Re: [PATCH 0/4] dma-buf: add revoke mechanism to invalidate shared buffers

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Jan 12, 2026 at 03:56:32PM +0100, Christian König wrote:
> > The problem revoke is designed to solve is that many importers have
> > hardware that can either be DMA'ing or failing. There is no fault
> > mechanims that can be used to implement the full "move around for no
> > reason" semantics that are implied by move_notify.
> 
> In this case just call dma_buf_pin(). We already support that
> approach for RDMA devices which can't do ODP.

That alone isn't good enough - the patch adding the non-ODP support
also contained this:

static void
ib_umem_dmabuf_unsupported_move_notify(struct dma_buf_attachment *attach)
{
	struct ib_umem_dmabuf *umem_dmabuf = attach->importer_priv;

	ibdev_warn_ratelimited(umem_dmabuf->umem.ibdev,
			       "Invalidate callback should not be called when memory is pinned\n");
}

static struct dma_buf_attach_ops ib_umem_dmabuf_attach_pinned_ops = {
	.allow_peer2peer = true,
	.move_notify = ib_umem_dmabuf_unsupported_move_notify,
};

So we can't just allow it to attach to exporters that are going to
start calling move_notify while pinned.

Looking around I don't see anyone else doing something like this, and
reading your remarks I think EFA guys got it wrong. So I'm wondering
if this should not have been allowed. Unfortunately 5 years later I'm
pretty sure it is being used in places where we don't have HW support
to invalidate at all, and it is now well established uAPI that we
can't just break.

Which is why we are coming to negotiation because at least the above
isn't going to work if move_notify is called for revoke reasons, and
we'd like to block attaching exporters that need revoke for the above.

So, would you be happier with this if we documented that move_notify
can be called for pinned importers for revoke purposes and figure out
something to mark the above as special so exporters can fail pin if
they are going to call move_notify?

Then this series would transform into documentation, making VFIO
accept pin and continue to call move_notify as it does right now, and
some logic to reject the RDMA non-ODP importer.

Jason