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Date: Thu, 21 Dec 2023 23:23:43 +0000
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Subject: Re: [PATCH net-next v3 2/3] net: introduce abstraction for network memory
From: Shakeel Butt <shakeelb@google.com>
To: Mina Almasry <almasrymina@google.com>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, kvm@vger.kernel.org, 
	virtualization@lists.linux.dev, "David S. Miller" <davem@davemloft.net>, 
	Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, 
	Stefan Hajnoczi <stefanha@redhat.com>, Stefano Garzarella <sgarzare@redhat.com>, 
	David Howells <dhowells@redhat.com>, Jason Gunthorpe <jgg@nvidia.com>, 
	"Christian =?utf-8?B?S8O2bmln?=" <christian.koenig@amd.com>, Yunsheng Lin <linyunsheng@huawei.com>, 
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>
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On Wed, Dec 20, 2023 at 01:45:01PM -0800, Mina Almasry wrote:
> Add the netmem_ref type, an abstraction for network memory.
> 
> To add support for new memory types to the net stack, we must first
> abstract the current memory type. Currently parts of the net stack
> use struct page directly:
> 
> - page_pool
> - drivers
> - skb_frag_t
> 
> Originally the plan was to reuse struct page* for the new memory types,
> and to set the LSB on the page* to indicate it's not really a page.
> However, for compiler type checking we need to introduce a new type.
> 
> netmem_ref is introduced to abstract the underlying memory type. Currently
> it's a no-op abstraction that is always a struct page underneath. In
> parallel there is an undergoing effort to add support for devmem to the
> net stack:
> 
> https://lore.kernel.org/netdev/20231208005250.2910004-1-almasrymina@google.com/
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
> 
> v3:
> 
> - Modify struct netmem from a union of struct page + new types to an opaque
>   netmem_ref type.  I went with:
> 
>   +typedef void *__bitwise netmem_ref;
> 
>   rather than this that Jakub recommended:
> 
>   +typedef unsigned long __bitwise netmem_ref;
> 
>   Because with the latter the compiler issues warnings to cast NULL to
>   netmem_ref. I hope that's ok.
> 

Can you share what the warning was? You might just need __force
attribute. However you might need this __force a lot. I wonder if you
can just follow struct encoded_page example verbatim here.