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

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Date:   Tue, 11 Jul 2023 10:06:28 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Ahern <dsahern@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
        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>
Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)

On Tue, Jul 11, 2023 at 9:32 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 11 Jul 2023 10:20:58 -0600 David Ahern wrote:
> > On 7/11/23 10:00 AM, Jakub Kicinski wrote:
> > >> RDMA works with the AMD and Intel intree drivers using DMABUF without
> > >> requiring struct pages using the DRM hacky scatterlist approach.
> > > I see, thanks. We need pages primarily for refcounting. Avoiding all
> > > the infamous problems with memory pins. Oh well.
> >
> > io_uring for example already manages the page pinning. An skb flag was
> > added for ZC Tx API to avoid refcounting in the core networking layer.
>
> Right, we can refcount in similar fashion. Still tracking explicitly
> when buffers are handed over to the NIC.
>
> > Any reason not to allow an alternative representation for skb frags than
> > struct page?
>
> I don't think there's a hard technical reason. We can make it work.

I also think we can switch the representation for skb frags to
something else. However - please do correct me if I'm wrong - I don't
think that is sufficient for device memory TCP. My understanding is
that we also need to modify any NIC drivers that want to use device
memory TCP to understand a new memory type, and the page pool as well
if that's involved. I think in particular modifying the memory type in
all the NIC drivers that want to do device memory TCP is difficult. Do
you think this is feasible?

--
Thanks,
Mina