[PATCH v3 1/7] iversion: update comments with info about atime updates

[Jeff Layton <jlayton@kernel.org>]

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 how a conformant
i_version implementation is expected to behave. This definition
suits the current users of i_version (NFSv4 and IMA), but is
loose enough to allow for a wide range of possible implementations.

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 | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 3bfebde5a1a6..45e93e1b4edc 100644
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -9,8 +9,19 @@
  * ---------------------------
  * 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.
+ *
+ * An explicit change is one that would ordinarily result in a change to the
+ * inode status change time (aka ctime). The 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.
+ *
+ * A conformant implementation is allowed to increment the counter in other
+ * cases, but this is not optimal. NFSv4 and IMA both use this value to determine
+ * whether caches are up to date. Spurious increments can cause false cache
+ * invalidations.
  *
  * 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
@@ -66,6 +77,14 @@
  * Storing the value to disk therefore does not count as a query, so those
  * filesystems should use inode_peek_iversion to grab the value to be stored.
  * There is no need to flag the value as having been queried in that case.
+ *
+ * Notes on atime updates
+ * ----------------------
+ * Access time (atime) updates due to reads or similar activity do not represent
+ * an explicit change to the inode data or metadata. If the only change to the
+ * inode is the atime, then i_version should not be incremented. If an observer
+ * cares about atime updates, it should plan to fetch and store the atime in
+ * conjunction with the i_version.
  */
 
 /*
-- 
2.37.2