Re: [RFC PATCH v2 0/4] ceph: safely use 'copy-from' Op on Octopus OSDs

[Jeff Layton <jlayton@kernel.org>]

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@suse.com/
> [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.

-- 
Jeff Layton <jlayton@kernel.org>