netdev - Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5] page_pool: remove PP_FLAG_PAGE

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Date: Wed, 12 Jul 2023 15:35:14 +0200
From: Christian König <christian.koenig@....com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Mina Almasry <almasrymina@...gle.com>, David Ahern <dsahern@...nel.org>,
 Samiullah Khawaja <skhawaja@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Christoph Hellwig <hch@....de>, John Hubbard <jhubbard@...dia.com>,
 Dan Williams <dan.j.williams@...el.com>,
 Jesper Dangaard Brouer <jbrouer@...hat.com>, brouer@...hat.com,
 Alexander Duyck <alexander.duyck@...il.com>,
 Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
 pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Lorenzo Bianconi <lorenzo@...nel.org>, Yisen Zhuang
 <yisen.zhuang@...wei.com>, Salil Mehta <salil.mehta@...wei.com>,
 Eric Dumazet <edumazet@...gle.com>, Sunil Goutham <sgoutham@...vell.com>,
 Geetha sowjanya <gakula@...vell.com>, Subbaraya Sundeep
 <sbhatta@...vell.com>, hariprasad <hkelam@...vell.com>,
 Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 Felix Fietkau <nbd@....name>, Ryder Lee <ryder.lee@...iatek.com>,
 Shayne Chen <shayne.chen@...iatek.com>, Sean Wang <sean.wang@...iatek.com>,
 Kalle Valo <kvalo@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>, linux-rdma@...r.kernel.org,
 linux-wireless@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org, Jonathan Lemon
 <jonathan.lemon@...il.com>, logang@...tatee.com,
 Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)

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