netdev - Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider

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Date: Thu, 16 Nov 2023 08:58:22 -0700
From: David Ahern <dsahern@...nel.org>
To: Yunsheng Lin <linyunsheng@...wei.com>, Jason Gunthorpe <jgg@...dia.com>,
 Mina Almasry <almasrymina@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net, pabeni@...hat.com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Willem de Bruijn <willemb@...gle.com>, Kaiyuan Zhang <kaiyuanz@...gle.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Eric Dumazet <edumazet@...gle.com>,
 Christian König <christian.koenig@....com>,
 Matthew Wilcox <willy@...radead.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider

On 11/16/23 4:12 AM, Yunsheng Lin wrote:
> On 2023/11/16 1:57, David Ahern wrote:
>> On 11/15/23 2:21 AM, Yunsheng Lin wrote:
>>> On 2023/11/14 21:16, Jason Gunthorpe wrote:
>>>> On Tue, Nov 14, 2023 at 04:21:26AM -0800, Mina Almasry wrote:
>>>>
>>>>> Actually because you put the 'strtuct page for devmem' in
>>>>> skb->bv_frag, the net stack will grab the 'struct page' for devmem
>>>>> using skb_frag_page() then call things like page_address(), kmap,
>>>>> get_page, put_page, etc, etc, etc.
>>>>
>>>> Yikes, please no. If net has its own struct page look alike it has to
>>>> stay entirely inside net. A non-mm owned struct page should not be
>>>> passed into mm calls. It is just way too hacky to be seriously
>>>> considered :(
>>>
>>> Yes, that is something this patchset is trying to do, defining its own
>>> struct page look alike for page pool to support devmem.
>>>
>>
>> Networking needs to be able to move away from struct page references.
>> The devmem and host memory for Rx use cases do not need to be page based.
> 
> Yes, I am agreed the ultimate goal is to move away from struct page
> references. But I am not sure if we can do it right away as there
> still are different types of existing 'struct page' in the netstack,
> see:
> 
> https://lore.kernel.org/all/8b7d25eb-1f10-3e37-8753-92b42da3fb34@huawei.com/

yes, that is the point of a blended approach -- pages and buffers (or
iov) -- leveraging the LSB of the address. That proposal is the right
direction to be moving for co-existence. Adding fake struct page
instances is the wrong direction.