Re: [PATCH v8 RESEND 2/8] fs: clarify when the i_version counter must be updated

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2023-01-26 at 12:36 +0100, Jan Kara wrote:
> On Thu 26-01-23 05:54:16, Jeff Layton wrote:
> > On Wed, 2023-01-25 at 17:06 +0100, Jan Kara wrote:
> > > On Tue 24-01-23 14:30:19, 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@verbum.org>
> > > > Cc: NeilBrown <neilb@suse.de>
> > > > Cc: Trond Myklebust <trondmy@hammerspace.com>
> > > > Cc: Dave Chinner <david@fromorbit.com>
> > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > 
> > > Looks good to me. But one note below:
> > > 
> > > > diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> > > > index 6755d8b4f20b..fced8115a5f4 100644
> > > > --- a/include/linux/iversion.h
> > > > +++ b/include/linux/iversion.h
> > > > @@ -9,8 +9,25 @@
> > > >   * ---------------------------
> > > >   * 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 or other relevant spec mandates that the
> > > > + * ctime must change due to an operation, then the i_version counter must be
> > > > + * incremented as well.
> > > > + *
> > > > + * Making the i_version update completely atomic with the operation itself would
> > > > + * be prohibitively expensive. Traditionally the kernel has updated the times on
> > > > + * directories after an operation that changes its contents. For regular files,
> > > > + * the ctime is usually updated before the data is copied into the cache for a
> > > > + * write. This means that there is a window of time when an observer can
> > > > + * associate a new timestamp with old file contents. Since the purpose of the
> > > > + * i_version is to allow for better cache coherency, the i_version must always
> > > > + * be updated after the results of the operation are visible. Updating it before
> > > > + * and after a change is also permitted.
> > > 
> > > This sounds good but it is not the case for any of the current filesystems, is
> > > it? Perhaps the documentation should mention this so that people are not
> > > confused?
> > 
> > Correct. Currently, all filesystems change the times and version before
> > a write instead of after. I'm hoping that situation will change soon
> > though, as I've been working on a patchset to fix this for tmpfs, ext4
> > and btrfs.
> 
> That is good but we'll see how long it takes to get merged. AFAIR it is not
> a complete nobrainer ;)
> 
> > If you still want to see something for this though, what would you
> > suggest for verbiage?
> 
> Sure:
> 
> ... the i_version must a be updated after the results of the operation are
> visible (note that none of the filesystems currently do this, it is a work
> in progress to fix this).
> 
> And once your patches are merged, you can also delete this note :).
> 
> 								Honza

Sounds good, I folded something similar to that into the patch and
pushed it into the branch I'm feeding into linux-next. I won't bother
re-posting for just that though:

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index fced8115a5f4..f174ff1b59ee 100644
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -27,7 +27,8 @@
  * associate a new timestamp with old file contents. Since the purpose of the
  * i_version is to allow for better cache coherency, the i_version must always
  * be updated after the results of the operation are visible. Updating it before
- * and after a change is also permitted.
+ * and after a change is also permitted. (Note that no filesystems currently do
+ * this. Fixing that is a work-in-progress).
  *
  * 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

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>