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

[Mina Almasry <almasrymina@google.com>]

On Fri, Nov 1, 2024 at 2:38 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 11/1/24 19:24, Mina Almasry wrote:
> > On Fri, Nov 1, 2024 at 11:34 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> ...
> >>> Huh, interesting. For devmem TCP we bind a region of memory to the
> >>> queue once, and after that we can create N connections all reusing the
> >>> same memory region. Is that not the case for io_uring? There are no
> >>
> >> Hmm, I think we already discussed the same question before. Yes, it
> >> does indeed support arbitrary number of connections. For what I was
> >> saying above, the devmem TCP analogy would be attaching buffers to the
> >> netlink socket instead of a tcp socket (that new xarray you added) when
> >> you give it to user space. Then, you can close the connection after a
> >> receive and the buffer you've got would still be alive.
> >>
> >
> > Ah, I see. You're making a tradeoff here. You leave the buffers alive
> > after each connection so the userspace can still use them if it wishes
> > but they are of course unavailable for other connections.
> >
> > But in our case (and I'm guessing yours) the process that will set up
> > the io_uring memory provider/RSS/flow steering will be a different
> > process from the one that sends/receive data, no? Because the former
> > requires CAP_NET_ADMIN privileges while the latter will not. If they
> > are 2 different processes, what happens when the latter process doing
> > the send/receive crashes? Does the memory stay unavailable until the
> > CAP_NET_ADMIN process exits? Wouldn't it be better to tie the lifetime
> > of the buffers of the connection? Sure, the buffers will become
>
> That's the tradeoff google is willing to do in the framework,
> which is fine, but it's not without cost, e.g. you need to
> store/erase into the xarray, and it's a design choice in other
> aspects, like you can't release the page pool if the socket you
> got a buffer from is still alive but the net_iov hasn't been
> returned.
>
> > unavailable after the connection is closed, but at least you don't
> > 'leak' memory on send/receive process crashes.
> >
> > Unless of course you're saying that only CAP_NET_ADMIN processes will
>
> The user can pass io_uring instance itself
>

Thanks, but sorry, my point still stands. If the CAP_NET_ADMIN passes
the io_uring instance to the process doing the send/receive, then the
latter process crashes, do the io_uring netmems leak until the
page_pool is destroyed? Or are they cleaned up because the io_uring
instance is destroyed with the process crashing, and the io_uring will
destroy the page_pool on exit?

-- 
Thanks,
Mina