netdev - Re: [PATCH net-next v3 4/4] net: gro: move L3 flush checks to tcp_gro

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Date: Mon, 11 Mar 2024 08:11:50 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Richard Gobert <richardbgobert@...il.com>, 
 Eric Dumazet <edumazet@...gle.com>
Cc: davem@...emloft.net, 
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Subject: Re: [PATCH net-next v3 4/4] net: gro: move L3 flush checks to
 tcp_gro_receive

Richard Gobert wrote:
> Eric Dumazet wrote:
> > On Sat, Mar 9, 2024 at 4:35 PM Richard Gobert <richardbgobert@...il.com> wrote:
> >>
> >> {inet,ipv6}_gro_receive functions perform flush checks (ttl, flags,
> >> iph->id, ...) against all packets in a loop. These flush checks are
> >> relevant only to tcp flows, and as such they're used to determine whether
> >> the packets can be merged later in tcp_gro_receive.
> >>
> >> These checks are not relevant to UDP packets.
> > 
> > I do not think this claim is true.
> > 
> > Incoming packets  ->  GRO -> GSO -> forwarded packets
> > 
> > The {GRO,GSO} step must be transparent, GRO is not LRO.
> 
> Sorry, I should rephrase myself. The patch preserves the
> current logic in GRO. These L3 checks (ttl, flags, etc.) are written to
> NAPI_GRO_CB(p)->{flush,flush_id}, and NAPI_GRO_CB(skb)->is_atomic - and
> all of these are currently used only in tcp_gro_receive.

That was perhaps an oversight when adding UDP GRO?

Simply because the flush is determined in the innermost callback.