Re: [PATCH v7 11/15] io_uring/zcrx: implement zerocopy receive pp memory provider

[Mina Almasry <almasrymina@google.com>]

On Mon, Nov 11, 2024 at 1:15 PM David Wei <dw@davidwei.uk> wrote:
> > Is it very complicated to napi_pp_put_page() niovs as the user puts
> > them in the refill queue without adding a new syscall? If so, is it
> > possible to do a niov equivalent of page_pool_put_page_bulk() of the
> > refill queue while/as you process the RX path?
> >
> > If you've tested the generic code paths to be performance deficient
> > and your recycling is indeed better, you could improve the page_pool
> > to pull netmems when it needs to like you're doing here, but in a
> > generic way that applies to the page allocator and other providers.
> > Not a one-off implementation that only applies to your provider.
> >
> > If you're absolutely set on ignoring the currently supported reffing
> > and implementing your own reffing and recycling for your use case,
> > sure, that could work, but please don't overload the
> > niov->pp_ref_count reserved for the generic use cases for this. Add
> > io_zcrx_area->io_uring_ref or something and do whatever you want with
> > it. Since it's not sharing the pp_ref_count with the generic code
> > paths I don't see them conflicting in the future.
>
> Why insist on this? Both page/niov and devmem/io_uring niov are mutually
> exclusive. There is no strong technical reason to not re-use
> pp_ref_count.
>

Conflict between devmem (specifically) and io_uring is not my concern.
My concern is possible future conflict between io_uring refcounting
and page/devmem refcounting.

Currently net_iov refcounting and page refcounting is unified (as much
as possible). net_iov->pp_ref_count is an exact mirror in usage of
page->pp_ref_count and the intention is for it to remain so. This
patch reuses net_iov->pp_ref_count in a way that doesn't really apply
to page->pp_ref_count and already some deviation in use case is
happening which may lead to conflicting requirements in the future
(and to be fair, may not).

But I've been bringing this up a lot in the past (and offering
alternatives that don't introduce this overloading) and I think this
conversation has run its course. I'm unsure about this approach and
this could use another pair of eyes. Jakub, sorry to bother you but
you probably are the one that reviewed the whole net_iov stuff most
closely. Any chance you would take a look and provide direction here?
Maybe my concern is overblown...

--
Thanks,
Mina