On Thu, Nov 14, 2019 at 2:28 PM Sage Weil <sweil@xxxxxxxxxx> wrote:
>
> On Thu, 14 Nov 2019, Jeff Layton wrote:
> > On Thu, 2019-11-14 at 10:57 +0000, Luis Henriques wrote:
> > > Hi!
> > >
> > > So, after the feedback I got from v1 [1] I've sent out a pull-request
> > > for the OSDs [2] which encodes require_osd_release into the OSDMap
> > > client data. This allows the client to figure out which ceph release
> > > the OSDs cluster is running and decide whether or not it's safe to use
> > > the copy-from Op for copy_file_range.
> > >
> > > This new patchset I'm sending simply adds enough functionality to the
> > > kernel client so that it can take advantage of this OSD patch:
> > >
> > > 0001 - adds the ability to decode TYPE_MSGR2 addresses. This is a
> > > required functionality for enabling SERVER_NAUTILUS in the
> > > client. I hope I got the new format right, as I couldn't figure
> > > out what the hard-coded values (see comments) really mean.
> > >
> >
> > nit: the first 3 patch subject lines should probably be prefixed with
> > "libceph:"
> >
> > > 0002 - allows the client to retrieve the new require_osd_release field
> > > from the OSDMap if available. This patch also adds SERVER_MIMIC,
> > > SERVER_NAUTILUS and SERVER_OCTOPUS to the supported features,
> > > which TBH I'm not sure if that's a safe thing to do -- the only
> > > issue I've seen was that Nautilus requires the ability to decode
> > > TYPE_MSGR2 address, but I may have missed others.
> > >
> >
> > Yes, this needs to be done with care. We have to ensure that the server
> > side isn't assuming that the client supports something that it doesn't.
> > I think that means just trawling through the code and verifying whether
> > this is safe.
> >
> > > 0003 - debug code to add require_osd_release to the osdmap debugfs file.
> > >
> > > 0004 - adds the truncate_{seq,size} fields to the 'copy-from' operation
> > > if the OSDs are >= Octopus.
> > >
> > > Also note that, as suggested by Ilya, I've dropped the patch that would
> > > change the default mount options to 'copyfrom'.
> > >
> > > These patches have been tested with the xfstests generic test suite, and
> > > with a couple of other (local) tests that exercise the cephfs
> > > copy_file_range syscall. I didn't saw any issues, but as I said above,
> > > I'm not really sure if adding the SERVER_* flags to the supported
> > > features have other side effects.
> > >
> > > [1] https://lore.kernel.org/lkml/20191108141555.31176-1-lhenriques@xxxxxxxx/
> > > [2] https://github.com/ceph/ceph/pull/31611
> > >
> >
> > I'm just getting caught up on the discussion here, but why was it
> > decided to do it this way instead of just adding a new OSD
> > "copy-from-no-truncseq" operation? Once you tried it once and an OSD
> > didn't support it, you could just give up on using it any longer? That
> > seems a lot simpler than trying to monkey with feature bits.
>
> I don't remember the original discussion either, but in retrospect that
> does seem much simpler--especially since hte client is conditioning
> sending this based on the the require_osd_release. It seems like passing
> a flag to the copy-from op would be more reasonable instead of conditional
> feature-based behavior.
Yeah, I suggested adding require_osd_release to the client portion just
because we are running into it more and more: Objecter relies on it for
RESEND_ON_SPLIT for example. It needs to be accessible so that patches
like that can be carried over to the kernel client without workarounds.
copy-from in its existing form is another example. AFAIU the problem
is that copy-from op doesn't reject unknown flags. Luis added a flag
in https://github.com/ceph/ceph/pull/25374, but it is simply ignored on
nautilus and older releases, potentially leading to data corruption.
Adding a new op that would be an alias for CEPH_OSD_OP_COPY_FROM with
CEPH_OSD_COPY_FROM_FLAG_TRUNCATE_SEQ like Jeff is suggesting, or a new
copy-from2 op that would behave just like copy-from, but reject unknown
flags to avoid similar compatibility issues in the future is probably
the best thing we can do from the client perspective.
Thanks,
Ilya
