[PATCH v2 net-next] udp: allow forwarding of plain (non-fraglisted) UDP GRO packets

[Alexander Lobakin]

Commit 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.") actually
not only added a support for fraglisted UDP GRO, but also tweaked
some logics the way that non-fraglisted UDP GRO started to work for
forwarding too.
Commit 2e4ef10f5850 ("net: add GSO UDP L4 and GSO fraglists to the
list of software-backed types") added GSO UDP L4 to the list of
software GSO to allow virtual netdevs to forward them as is up to
the real drivers.

Tests showed that currently forwarding and NATing of plain UDP GRO
packets are performed fully correctly, regardless if the target
netdevice has a support for hardware/driver GSO UDP L4 or not.
Add the last element and allow to form plain UDP GRO packets if
there is no socket -> we are on forwarding path.

Plain UDP GRO forwarding even shows better performance than fraglisted
UDP GRO in some cases due to not wasting one skbuff_head per every
segment.

Since v1 [0]:
 - drop redundant 'if (sk)' check (Alexander Duyck);
 - add a ref in the commit message to one more commit that was
   an important step for UDP GRO forwarding.

[0] https://lore.kernel.org/netdev/20210112211536.261172-1-alobakin@xxxxx

Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
---
 net/ipv4/udp_offload.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index ff39e94781bf..6f5237a0d181 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -460,12 +460,12 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
 	if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
 		NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
 
-	if ((sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist) {
+	if (!sk || udp_sk(sk)->gro_enabled || NAPI_GRO_CB(skb)->is_flist) {
 		pp = call_gro_receive(udp_gro_receive_segment, head, skb);
 		return pp;
 	}
 
-	if (!sk || NAPI_GRO_CB(skb)->encap_mark ||
+	if (NAPI_GRO_CB(skb)->encap_mark ||
 	    (skb->ip_summed != CHECKSUM_PARTIAL &&
 	     NAPI_GRO_CB(skb)->csum_cnt == 0 &&
 	     !NAPI_GRO_CB(skb)->csum_valid) ||
-- 
2.30.0