Date: Mon, 17 Jun 2024 20:15:17 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: David Ahern <dsahern@...nel.org>, David Wei <dw@...idwei.uk>,
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Subject: Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for
 custom page providers

On 6/10/24 23:15, Jason Gunthorpe wrote:
> On Mon, Jun 10, 2024 at 08:20:08PM +0100, Pavel Begunkov wrote:
>> On 6/10/24 16:16, David Ahern wrote:
> 
>>>> 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.
>>
>> Let's not mix up devmem TCP and dmabuf specifically, as I see it
>> your question was concerning the latter: "... DMABUF memory registered
>> through Mina's mechanism". io_uring's zcrx can trivially get dmabuf
>> support in future, as mentioned it's mostly the setup side. ABI,
>> buffer workflow and some details is a separate issue, and I don't
>> see how further integration aside from what we're already sharing
>> is beneficial, on opposite it'll complicate things.
> 
> Again, I am talking about composability here, duplicating the DMABUF
> stuff into io_uring is not composable, it is just duplicating things.

Ok, then registering, say, a dmabuf via devmem TCP and then using it
in io_uring. Let's say we make devmem TCP API to be able to register
a dmabuf without using it, from where io_uring can take ownership
over it and use in the flow. And I strongly believe the same memory
region/dmabuf should never be used by both at the same time and hence
lifetime of any such memory should be exclusively bound to io_uring.

That leaves the user api, where to add memory you need to create
a netlink socket and pass everything through it, which is an extra
step, and then letting know io_uring that it can use the memory, not
forgetting to eject it from netlink. That's not a good api as far as
it goes with io_uring.

I don't think slight duplicating of registration is a problem when
the upside is much cleaner API. Internals, however, can be easily
shared. We can even say that the net stack should provide helpers
like init_page_pool_from_dmabuf_fd() and now allow poking into
related bits aside from it (initialising net_iov / etc.).

> It does not match the view that there should be two distinct layers
> here, one that provides the pages and one that manages the
> lifecycle. As HCH pushes for pages either come from the allocator and
> get to use the struct folio or the come from a dmabuf and they
> don't. That is it, the only two choices.
> 
> The iouring stuff is trying to confuse the source of the pages with
> the lifecycle - which is surely convenient, but is why Christoph is
> opposing it.

-- 
Pavel Begunkov

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