Re: [PATCH RFC 03/26] dma-buf: Add dma_buf_mapping_attach()

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Mon, 2026-10-19 at 10:35 +0200, Christian König wrote:
> On 10/16/26 22:26, Karl Mehltretter wrote:
> > On Tue, Feb 17, 2026 at 08:11:34PM -0400, Jason Gunthorpe wrote:
> > > +	match_args.attach = attach;
> > > +	if (dmabuf->ops->match_mapping) {
> > > +		ret = dmabuf->ops->match_mapping(&match_args);
> > > +	}
> > > +
> > >  	attach->dmabuf = dmabuf;
> > >  	attach->importer_ops = importer_ops;
> > >  	attach->importer_priv = importer_priv;
> > 
> > I may misunderstand the intended contract. match_mapping() runs
> > while the
> > attachment is still zeroed and without dmabuf->resv held.
> > 
> > Patch 11's amdgpu callback needs importer_ops and importer_priv, so
> > its XGMI
> > match cannot work here. Patch 12's VFIO callback asserts dmabuf-
> > >resv and
> > reads priv->vdev, which cleanup changes under that lock.
> > 
> > Should the core initialize the attachment and hold dmabuf->resv for
> > match_mapping(), or should callbacks avoid attachment and lock-
> > protected
> > state?
> 
> Right. This completely misplaced.
> 
> The matching can't be done at attachment time, but rather needs to be
> done before creating a mapping while holding the resv lock so that
> the matching decision can't change.

So question is what exactly should the contract be?
Looking at the pcie_p2p negotiation today it looks somewhat like.

1) Importer and exporter agree at attach time what mappings are allowed
pending placement conditions and other attachers.
2) At map-time the mapping is negotiated taking all constraints into
account.
3) The negotiated mapping is valid until an invalidate_mappings()
callback is received.

Following this would allow a re-implementation of pcie_p2p as a mapping
type and would allow a controlled link-severing to send an
invalidate_mappings() notifier and the mappings to be re-negotiated.

Importers requiring an early hint as to what the actual mapping would
look like would need to be prepared to re-consider at map-time, with a
reasonable assumption that the reconsideration and re-mapping would
happen seldom enough.

Thanks,
Thomas



> 
> Regards,
> Christian.
> 
> > 
> > Thanks,
> > Karl