Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for custom page providers

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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 don't see how dmabuf would
help here. Adding dmabuf in the middle (internally wrapping pages) would
add more setup code with the same final result, that is a format that
page pool can work with. And for io_uring it's normal user memory. We'll
have to use dmabuf when we'd want to extend to peer-to-peer and all that
fun, but that's a small fraction of it, and we'll hopefully reuse some
setup helpers from devmem tcp.

Pavel Begunkov

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