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Date: Wed, 8 May 2024 12:35:52 +0100
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Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
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From: Pavel Begunkov <asml.silence@gmail.com>
In-Reply-To: <Zjsm3vO6rIY_sw5A@phenom.ffwll.local>
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On 5/8/24 08:16, Daniel Vetter wrote:
> On Tue, May 07, 2024 at 08:32:47PM -0300, 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.
>>
>> 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
>> topic to me, and honestly hacking into the allocator free function
>> seems a bit weird..
> 
> Also I don't see how it's an argument against dma-buf as the interface for

It's not, neither I said it is, but it is an argument against removing
the network's page pool ops.

> all these, because e.g. ttm internally does have a page pool because
> depending upon allocator, that's indeed beneficial. Other drm drivers have
> more buffer-based concepts for opportunistically memory around, usually
> by marking buffers that are just kept as cache as purgeable (which is a
> concept that goes all the way to opengl/vulkan).

Because in this case it solves nothing and helps with nothing, quite
the opposite. Just as well we can ask why NVMe doesn't wrap user pages
into a dmabuf while doing IO.

> But these are all internals of the dma-buf exporter, the dma-buf api users
> don't ever need to care.
> -Sima

-- 
Pavel Begunkov