fs: clarify when the i_version counter must be updated

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 <[e-mail]>
Cc: NeilBrown <[e-mail]>
Cc: Trond Myklebust <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>

include/linux/iversion.h

···
  * ---------------------------
  * 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. (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