Re: [PATCH v6 2/9] iversion: clarify when the i_version counter must be updated

[NeilBrown]

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.
> 
> Cc: Colin Walters <walters@xxxxxxxxxx>
> 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 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 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.

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@xxxxxxx>

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
> -- 
> 2.37.3
> 
>