[RFC] Make use of non-dynamic dmabuf in RDMA

[Alex Deucher]

On Tue, Aug 24, 2021 at 3:16 PM Dave Airlie <airlied at gmail.com> wrote:
>
> On Wed, 25 Aug 2021 at 03:36, John Hubbard <jhubbard at nvidia.com> wrote:
> >
> > On 8/24/21 10:32 AM, Jason Gunthorpe wrote:
> > ...
> > >>> And yes at least for the amdgpu driver we migrate the memory to host
> > >>> memory as soon as it is pinned and I would expect that other GPU drivers
> > >>> do something similar.
> > >>
> > >> Well...for many topologies, migrating to host memory will result in a
> > >> dramatically slower p2p setup. For that reason, some GPU drivers may
> > >> want to allow pinning of video memory in some situations.
> > >>
> > >> Ideally, you've got modern ODP devices and you don't even need to pin.
> > >> But if not, and you still hope to do high performance p2p between a GPU
> > >> and a non-ODP Infiniband device, then you would need to leave the pinned
> > >> memory in vidmem.
> > >>
> > >> So I think we don't want to rule out that behavior, right? Or is the
> > >> thinking more like, "you're lucky that this old non-ODP setup works at
> > >> all, and we'll make it work by routing through host/cpu memory, but it
> > >> will be slow"?
> > >
> > > I think it depends on the user, if the user creates memory which is
> > > permanently located on the GPU then it should be pinnable in this way
> > > without force migration. But if the memory is inherently migratable
> > > then it just cannot be pinned in the GPU at all as we can't
> > > indefinately block migration from happening eg if the CPU touches it
> > > later or something.
> > >
> >
> > OK. I just want to avoid creating any API-level assumptions that dma_buf_pin()
> > necessarily implies or requires migrating to host memory.
>
> I'm not sure we should be allowing dma_buf_pin at all on
> non-migratable memory, what's to stop someone just pinning all the
> VRAM and making the GPU unuseable?

In a lot of cases we have GPUs with more VRAM than system memory, but
we allow pinning in system memory.

Alex

>
> I understand not considering more than a single user in these
> situations is enterprise thinking, but I do worry about pinning is
> always fine type of thinking when things are shared or multi-user.
>
> My impression from this is we've designed hardware that didn't
> consider the problem, and now to let us use that hardware in horrible
> ways we should just allow it to pin all the things.
>
> Dave.