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    SubjectRe: [PATCH] iversion: update comments with info about atime updates
    Date
    On Mon, 22 Aug 2022, Jeff Layton wrote:
    > Add an explicit paragraph codifying that atime updates due to reads
    > should not be counted against the i_version counter. None of the
    > existing subsystems that use the i_version want those counted, and
    > there is an easy workaround for those that do.
    >
    > 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..da6cc1cc520a 100644
    > --- a/include/linux/iversion.h
    > +++ b/include/linux/iversion.h
    > @@ -9,8 +9,8 @@
    > * ---------------------------
    > * 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.

    Should rename change the i_version?
    It does not explicitly change data or metadata, though it seems to
    implicitly change the ctime.

    > *
    > * 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
    > @@ -18,6 +18,12 @@
    > * anything about the nature or magnitude of the changes from the value, only
    > * that the inode has changed in some fashion.
    > *
    > + * Note that atime updates due to reads or similar activity do _not_ represent
    > + * an explicit change to the inode. If the only change is to the atime and it
    > + * wasn't set via utimes() or a similar mechanism, then i_version should not be
    > + * incremented. If an observer cares about atime updates, it should plan to
    > + * fetch and store them in conjunction with the i_version.
    > + *

    If an implicit atime update happened to make the atime go backwards
    (possible, but not common), the updating i_version should be permitted,
    and possibly should be preferred.

    NeilBrown


    > * Not all filesystems properly implement the i_version counter. Subsystems that
    > * want to use i_version field on an inode should first check whether the
    > * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION macro).
    > --
    > 2.37.2
    >
    >

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