netdev - Re: [PATCH net] net: Fix gro aggregation for udp encaps with zero csum

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Date:   Sun, 28 Feb 2021 12:02:08 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     John Fastabend <john.fastabend@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net] net: Fix gro aggregation for udp encaps with zero
 csum

On Sat, 27 Feb 2021 09:16:38 -0800 John Fastabend wrote:
> Willem de Bruijn wrote:
> > On Fri, Feb 26, 2021 at 4:23 PM Daniel Borkmann <daniel@...earbox.net> wrote:  
> > >
> > > We noticed a GRO issue for UDP-based encaps such as vxlan/geneve when the
> > > csum for the UDP header itself is 0. In that case, GRO aggregation does
> > > not take place on the phys dev, but instead is deferred to the vxlan/geneve
> > > driver (see trace below).
> > >
> > > The reason is essentially that GRO aggregation bails out in udp_gro_receive()
> > > for such case when drivers marked the skb with CHECKSUM_UNNECESSARY (ice, i40e,
> > > others) where for non-zero csums 2abb7cdc0dc8 ("udp: Add support for doing
> > > checksum unnecessary conversion") promotes those skbs to CHECKSUM_COMPLETE
> > > and napi context has csum_valid set. This is however not the case for zero
> > > UDP csum (here: csum_cnt is still 0 and csum_valid continues to be false).
> > >
> > > At the same time 57c67ff4bd92 ("udp: additional GRO support") added matches
> > > on !uh->check ^ !uh2->check as part to determine candidates for aggregation,
> > > so it certainly is expected to handle zero csums in udp_gro_receive(). The
> > > purpose of the check added via 662880f44203 ("net: Allow GRO to use and set
> > > levels of checksum unnecessary") seems to catch bad csum and stop aggregation
> > > right away.
> > >
> > > One way to fix aggregation in the zero case is to only perform the !csum_valid
> > > check in udp_gro_receive() if uh->check is infact non-zero.
> >
> > Acked-by: Willem de Bruijn <willemb@...gle.com>  
> 
> Acked-by: John Fastabend <john.fastabend@...il.com>

Applied, thanks!