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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-integrity@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        "Trond Myklebust" <trondmy@hammerspace.com>,
        "Dave Chinner" <david@fromorbit.com>
Subject: Re: [PATCH] iversion: update comments with info about atime updates
In-reply-to: <20220822133309.86005-1-jlayton@kernel.org>
References: <20220822133309.86005-1-jlayton@kernel.org>
Date:   Tue, 23 Aug 2022 08:42:15 +1000
Message-id: <166120813594.23264.3095357572943917078@noble.neil.brown.name>
Precedence: bulk

On Mon, 22 Aug 2022, Jeff Layton wrote:
> Add an explicit paragraph codifying that atime updates due to reads
> should not be counted against the i_version counter. None of the
> existing subsystems that use the i_version want those counted, and
> there is an easy workaround for those that do.
>=20
> 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 3bfebde5a1a6..da6cc1cc520a 100644
> --- a/include/linux/iversion.h
> +++ b/include/linux/iversion.h
> @@ -9,8 +9,8 @@
>   * ---------------------------
>   * 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 different to observers if there was an explicit change to the in=
ode's
> + * data or metadata since it was last queried.

Should rename change the i_version?
It does not explicitly change data or metadata, though it seems to
implicitly change the ctime.

>   *
>   * 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
> @@ -18,6 +18,12 @@
>   * anything about the nature or magnitude of the changes from the value, o=
nly
>   * that the inode has changed in some fashion.
>   *
> + * Note that atime updates due to reads or similar activity do _not_ repre=
sent
> + * an explicit change to the inode. If the only change is to the atime and=
 it
> + * wasn't set via utimes() or a similar mechanism, then i_version should n=
ot be
> + * incremented. If an observer cares about atime updates, it should plan to
> + * fetch and store them in conjunction with the i_version.
> + *

If an implicit atime update happened to make the atime go backwards
(possible, but not common), the updating i_version should be permitted,
and possibly should be preferred.

NeilBrown


>   * Not all filesystems properly implement the i_version counter. Subsystem=
s that
>   * want to use i_version field on an inode should first check whether the
>   * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION mac=
ro).
> --=20
> 2.37.2
>=20
>=20