The i_version field in the kernel has had different semantics over
the decades, but we're now proposing to expose it to userland via
statx. This means that we need a clear, consistent definition of
what it means and when it should change.

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>
Link: https://lore.kernel.org/linux-xfs/166086932784.5425.17134712694961326033@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(-)

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 3bfebde5a1a6..0555a3851dbf 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.
  *
  * 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