While testing UDP GSO fraglists forwarding through driver that uses Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order iperf packets:
[ ID] Interval Transfer Bitrate Jitter [SUM] 0.0-40.0 sec 12106 datagrams received out-of-order Simple switch to napi_gro_receive() any other method without frag0 shortcut completely resolved them. I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive() callback. While it's probably OK for non-frag0 paths (when all headers or even the entire frame are already in skb->data), this inline points to junk when using Fast GRO (napi_gro_frags() or napi_gro_receive() with only Ethernet header in skb->data and all the rest in shinfo->frags) and breaks GRO packet compilation and the packet flow itself. To support both modes, skb_gro_header_fast() + skb_gro_header_slow() are typically used. UDP even has an inline helper that makes use of them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr() to get rid of the out-of-order delivers. Present since the introduction of plain UDP GRO in 5.0-rc1. Since v1 [1]: - added a NULL pointer check for "uh" as suggested by Willem. [1] https://lore.kernel.org/netdev/yazu6gezbdpyzmdmwjirxdx7b4sualpdg68adzy...@cp4-web-034.plabs.ch Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.") Signed-off-by: Alexander Lobakin <aloba...@pm.me> --- net/ipv4/udp_offload.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index e67a66fbf27b..7f6bd221880a 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c @@ -366,13 +366,18 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb, static struct sk_buff *udp_gro_receive_segment(struct list_head *head, struct sk_buff *skb) { - struct udphdr *uh = udp_hdr(skb); + struct udphdr *uh = udp_gro_udphdr(skb); struct sk_buff *pp = NULL; struct udphdr *uh2; struct sk_buff *p; unsigned int ulen; int ret = 0; + if (unlikely(!uh)) { + NAPI_GRO_CB(skb)->flush = 1; + return NULL; + } + /* requires non zero csum, for symmetry with GSO */ if (!uh->check) { NAPI_GRO_CB(skb)->flush = 1; -- 2.29.2
