On 6/7/24 17:59, Mina Almasry wrote:

On Fri, Jun 7, 2024 at 8:47 AM Pavel Begunkov [off-list ref] wrote:

quoted

On 6/7/24 16:42, Pavel Begunkov wrote:

quoted

On 6/7/24 15:27, David Ahern wrote:

quoted

On 6/7/24 7:42 AM, Pavel Begunkov wrote:

quoted

I haven't seen any arguments against from the (net) maintainers so
far. Nor I see any objection against callbacks from them (considering
that either option adds an if).

I have said before I do not understand why the dmabuf paradigm is not
sufficient for both device memory and host memory. A less than ideal
control path to put hostmem in a dmabuf wrapper vs extra checks and
changes in the datapath. The former should always be preferred.

If we're talking about types of memory specifically, I'm not strictly
against wrapping into dmabuf in kernel, but that just doesn't give
anything.

And the reason I don't have too strong of an opinion on that is
mainly because it's just setup/cleanup path.

I agree wrapping io uring in dmabuf seems to be an unnecessary detour.
I never understood the need or upside to do that, but it could be a
lack of understanding on my part.

However, the concern that David brings up may materialize. I've had to
spend a lot of time minimizing or justifying checks to the code with
page pool benchmarks that detect even 1 cycle regressions. You may be
asked to run the same benchmarks and minimize similar overhead.

The benchmark in question is Jesper's bench_page_pool_simple. I've
forked it and applied it on top of net-next here:
https://github.com/mina/linux/commit/927596f87ab5791a8a6ba8597ba2189747396e54

As io_uring ZC comes close to merging, I suspect it would be good to
run this to understand the regression in the fast path, if any. If
there are no to little regressions, I have no concerns over io uring
memory not being wrapped in dmabufs, and David may agree as well.

That's the easiest part as io_uring only reusing call points
you added for devmem and thus doesn't add anything new on top
to hot paths.

-- 
Pavel Begunkov