From patchwork Tue Sep 27 20:35:50 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 611006 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 7FBCBC6FA83 for ; Tue, 27 Sep 2022 20:36:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233175AbiI0UgF (ORCPT ); Tue, 27 Sep 2022 16:36:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37506 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233113AbiI0UgD (ORCPT ); Tue, 27 Sep 2022 16:36:03 -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 A54C17C74D; Tue, 27 Sep 2022 13:35:54 -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 262C761B9D; Tue, 27 Sep 2022 20:35:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 89516C433D6; Tue, 27 Sep 2022 20:35:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1664310953; bh=KI+o4v/0Z/0uEBUC8hWcTFsL16DhcnsX1IF8KdH9XQM=; h=From:To:Cc:Subject:Date:From; b=Canh2CnpYzR+f3P5Nz3xhwztWCpngQuI14I5Ic++NDfv6vmn/5QJOR+A9EKg6rKDV 0n8EQeDpiiURLNHEfjRG1vK3GqupNX2OZ84seYJOyeW1VAqpz7vpmlA01Ihm6ipRPT juAFBAgqrlmKzMFmZMwx+DW5iT22u021L7RRrXbPkVbmpR8Id6Sa5WdEFTUbRl2xQv ceTLua4ntz0NDvQ0upni7o/SfSi+LYf6OnnX97FGV9pU2K5SUmSYmFHe3ISaglYE+U hi7OXFCTJaQN+qjOLGNsflRnlwWyFDs5a4DInzF0PsCMJjbg8HUsLHkZpigqtlEJ9E Lxb8bORjtbAGg== From: Jeff Layton To: tytso@mit.edu, adilger.kernel@dilger.ca, djwong@kernel.org, david@fromorbit.com, trondmy@hammerspace.com, neilb@suse.de, viro@zeniv.linux.org.uk, zohar@linux.ibm.com, xiubli@redhat.com, chuck.lever@oracle.com, lczerner@redhat.com, jack@suse.cz, bfields@fieldses.org, brauner@kernel.org, fweimer@redhat.com, linux-man@vger.kernel.org Cc: linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-nfs@vger.kernel.org, linux-xfs@vger.kernel.org Subject: [PATCH] statx, inode: document the new STATX_VERSION field Date: Tue, 27 Sep 2022 16:35:50 -0400 Message-Id: <20220927203550.331261-1-jlayton@kernel.org> X-Mailer: git-send-email 2.37.3 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: ceph-devel@vger.kernel.org I'm proposing to expose the inode change attribute via statx [1]. Document what this value means and what an observer can infer from it changing. Signed-off-by: Jeff Layton [1]: https://lore.kernel.org/linux-nfs/d9c065939af2728b1c0768d5ef7526995b634902.camel@kernel.org/T/#t --- man2/statx.2 | 13 +++++++++++++ man7/inode.7 | 31 +++++++++++++++++++++++++++++++ 2 files changed, 44 insertions(+) Another RFC posting to hopefully nail down the desired semantics. I purposefully left out verbiage around atomicity, with the expectation that we should be able to make the existing filesystems that support i_version bump the counter after a write instead of before. Also, for v5: - drop _INO/_ino from the name (it's redunant) - add STATX_ATTR_VERSION_MONOTONIC diff --git a/man2/statx.2 b/man2/statx.2 index 0d1b4591f74c..b2fdb5ddf97a 100644 --- a/man2/statx.2 +++ b/man2/statx.2 @@ -62,6 +62,7 @@ struct statx { __u32 stx_dev_major; /* Major ID */ __u32 stx_dev_minor; /* Minor ID */ __u64 stx_mnt_id; /* Mount ID */ + __u64 stx_version; /* Inode change attribute */ }; .EE .in @@ -247,6 +248,7 @@ STATX_BTIME Want stx_btime STATX_ALL The same as STATX_BASIC_STATS | STATX_BTIME. It is deprecated and should not be used. STATX_MNT_ID Want stx_mnt_id (since Linux 5.8) +STATX_VERSION Want stx_version (DRAFT) .TE .in .PP @@ -407,10 +409,16 @@ This is the same number reported by .BR name_to_handle_at (2) and corresponds to the number in the first field in one of the records in .IR /proc/self/mountinfo . +.TP +.I stx_version +The inode version, also known as the inode change attribute. See +.BR inode (7) +for details. .PP For further information on the above fields, see .BR inode (7). .\" +.TP .SS File attributes The .I stx_attributes @@ -489,6 +497,11 @@ without an explicit See .BR mmap (2) for more information. +.TP +.BR STATX_ATTR_VERSION_MONOTONIC " (since Linux 6.?)" +The stx_version value monotonically increases over time and will never appear +to go backward, even in the event of a crash. This can allow an application to +make a better determination about ordering when viewing different versions. .SH RETURN VALUE On success, zero is returned. On error, \-1 is returned, and diff --git a/man7/inode.7 b/man7/inode.7 index 9b255a890720..ec7f80dacaa8 100644 --- a/man7/inode.7 +++ b/man7/inode.7 @@ -184,6 +184,12 @@ Last status change timestamp (ctime) This is the file's last status change timestamp. It is changed by writing or by setting inode information (i.e., owner, group, link count, mode, etc.). +.TP +Inode version (version) +(not returned in the \fIstat\fP structure); \fIstatx.stx_version\fP +.IP +This is the inode change counter. See the discussion of +\fBthe inode version counter\fP, below. .PP The timestamp fields report time measured with a zero point at the .IR Epoch , @@ -424,6 +430,31 @@ on a directory means that a file in that directory can be renamed or deleted only by the owner of the file, by the owner of the directory, and by a privileged process. +.SS The inode version counter +.PP +The +.I statx.stx_version +field is the inode change counter. Any operation that would result in a +change to \fIstatx.stx_ctime\fP must result in an increase to this value. +The value must increase even in the case where the ctime change is not +evident due to coarse timestamp granularity. +.PP +An observer cannot infer anything from amount of increase about the +nature or magnitude of the change. If the returned value is different +from the last time it was checked, then something has made an explicit +data and/or metadata change to the inode. +.PP +In the event of a system crash this value can appear to go backward, +if it were queried before being written to the backing store. If +the value were then incremented again after restart, then an observer +could miss noticing a change. Applications that persist stx_version values +across a reboot should take care to mitigate this problem. If the filesystem +reports \fISTATX_ATTR_VERSION_MONOTONIC\fP in stx_attributes, then it is not +subject to this problem. +.PP +The stx_version is a Linux extension and is not supported by all filesystems. +The application must verify that the \fISTATX_VERSION\fP bit is set in the +returned \fIstatx.stx_mask\fP before relying on this field. .SH STANDARDS If you need to obtain the definition of the .I blkcnt_t