edu> In-Reply-To: From: Amir Goldstein Date: Fri, 29 Sep 2023 06:50:13 +0300 Message-ID: Subject: Re: [PATCH 86/87] fs: switch timespec64 fields in inode to discrete integers To: Linus Torvalds Cc: "Theodore Ts'o" , Jeff Layton , "Darrick J. Wong" , Arnd Bergmann , Alexander Viro , Christian Brauner , David Sterba , "Eric W. Biederman" , Kees Cook , Jeremy Kerr , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Greg Kroah-Hartman , =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan , Mattia Dongili , Dennis Dalessandro , Jason Gunthorpe , Leon Romanovsky , Brad Warrum , Ritu Agarwal , Hans de Goede , =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= , Mark Gross , Jiri Slaby , Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , David Sterba , David Howells , Marc Dionne , Ian Kent , Luis de Bethencourt , Salah Triki , "Tigran A. Aivazian" , Chris Mason , Josef Bacik , Xiubo Li , Ilya Dryomov , Jan Harkes , coda@cs.cmu.edu, Joel Becker , Christoph Hellwig , Nicolas Pitre , "Rafael J . Wysocki" , Ard Biesheuvel , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Namjae Jeon , Sungjong Seo , Jan Kara , Andreas Dilger , Jaegeuk Kim , OGAWA Hirofumi , Christoph Hellwig , Miklos Szeredi , Bob Peterson , Andreas Gruenbacher , Richard Weinberger , Anton Ivanov , Johannes Berg , Mikulas Patocka , Mike Kravetz , Muchun Song , Jan Kara , David Woodhouse , Dave Kleikamp , Tejun Heo , Trond Myklebust , Anna Schumaker , Chuck Lever , Neil Brown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Ryusuke Konishi , Anton Altaparmakov , Konstantin Komarov , Mark Fasheh , Joseph Qi , Bob Copeland , Mike Marshall , Martin Brandenburg , Luis Chamberlain , Iurii Zaikin , Tony Luck , "Guilherme G. Piccoli" , Anders Larsen , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Sergey Senozhatsky , Phillip Lougher , Steven Rostedt , Masami Hiramatsu , Evgeniy Dushistov , Chandan Babu R , Damien Le Moal , Naohiro Aota , Johannes Thumshirn , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Hugh Dickins , Andrew Morton , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , John Johansen , Paul Moore , James Morris , "Serge E. Hallyn" , Stephen Smalley , Eric Paris , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, platform-driver-x86@vger.kernel.org, linux-rdma@vger.kernel.org, linux-serial@vger.kernel.org, linux-usb@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, autofs@vger.kernel.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@telemann.coda.cs.cmu.edu, linux-efi@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, gfs2@lists.linux.dev, linux-um@lists.infradead.org, linux-mtd@lists.infradead.org, jfs-discussion@lists.sourceforge.net, linux-nfs@vger.kernel.org, linux-nilfs@vger.kernel.org, linux-ntfs-dev@lists.sourceforge.net, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, linux-karma-devel@lists.sourceforge.net, devel@lists.orangefs.org, linux-unionfs@vger.kernel.org, linux-hardening@vger.kernel.org, reiserfs-devel@vger.kernel.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-trace-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, bpf@vger.kernel.org, Netdev , apparmor@lists.ubuntu.com, linux-security-module@vger.kernel.org, selinux@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: DD177140008 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: gituw11d4odpmopxreufw5idoinfu91q X-HE-Tag: 1695959425-859017 X-HE-Meta: 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there are certainly filesystems > out there that don't do nanosecond resolution (and other operations > like copying trees around will obviously also change times). > > Anybody doing steganography in the timestamps is already not going to > have a great time, really. > Your thesis implies that all applications are portable across different filesystems and all applications are expected to cope with copying trees around. There are applications that work on specific filesystems and those applications are very much within sanity if they expect that past observed values of nsec will not to change if the file was not changed. But even if we agree that will "only" hurt performance, your example of performance hit (10s of git diff) is nowhere close to the performance hit of invalidating the mtime cache of billions of files at once (i.e. afte= r kernel upgrade), which means that rsync-like programs need to re-read all the data from remote locations. I am not saying that filesystems cannot decide to *stop storing nsec granularity* from this day forth, but like btrfs pre-historic timestamps, those fs have an obligation to preserve existing metadata, unless users opted to throw it away. OTOH, it is perfectly fine if the vfs wants to stop providing sub 100ns services to filesystems. It's just going to be the fs problem and the preserved pre-historic/fine-grained time on existing files would only need to be provided in getattr(). It does not need to be in __i_mtime. Thanks, Amir.