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From:   Jeff Layton <jlayton@kernel.org>
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,
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        bfields@fieldses.org, brauner@kernel.org, fweimer@redhat.com
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, Colin Walters <walters@verbum.org>
Subject: [PATCH v7 2/9] fs: clarify when the i_version counter must be updated
Date:   Mon, 17 Oct 2022 06:57:02 -0400
Message-Id: <20221017105709.10830-3-jlayton@kernel.org>
In-Reply-To: <20221017105709.10830-1-jlayton@kernel.org>
References: <20221017105709.10830-1-jlayton@kernel.org>
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The i_version field in the kernel has had different semantics over
the decades, but NFSv4 has certain expectations. Update the comments
in iversion.h to describe when the i_version must change.

Cc: Colin Walters <walters@verbum.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Trond Myklebust <trondmy@hammerspace.com>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/iversion.h | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 6755d8b4f20b..94f4dc620d01 100644
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -9,8 +9,24 @@
  * ---------------------------
  * The change attribute (i_version) is mandated by NFSv4 and is mostly for
  * knfsd, but is also used for other purposes (e.g. IMA). The i_version must
- * appear different to observers if there was a change to the inode's data or
- * metadata since it was last queried.
+ * appear larger to observers if there was an explicit change to the inode's
+ * data or metadata since it was last queried.
+ *
+ * An explicit change is one that would ordinarily result in a change to the
+ * inode status change time (aka ctime). i_version must appear to change, even
+ * if the ctime does not (since the whole point is to avoid missing updates due
+ * to timestamp granularity). If POSIX mandates that the ctime must change due
+ * to an operation, then the i_version counter must be incremented as well.
+ *
+ * Making the i_version update completely atomic with the operation itself would
+ * be prohibitively expensive. Traditionally the kernel has updated the times on
+ * directories after an operation that changes its contents. For regular files,
+ * the ctime is usually updated before the data is copied into the cache for a
+ * write. This means that there is a window of time when an observer can
+ * associate a new timestamp with old file contents. Since the purpose of the
+ * i_version is to allow for better cache coherency, the i_version must always
+ * be updated after the results of the operation are visible. Updating it before
+ * and after a change is also permitted.
  *
  * Observers see the i_version as a 64-bit number that never decreases. If it
  * remains the same since it was last checked, then nothing has changed in the
-- 
2.37.3