Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5] page_pool: remove PP_FLAG_PAGE_FRAG flag)

[Christian König]

Am 12.07.23 um 15:03 schrieb Jason Gunthorpe:
> On Wed, Jul 12, 2023 at 09:55:51AM +0200, Christian König wrote:
>
> > > Anyone see any glaring issues with this approach? I plan on trying to
> > > implement a PoC and sending an RFC v2.
> > Well we already have DMA-buf as user API for this use case, which is
> > perfectly supported by RDMA if I'm not completely mistaken.
> >
> > So what problem do you try to solve here actually?
> In a nutshell, netdev's design currently needs struct pages to do DMA
> to it's packet buffers.
>
> So it cannot consume the scatterlist that dmabuf puts out
>
> RDMA doesn't need struct pages at all, so it is fine.
>
> If Mina can go down the path of changing netdev to avoid needing
> struct pages then no changes to DRM side things.
>
> Otherwise a P2P struct page and a co-existance with netmem on a
> ZONE_DEVICE page would be required. :\

Uff, depending on why netdev needs struct page (I think I have a good 
idea why) this isn't really going to work generically either way.

What we maybe able to do is to allow copy_file_range() between DMA-buf 
file descriptor and a TCP socket.

If I'm not completely mistaken that should then end up in DMA-bufs 
file_operations->copy_file_range callback (maybe with some minor change 
to allows this).

The DMA-buf framework could then forward this to the exporter of the 
memory which owns the backing memory could then do the necessary steps.

Regards,
Christian.

> Jason