netdev - Re: [PATCH net-next v9 2/9] net: add get_netmem/put

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Message-ID: <CAHS8izM8+zG6KOhV7ysTsCj_PEty5eL+P+uUxTZhdsOSZTwmow@mail.gmail.com>
Date: Tue, 22 Apr 2025 11:30:54 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, io-uring@...r.kernel.org, 
	virtualization@...ts.linux.dev, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
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	Kuniyuki Iwashima <kuniyu@...zon.com>, Willem de Bruijn <willemb@...gle.com>, Jens Axboe <axboe@...nel.dk>, 
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	sdf@...ichev.me, dw@...idwei.uk, Jamal Hadi Salim <jhs@...atatu.com>, 
	Victor Nogueira <victor@...atatu.com>, Pedro Tammela <pctammela@...atatu.com>, 
	Samiullah Khawaja <skhawaja@...gle.com>
Subject: Re: [PATCH net-next v9 2/9] net: add get_netmem/put_netmem support

On Tue, Apr 22, 2025 at 11:19 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 4/22/25 14:56, Mina Almasry wrote:
> > On Tue, Apr 22, 2025 at 1:43 AM Pavel Begunkov <asml.silence@...il.com> wrote:
> >>
> >> On 4/18/25 00:15, Mina Almasry wrote:
> >>> Currently net_iovs support only pp ref counts, and do not support a
> >>> page ref equivalent.
> >>
> >> Makes me wonder why it's needed. In theory, nobody should ever be
> >> taking page references without going through struct ubuf_info
> >> handling first, all in kernel users of these pages should always
> >> be paired with ubuf_info, as it's user memory, it's not stable,
> >> and without ubuf_info the user is allowed to overwrite it.
> >>
> >
> > The concern about the stability of the from-userspace data is already
> > called out in the MSG_ZEROCOPY documentation that we're piggybacking
> > devmem TX onto:
>
> Sure, I didn't object that. There is no problem as long as the
> ubuf_info semantics is followed, which by extension mean that
> any ref manipulation should already be gated on ubuf_info, and
> there should be no need in changing generic paths.
>

I'm sorry I'm not following. skb_frag_ref is how the net stack obtains
references on an skb_frag, regardless on whether the frag is a
MSG_ZEROCOPY one with ubuf info, or a regular tx frag without a
ubuf_info, or even an io_uring frag which I think have the
msg->ubuf_info like we discussed previously. I don't see the net stack
in the current code special casing how it obtains refs on frags, and I
don't see the need to add special casing. Can you elaborate in more
detail what is the gating you expect, and why? Are you asking that I
check the skb has a ubuf_info before allowing to grab the reference on
the dmabuf binding? Or something else?


-- 
Thanks,
Mina