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@xxxxxxxxxx> Cc: NeilBrown <neilb@xxxxxxx> Cc: Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> Cc: Dave Chinner <david@xxxxxxxxxxxxx> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> --- include/linux/iversion.h | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) 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. * * 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 -- 2.39.1
