netdev - Re: [net-next v1 08/16] memory-provider: dmabuf devmem memory provider

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Date: Tue, 12 Dec 2023 06:26:51 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Shailend Chand <shailend@...gle.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	bpf@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Jeroen de Borst <jeroendb@...gle.com>, 
	Praveen Kaligineedi <pkaligineedi@...gle.com>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, Arnd Bergmann <arnd@...db.de>, 
	David Ahern <dsahern@...nel.org>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	Shuah Khan <shuah@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, 
	Yunsheng Lin <linyunsheng@...wei.com>, Harshitha Ramamurthy <hramamurthy@...gle.com>, 
	Shakeel Butt <shakeelb@...gle.com>, Willem de Bruijn <willemb@...gle.com>, 
	Kaiyuan Zhang <kaiyuanz@...gle.com>, Christoph Hellwig <hch@...radead.org>
Subject: Re: [net-next v1 08/16] memory-provider: dmabuf devmem memory provider

On Tue, Dec 12, 2023 at 4:25 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Thu, Dec 07, 2023 at 04:52:39PM -0800, Mina Almasry wrote:
>
> > +static inline struct page_pool_iov *page_to_page_pool_iov(struct page *page)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return (struct page_pool_iov *)((unsigned long)page & ~PP_IOV);
> > +
> > +     DEBUG_NET_WARN_ON_ONCE(true);
> > +     return NULL;
> > +}
>
> We already asked not to do this, please do not allocate weird things
> can call them 'struct page' when they are not. It undermines the
> maintainability of the mm to have things mis-typed like
> this. Introduce a new type for your thing so the compiler can check it
> properly.
>

There is a new type introduced, it's the page_pool_iov. We set the LSB
on page_pool_iov* and cast it to page* only to avoid the churn of
renaming page* to page_pool_iov* in the page_pool and all the net
drivers using it. Is that not a reasonable compromise in your opinion?
Since the LSB is set on the resulting page pointers, they are not
actually usuable as pages, and are never passed to mm APIs per your
requirement.

-- 
Thanks,
Mina