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

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Date: Thu, 6 Jul 2023 09:50:28 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: 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 Thu, 29 Jun 2023 19:27:46 -0700 Mina Almasry wrote:
> I've discussed your page pool frontend idea with our gve owners and
> the idea is attractive. In particular it would be good not to insert
> much custom code into the driver to support device memory pages or
> other page types. I plan on trying to change my approach to match the
> page pool provider you have in progress here:
> https://github.com/kuba-moo/linux/tree/pp-providers
> 
> In particular the main challenge right now seems to be that my device
> memory pages are ZONE_DEVICE pages, which can't be inserted to the
> page pool as-is due to the union in struct page between the page pool
> entries and the ZONE_DEVICE entries. I have some ideas on how to work
> around that I'm looking into.

Right, have you talked to Willem? I mentioned to him that I initially
pursued the idea of using the page pool as an abstraction but I
realized that fitting both the dma addr and the frag reference count
into struct page which isn't just a basic page will be a challenge.

So the second best thing seems to be to create an API which matches 
the page pool API, and have it select between a page pool and another
provider based on user configuration. As you said the main goal is to
be able to feed kernel / user / device memory to the driver without
having to modify driver code.

I thought this would live "above" the page pool (perhaps that's what
you mean by gen_pool) but Jesper brought up integrating it into the
(middle of?) page pool and have page pool switch on the pp_magic.
No strong preference on which on is better, seems like something that
can only be ironed out by modifying a couple of drivers to find out
what fits best :(

I'm hoping to jump back into the pp_provider work and finish that up
over the next few days, to at least post a PoC. Even if it doesn't work
for user / device memory - being able to feed the page pool from huge
pages is already a big win for the IOTLB.