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Subject: Re: [PATCH RFC 3/8] memory-provider: dmabuf devmem memory provider
To:     Jason Gunthorpe <jgg@nvidia.com>
CC:     Mina Almasry <almasrymina@google.com>,
        Jakub Kicinski <kuba@kernel.org>, <davem@davemloft.net>,
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        Jesper Dangaard Brouer <hawk@kernel.org>,
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        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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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <0a1cdd5a-c4c5-3d77-20a2-2beb8e3a6411@huawei.com>
Date:   Thu, 16 Nov 2023 19:10:01 +0800
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On 2023/11/15 21:38, Jason Gunthorpe wrote:
> On Wed, Nov 15, 2023 at 05:21:02PM +0800, Yunsheng Lin wrote:
> 
>>>>> I would expect net stack, page pool, driver still see the 'struct page',
>>>>> only memory provider see the specific struct for itself, for the above,
>>>>> devmem memory provider sees the 'struct page_pool_iov'.
>>>>>
>>>>> The reason I still expect driver to see the 'struct page' is that driver
>>>>> will still need to support normal memory besides devmem.
>>>
>>> I wouldn't say this approach is unreasonable, but it does have to be
>>> done carefully to isolate the mm. Keeping the struct page in the API
>>> is going to make this very hard.
>>
>> I would expect that most of the isolation is done in page pool, as far as
>> I can see:
> 
> It is the sort of thing that is important enough it should have
> compiler help via types to prove that it is being done
> properly. Otherwise it will be full of mistakes over time.

Yes, agreed.

I have done something similar as willy has done for some of
folio conversion as below:

+#define PAGE_POOL_MATCH(pg, iov)				\
+	static_assert(offsetof(struct page, pg) ==		\
+		      offsetof(struct page_pool_iov, iov))
+PAGE_POOL_MATCH(flags, res0);
+PAGE_POOL_MATCH(pp_magic, pp_magic);
+PAGE_POOL_MATCH(pp, pp);
...

Not sure if we need to add new API for driver to use when the
driver need the devmem support yet.


> 
> Jason
> .
>