linux-kernel - 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 09:13:50 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Richard Gobert <richardbgobert@...il.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Eric Dumazet <edumazet@...gle.com>
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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:
> Willem de Bruijn wrote:
> > 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.
> 
> It might have been an oversight. From what I have seen it's only relevant
> to GRO's UDP fraglist path (it was added in 9fd1ff5d ("udp: Support UDP
> fraglist GRO/GSO.")). That's the only UDP path that calls skb_gro_receive -
> which may alter the forwarded packets and make GRO/GSO not transparent.
> 
> AFAIU NAPI_GRO_CB(p)->flush value is not overwritten in encapsulation - it
> is determined by both outer and inner callbacks.

Thanks for the context

> I tried to preserve the current behaviour in GRO - if we want to change
> this behaviour I'll gladly do it, although I'd prefer to address it in a
> different patch series. What do you think?

Yes, it's entirely reasonable to leave that out of this series.