netdev - Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

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Date:   Mon, 29 Mar 2021 18:23:15 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow
 path

On Mon, 2021-03-29 at 11:24 -0400, Willem de Bruijn wrote:
> On Mon, Mar 29, 2021 at 11:01 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > On Mon, 2021-03-29 at 09:52 -0400, Willem de Bruijn wrote:
> > > > +       if (skb->ip_summed == CHECKSUM_NONE && !skb->csum_valid)
> > > > +               skb->csum_valid = 1;
> > > 
> > > Not entirely obvious is that UDP packets arriving on a device with rx
> > > checksum offload off, i.e., with CHECKSUM_NONE, are not matched by
> > > this test.
> > > 
> > > I assume that such packets are not coalesced by the GRO layer in the
> > > first place. But I can't immediately spot the reason for it..
> 
> As you point out, such packets will already have had their checksum
> verified at this point, so this branch only matches tunneled packets.
> That point is just not immediately obvious from the code.

I understand is a matter of comment clarity ?!?

I'll rewrite the related code comment - in udp_post_segment_fix_csum()
- as:

	/* UDP packets generated with UDP_SEGMENT and traversing:
	 *
         * UDP tunnel(xmit) -> veth (segmentation) -> veth (gro) -> UDP tunnel (rx)
	 * 
         * land here with CHECKSUM_NONE, because __iptunnel_pull_header() converts
         * CHECKSUM_PARTIAL into NONE.
	 * SKB_GSO_UDP_L4 or SKB_GSO_FRAGLIST packets with no UDP tunnel will land
	 * here with valid checksum, as the GRO engine validates the UDP csum
	 * before the aggregation and nobody strips such info in between.
	 * Instead of adding another check in the tunnel fastpath, we can force
	 * a valid csum here.
         * Additionally fixup the UDP CB.
         */

Would that be clear enough?

> > I do see checksum validation in the GRO engine for CHECKSUM_NONE UDP
> > packet prior to this series.
> > 
> > I *think* the checksum-and-copy optimization is lost
> > since 573e8fca255a27e3573b51f9b183d62641c47a3d.
> 
> Wouldn't this have been introduced with UDP_GRO?

Uhmm.... looks like the checksum-and-copy optimization has been lost
and recovered a few times. I think the last one
with 9fd1ff5d2ac7181844735806b0a703c942365291, which move the csum
validation before the static branch on udp_encap_needed_key.

Can we agree re-introducing the optimization is independent from this
series?

Thanks!

Paolo