Re: [PATCH v1 06/15] net: page_pool: add ->scrub mem provider callback

Mina Almasry
2024-10-10T17:54:19Z

On Wed, Oct 9, 2024 at 2:58 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 10/9/24 22:00, Mina Almasry wrote:
> > On Mon, Oct 7, 2024 at 3:16 PM David Wei <dw@davidwei.uk> wrote:
> >>
> >> From: Pavel Begunkov <asml.silence@gmail.com>
> >>
> >> page pool is now waiting for all ppiovs to return before destroying
> >> itself, and for that to happen the memory provider might need to push
> >> some buffers, flush caches and so on.
> >>
> >> todo: we'll try to get by without it before the final release
> >>
> >
> > Is the intention to drop this todo and stick with this patch, or to
> > move ahead with this patch?
>
> Heh, I overlooked this todo. The plan is to actually leave it
> as is, it's by far the simplest way and doesn't really gets
> into anyone's way as it's a slow path.
>
> > To be honest, I think I read in a follow up patch that you want to
> > unref all the memory on page_pool_destory, which is not how the
> > page_pool is used today. Tdoay page_pool_destroy does not reclaim
> > memory. Changing that may be OK.
>
> It doesn't because it can't (not breaking anything), which is a
> problem as the page pool might never get destroyed. io_uring
> doesn't change that, a buffer can't be reclaimed while anything
> in the kernel stack holds it. It's only when it's given to the
> user we can force it back out of there.
>
> And it has to happen one way or another, we can't trust the
> user to put buffers back, it's just devmem does that by temporarily
> attaching the lifetime of such buffers to a socket.
>

(noob question) does io_uring not have a socket equivalent that you
can tie the lifetime of the buffers to? I'm thinking there must be
one, because in your patches IIRC you have the fill queues and the
memory you bind from the userspace, there should be something that
tells you that the userspace has exited/crashed and it's time to now
destroy the fill queue and unbind the memory, right?

I'm thinking you may want to bind the lifetime of the buffers to that,
instead of the lifetime of the pool. The pool will not be destroyed
until the next driver/reset reconfiguration happens, right? That could
be long long after the userspace has stopped using the memory.

--
Thanks,
Mina