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 10:11:25 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Christoph Hellwig <hch@....de>, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        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 Mon, Jul 10, 2023 at 05:45:05PM -0700, Mina Almasry wrote:

> > At least from my position I want to see MEMORY_DEVICE_PCI_P2PDMA used
> > to represent P2P memory.
> 
> Would using p2pdma API instead of dmabuf be an acceptable direction?

"p2pdma API" is really just using MEMORY_DEVICE_PCI_P2PDMA and
teaching the pagepool how to work with ZONE_DEVICE pages.

I suspect this will clash badly with Matthew's work here:

https://lore.kernel.org/all/20230111042214.907030-1-willy@infradead.org/

As from a mm side we haven't ever considered that ZONE_DEVICE and
"netmem" can be composed together. The entire point of netmem like
stuff is that the allocator hands over the majority of struct page to
the allocatee, and ZONE_DEVICE can't work like that. 

However, assuming that can be solved in some agreeable way then it
would be OK to go down this path.

But, I feel like this is just overall too hard a direction from the mm
perspective.

I don't know anything about page pool, but the main sticking point is
its reliance on struct page. If it can find another way to locate its
meta data (eg an xarray), at least for some cases, it would make
things alot easier.

Jason