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

[Richard Gobert]

Eric Dumazet wrote:
> On Sat, Mar 9, 2024 at 4:35 PM Richard Gobert <richardbgobert@xxxxxxxxx> 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.