Re: [PATCH v20 09/12] dm: Add support for copy offload

[Nitesh Shetty]

On 22/05/24 08:22AM, Hannes Reinecke wrote:
> On 5/21/24 16:08, Nitesh Shetty wrote:
> > On 21/05/24 09:11AM, Hannes Reinecke wrote:
> > > On 5/20/24 12:20, Nitesh Shetty wrote:
> > > > Before enabling copy for dm target, check if underlying devices and
> > > > dm target support copy. Avoid split happening inside dm target.
> > > > Fail early if the request needs split, currently splitting copy
> > > > request is not supported.
> > > >
> > > > Signed-off-by: Nitesh Shetty <nj.shetty@xxxxxxxxxxx>
> > > > ---
> > > > @@ -397,6 +397,9 @@ struct dm_target {
> > > >       * bio_set_dev(). NOTE: ideally a target should _not_ need this.
> > > >       */
> > > >      bool needs_bio_set_dev:1;
> > > > +
> > > > +    /* copy offload is supported */
> > > > +    bool copy_offload_supported:1;
> > > >  };
> > > >  void *dm_per_bio_data(struct bio *bio, size_t data_size);
> > >
> > > Errm. Not sure this will work. DM tables might be arbitrarily, 
> > > requiring us to _split_ the copy offload request according to the 
> > > underlying component devices. But we explicitly disallowed a split 
> > > in one of the earlier patches.
> > > Or am I wrong?
> > Yes you are right w.r.to split, we disallow split.
> > But this flag indicates whether we support copy offload in dm-target or
> > not. At present we support copy offload only in dm-linear.
> > For other dm-target, eventhough underlaying device supports copy
> > offload, dm-target based on it wont support copy offload.
> > If the present series get merged, we can test and integrate more
> > targets.
> But dm-linear can be concatenated, too; you can easily use dm-linear
> to tie several devices together.
> Which again would require a copy-offload range to be split.
> Hmm?
Sorry, I dont understand the concern here. I see 3 possibilites here.

1. Both src and dst IO lies in same underlying device. This will succeed.
2. src and dst lie in different devices. This will fail.
	a. src or dst needs to be split, if one or both of them
	spans across the underlying block device boundary. In this case we
	fail the IO in dm layer(refer patch 9).
	b. src and dst doesn't split in dm,
	but they wont be merged in request later as they belong to
	different block device.
	Hence the request reaches the driver with single bio and will fail
	in driver(refer patch 7)

Does this address your concern, or do you have something else in mind ?

Thank you,
Nitesh Shetty