From: Eric Dumazet <eric.duma...@gmail.com> Date: Mon, 9 Nov 2020 18:37:36 +0100
> On 11/9/20 5:56 PM, Alexander Lobakin wrote: >> 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)) { > > How uh could be NULL here ? > > My understanding is that udp_gro_receive() is called > only after udp4_gro_receive() or udp6_gro_receive() > validated that udp_gro_udphdr(skb) was not NULL. Right, but only after udp{4,6}_lib_lookup_skb() in certain cases. I don't know for sure if their logic can actually edit skb->data, so it's better to check from my point of view. >> + 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; >> > >Why uh2 is left unchanged ? > > uh2 = udp_hdr(p); > >... Packets from list_head *head have their headers already pulled to skb->data in 100% cases, no need to change anything here. I double-checked that udp_hdr(p) always returns the same pointer as "p->data + network offset" and left it as it is. Thanks, Al
