linux-kernel - Re: [PATCH net-next v9 1/9] netmem: add niov->type attribute to distinguish different net

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Message-ID: <CAHS8izMFxDG5E07ZdqnDH_2D_g1fW8X0M7u3gGyV8efzxDNZbg@mail.gmail.com>
Date: Tue, 22 Apr 2025 07:03:26 -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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	Jeroen de Borst <jeroendb@...gle.com>, Harshitha Ramamurthy <hramamurthy@...gle.com>, 
	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 1/9] netmem: add niov->type attribute to
 distinguish different net_iov types

On Tue, Apr 22, 2025 at 1:16 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 4/18/25 00:15, Mina Almasry wrote:
> > Later patches in the series adds TX net_iovs where there is no pp
> > associated, so we can't rely on niov->pp->mp_ops to tell what is the
> > type of the net_iov.
>
> That's fine, but that needs a NULL pp check in io_uring as well,
> specifically in io_zcrx_recv_frag().
>

I think you mean this update in the code:

if (!niov->pp || niov->pp->mp_ops != &io_uring_pp_zc_ops ||
    io_pp_to_ifq(niov->pp) != ifq)
return -EFAULT;

Yes, thanks, will do.

> You can also move it to struct net_iov_area and check niov->owner->type
> instead. It's a safer choice than aliasing with struct page, there is
> no cost as you're loading ->owner anyway (e.g. for
> net_iov_virtual_addr()), and it's better in terms of normalisation /
> not unnecessary duplicating it, assuming we'll never have niovs of
> different types bound to the same struct net_iov_area.
>

Putting it in niov->owner->type is an alternative approach. I don't
see a strong reason to go with one over the other. I'm thinking there
will be fast code paths that want to know the type of the frag or skb
and don't need the owner, so it will be good to save loading another
cacheline. We have more space in struct net_iov than we know what to
do with anyway.


--
Thanks,
Mina