Re: [PATCH v3 4/7] xfs: don't bump the i_version on an atime update in xfs_vn_update_time

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Aug 27, 2022 at 10:26 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Sat, Aug 27, 2022 at 12:49 AM Jeff Layton <jlayton@kernel.org> wrote:
> >
> > xfs will update the i_version when updating only the atime value, which
> > is not desirable for any of the current consumers of i_version. Doing so
> > leads to unnecessary cache invalidations on NFS and extra measurement
> > activity in IMA.
> >
> > Add a new XFS_ILOG_NOIVER flag, and use that to indicate that the
> > transaction should not update the i_version. Set that value in
> > xfs_vn_update_time if we're only updating the atime.
> >
> > Cc: Dave Chinner <david@fromorbit.com>
> > Cc: NeilBrown <neilb@suse.de>
> > Cc: Trond Myklebust <trondmy@hammerspace.com>
> > Cc: David Wysochanski <dwysocha@redhat.com>
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  fs/xfs/libxfs/xfs_log_format.h  |  2 +-
> >  fs/xfs/libxfs/xfs_trans_inode.c |  2 +-
> >  fs/xfs/xfs_iops.c               | 11 +++++++++--
> >  3 files changed, 11 insertions(+), 4 deletions(-)
> >
> > Dave has NACK'ed this patch, but I'm sending it as a way to illustrate
> > the problem. I still think this approach should at least fix the worst
> > problems with atime updates being counted. We can look to carve out
> > other "spurious" i_version updates as we identify them.
> >
>
> AFAIK, "spurious" is only inode blocks map changes due to writeback
> of dirty pages. Anybody know about other cases?
>
> Regarding inode blocks map changes, first of all, I don't think that there is
> any practical loss from invalidating NFS client cache on dirty data writeback,
> because NFS server should be serving cold data most of the time.
> If there are a few unneeded cache invalidations they would only be temporary.
>

Unless there is an issue with a writer NFS client that invalidates its
own attribute
caches on server data writeback?

> One may even consider if NFSv4 server should not flush dirty data of an inode
> before granting a read lease to client.
> After all, if read lease was granted, client cached data and then server crashed
> before persisting the dirty data, then client will have cached a
> "future" version
> of the data and if i_version on the server did not roll back in that situation,
> we are looking at possible data corruptions.
>
> Same goes for IMA. IIUC, IMA data checksum would be stored in xattr?
> Storing in xattr a data checksum for data that is not persistent on disk
> would be an odd choice.
>
> So in my view, I only see benefits to current i_version users in the xfs
> i_version implementations and I don't think that it contradicts the
> i_version definition in the man page patch.
>
> > If however there are offline analysis tools that require atime updates
> > to be counted, then we won't be able to do this. If that's the case, how
> > can we fix this such that serving xfs via NFSv4 doesn't suck?
> >
>
> If I read the arguments correctly, implicit atime updates could be relaxed
> as long as this behavior is clearly documented and coherent on all
> implementations.
>
> Forensics and other applications that care about atime updates can and
> should check atime and don't need i_version to know that it was changed.
> The reliability of atime as an audit tool has dropped considerably since
> the default in relatime.
> If we want to be paranoid, maybe we can leave i_version increment on
> atime updates in case the user opted-in to strict '-o atime' updates, but
> IMO, there is no need for that.
>
> Thanks,
> Amir.