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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca, djwong@kernel.org,
        david@fromorbit.com, trondmy@hammerspace.com,
        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-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: Re: [PATCH v6 2/9] iversion: clarify when the i_version counter must
 be updated
In-reply-to: <20220930111840.10695-3-jlayton@kernel.org>
References: <20220930111840.10695-1-jlayton@kernel.org>,
 <20220930111840.10695-3-jlayton@kernel.org>
Date:   Tue, 04 Oct 2022 10:10:14 +1100
Message-id: <166483861470.14457.7243696062075946548@noble.neil.brown.name>
Precedence: bulk

On Fri, 30 Sep 2022, Jeff Layton wrote:
> 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.
>=20
> Cc: Colin Walters <walters@verbum.org>
> Cc: NeilBrown <neilb@suse.de>
> Cc: Trond Myklebust <trondmy@hammerspace.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Link: https://lore.kernel.org/linux-xfs/166086932784.5425.17134712694961326=
033@noble.neil.brown.name/#t
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  include/linux/iversion.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>=20
> diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> index 6755d8b4f20b..9925cac1fa94 100644
> --- a/include/linux/iversion.h
> +++ b/include/linux/iversion.h
> @@ -9,8 +9,14 @@
>   * ---------------------------
>   * 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 mu=
st
> - * 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 t=
he
> + * 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 update=
s 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.

POSIX doesn't (that I can see) describe when the ctime changes w.r.t
when the file changes.  For i_version we do want to specify that
i_version change is not visible before the file change is visible.
So this goes beyond the POSIX mandate.  I might be worth making that
explicit.
But this patch is nonetheless an improvement, so:

Reviewed-by: NeilBrown <neilb@suse.de>

Thanks,
NeilBrown


>   *
>   * 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
> --=20
> 2.37.3
>=20
>=20