@@ -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.
*
* 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
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> --- include/linux/iversion.h | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-)