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

[Jeff Layton]

On Tue, 2022-10-04 at 10:10 +1100, NeilBrown wrote:
> 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,

Now that we're looking at setting the ctime and i_version separately,
I'll plan to add something that makes this explict.

> >   *
> >   * 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
> > 
> > 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>