Date: Wed, 8 May 2024 12:30:07 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Mina Almasry <almasrymina@...gle.com>,
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Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
 custom page providers

On 5/8/24 00:32, Jason Gunthorpe wrote:
> On Tue, May 07, 2024 at 08:35:37PM +0100, Pavel Begunkov wrote:
>> On 5/7/24 18:56, Jason Gunthorpe wrote:
>>> On Tue, May 07, 2024 at 06:25:52PM +0100, Pavel Begunkov wrote:
>>>> On 5/7/24 17:48, Jason Gunthorpe wrote:
>>>>> On Tue, May 07, 2024 at 09:42:05AM -0700, Mina Almasry wrote:
>>>>>
>>>>>> 1. Align with devmem TCP to use udmabuf for your io_uring memory. I
>>>>>> think in the past you said it's a uapi you don't link but in the face
>>>>>> of this pushback you may want to reconsider.
>>>>>
>>>>> dmabuf does not force a uapi, you can acquire your pages however you
>>>>> want and wrap them up in a dmabuf. No uapi at all.
>>>>>
>>>>> The point is that dmabuf already provides ops that do basically what
>>>>> is needed here. We don't need ops calling ops just because dmabuf's
>>>>> ops are not understsood or not perfect. Fixup dmabuf.
>>>>
>>>> Those ops, for example, are used to efficiently return used buffers
>>>> back to the kernel, which is uapi, I don't see how dmabuf can be
>>>> fixed up to cover it.
>>>
>>> Sure, but that doesn't mean you can't use dma buf for the other parts
>>> of the flow. The per-page lifetime is a different topic than the
>>> refcounting and access of the entire bulk of memory.
>>
>> Ok, so if we're leaving uapi (and ops) and keep per page/sub-buffer as
>> is, the rest is resolving uptr -> pages, and passing it to page pool in
>> a convenient to page pool format (net_iov).
> 
> I'm not going to pretend to know about page pool details, but dmabuf
> is the way to get the bulk of pages into a pool within the net stack's
> allocator and keep that bulk properly refcounted while.> 
> An object like dmabuf is needed for the general case because there are
> not going to be per-page references or otherwise available.

They are already pinned, memory is owned by the provider, io_uring
in this case, and it should not be freed circumventing io_uring,
and at this stage calling release_pages() is not such a hassle,
especially comparing to introducing an additional object.

My question is how having an intermediary dmabuf benefits the net
stack or io_uring ? For now IMO it doesn't solve anything but adds
extra complexity. Adding dmabuf for the sake of adding dmabuf is
not a great choice.

> What you seem to want is to alter how the actual allocation flow works
> from that bulk of memory and delay the free. It seems like a different
For people who jumped here without looking what this patchset is
about, that's the entire point of the io_uring zero copy approach
as well as this set. Instead of using kernel private pages that you
have no other option but to copy/mmap (and then free), it hands
buffers to the user while using memory accessible/visible in some
way by the user.

That "delay free" is taking a reference while user is reading data
(slightly different for devmem tcp). And note, it's not a page/dmabuf
reference, kernel can forcibly take it back and release pages.

> topic to me, and honestly hacking into the allocator free function
> seems a bit weird..

Do you also think that DMA_BUF_IOCTL_SYNC is a weird hack, because
it "delays free" by pinning the dmabuf object and letting the user
read memory instead of copying it? I can find many examples

-- 
Pavel Begunkov

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