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Date:   Thu, 16 Nov 2023 08:58:22 -0700
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Subject: Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider
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To:     Yunsheng Lin <linyunsheng@huawei.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mina Almasry <almasrymina@google.com>
Cc:     Jakub Kicinski <kuba@kernel.org>, davem@davemloft.net,
        pabeni@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Willem de Bruijn <willemb@google.com>,
        Kaiyuan Zhang <kaiyuanz@google.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Eric Dumazet <edumazet@google.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Linux-MM <linux-mm@kvack.org>
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 <20231113130041.58124-4-linyunsheng@huawei.com>
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 <ee10d050-ef24-49b2-8712-c9bc8a911c2a@kernel.org>
 <15c404e4-8efa-cc1c-174f-0752005b6755@huawei.com>
From:   David Ahern <dsahern@kernel.org>
In-Reply-To: <15c404e4-8efa-cc1c-174f-0752005b6755@huawei.com>
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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.