linux-kernel - Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for custom page providers

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Date: Mon, 10 Jun 2024 14:38:18 +0200
From: Christian König <christian.koenig@....com>
To: Jason Gunthorpe <jgg@...pe.ca>, Pavel Begunkov <asml.silence@...il.com>
Cc: David Wei <dw@...idwei.uk>, David Ahern <dsahern@...nel.org>,
 Mina Almasry <almasrymina@...gle.com>, Christoph Hellwig
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Subject: Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for
 custom page providers

Am 10.06.24 um 14:16 schrieb Jason Gunthorpe:
> On Mon, Jun 10, 2024 at 02:07:01AM +0100, Pavel Begunkov wrote:
>> On 6/10/24 01:37, David Wei wrote:
>>> On 2024-06-07 17:52, Jason Gunthorpe wrote:
>>>> IMHO it seems to compose poorly if you can only use the io_uring
>>>> lifecycle model with io_uring registered memory, and not with DMABUF
>>>> memory registered through Mina's mechanism.
>>> By this, do you mean io_uring must be exclusively used to use this
>>> feature?
>>>
>>> And you'd rather see the two decoupled, so userspace can register w/ say
>>> dmabuf then pass it to io_uring?
>> Personally, I have no clue what Jason means. You can just as
>> well say that it's poorly composable that write(2) to a disk
>> cannot post a completion into a XDP ring, or a netlink socket,
>> or io_uring's main completion queue, or name any other API.
> There is no reason you shouldn't be able to use your fast io_uring
> completion and lifecycle flow with DMABUF backed memory. Those are not
> widly different things and there is good reason they should work
> together.

Well there is the fundamental problem that you can't use io_uring to 
implement the semantics necessary for a dma_fence.

That's why we had to reject the io_uring work on DMA-buf sharing from 
Google a few years ago.

But this only affects the dma_fence synchronization part of DMA-buf, but 
*not* the general buffer sharing.

Regards,
Christian.

>
> Pretending they are totally different just because two different
> people wrote them is a very siloed view.
>
>> The devmem TCP callback can implement it in a way feasible to
>> the project, but it cannot directly post events to an unrelated
>> API like io_uring. And devmem attaches buffers to a socket,
>> for which a ring for returning buffers might even be a nuisance.
> If you can't compose your io_uring completion mechanism with a DMABUF
> provided backing store then I think it needs more work.
>
> Jason