netdev - Re: [RFC PATCH v2 06/11] page-pool: add device memory support

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Message-ID: <20230821143131.47de8f8f@kernel.org>
Date: Mon, 21 Aug 2023 14:31:31 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: David Ahern <dsahern@...nel.org>, Jesper Dangaard Brouer
 <jbrouer@...hat.com>, brouer@...hat.com, Mina Almasry
 <almasrymina@...gle.com>, netdev@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, Arnd Bergmann <arnd@...db.de>,
 Sumit Semwal <sumit.semwal@...aro.org>, Christian König
 <christian.koenig@....com>, Jason Gunthorpe <jgg@...pe.ca>, Hari
 Ramakrishnan <rharix@...gle.com>, Dan Williams <dan.j.williams@...el.com>,
 Andy Lutomirski <luto@...nel.org>, stephen@...workplumber.org,
 sdf@...gle.com
Subject: Re: [RFC PATCH v2 06/11] page-pool: add device memory support

On Sat, 19 Aug 2023 12:12:16 -0400 Willem de Bruijn wrote:
> :-) For the record, there is a prior version that added a separate type.
> 
> I did not like the churn it brought and asked for this.

It does end up looking cleaner that I personally expected, FWIW.

> > Use of the LSB (or bits depending on alignment expectations) is a common
> > trick and already done in quite a few places in the networking stack.
> > This trick is essential to any realistic change here to incorporate gpu
> > memory; way too much code will have unnecessary churn without it.

We'll end up needing the LSB trick either way, right? The only question
is whether the "if" is part of page pool or the caller of page pool.

Having seen zctap I'm afraid if we push this out of pp every provider
will end up re-implementing page pool's recycling/caching functionality
:(

Maybe we need to "fork" the API? The device memory "ifs" are only needed
for data pages. Which means that we can retain a faster, "if-less" API
for headers and XDP. Or is that too much duplication?