Re: [PATCH] iversion: update comments with info about atime updates

[NeilBrown]

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.
> 
> Cc: NeilBrown <neilb@xxxxxxx>
> Cc: Trond Myklebust <trondmy@xxxxxxxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-xfs/166086932784.5425.17134712694961326033@xxxxxxxxxxxxxxxxxxxxx/#t
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  include/linux/iversion.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> 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 must
> - * 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 inode'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, only
>   * that the inode has changed in some fashion.
>   *
> + * Note that atime updates due to reads or similar activity do _not_ represent
> + * 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 not 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. Subsystems 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 macro).
> -- 
> 2.37.2
> 
>