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

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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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