Re: [PATCH net-next v6 1/8] net: add get_netmem/put_netmem support

[Mina Almasry <almasrymina@google.com>]

On Fri, Feb 28, 2025 at 4:38 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Thu, 27 Feb 2025 04:12:02 +0000 Mina Almasry wrote:
> >  static inline void __skb_frag_ref(skb_frag_t *frag)
> >  {
> > -     get_page(skb_frag_page(frag));
> > +     get_netmem(skb_frag_netmem(frag));
> >  }
>
> Silently handling types of memory the caller may not be expecting
> always worries me.

Sorry, I'm not following. What caller is not expecting netmem? Here
we're making sure __skb_frag_ref() handles netmem correctly, i.e. we
were not expecting netmem here before, and after this patch we'll
handle it correctly.

> Why do we need this?
>

The MSG_ZEROCOPY TX path takes a page reference on the passed memory
in zerocopy_fill_skb_from_iter() that kfree_skb() later drops when the
skb is sent. We need an equivalent for netmem, which only supports pp
refs today. This is my attempt at implementing a page_ref equivalent
to net_iov and generic netmem.

I think __skb_frag_[un]ref is used elsewhere in the TX path too,
tcp_mtu_probe for example calls skb_frag_ref eventually.

> In general, I'm surprised by the lack of bug reports for devmem.

I guess we did a good job making sure we don't regress the page paths.

The lack of support in any driver that qemu will run is an issue. I
wonder if also the fact that devmem needs some setup is also an issue.
We need headersplit enabled, udmabuf created, netlink API bound, and
then a connection referring to created and we don't support loopback.
I think maybe it all may make it difficult for syzbot to repro. I've
had it on my todo list to investigate this more.

> Can you think of any way we could expose this more to syzbot?
> First thing that comes to mind is a simple hack in netdevsim,
> to make it insert a netmem handle (allocated locally, not a real
> memory provider), every N packets (controllable via debugfs).
> Would that work?

Yes, great idea. I don't see why it wouldn't work.

We don't expect mixing of net_iovs and pages in the same skb, but
netdevsim could create one net_iov skb every N skbs.

I guess I'm not totally sure something is discoverable to syzbot. Is a
netdevsim hack toggleable via a debugfs sufficient for syzbot? I'll
investigate and ask.

--
Thanks,
Mina