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

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2022-08-27 at 11:01 +0300, Amir Goldstein wrote:
> 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?
> 

The client just looks at the file attributes (of which i_version is but
one), and if certain attributes have changed (mtime, ctime, i_version,
etc...) then it invalidates its cache.

In the case of blocks map changes, could that mean a difference in the
observable sparse regions of the file? If so, then a READ_PLUS before
the change and a READ_PLUS after could give different results. Since
that difference is observable by the client, I'd think we'd want to bump
i_version for that anyway.

> > 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.
> > 

We don't hand out read leases if there are file descriptions open for
write. NFS clients usually issue a COMMIT before closing a stateid in
order to satisfy close-to-open cache coherency.

So in most cases, this is probably not an issue. It might still be
worthwhile to make sure of it by doing a filemap_write_and_wait before
we hand out a delegation, but that's likely to be a no-op in most cases
anyway.

Note too that the client will still revalidate its caches when it
receives attributes even when it holds a read delegation. In fact, this
behavior mostly papered over a rather nasty knfsd bug we found recently
where it was allowing conflicting activity to proceed even when there
was a read delegation outstanding.
 
> > 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,
-- 
Jeff Layton <jlayton@kernel.org>