Messages in this thread | | | Date | Mon, 09 Nov 2020 19:48:04 +0000 | From | Alexander Lobakin <> | Subject | Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO |
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From: Willem de Bruijn <willemdebruijn.kernel@gmail.com> Date: Mon, 9 Nov 2020 14:06:07 -0500
> On Mon, Nov 9, 2020 at 1:54 PM Eric Dumazet <eric.dumazet@gmail.com> wrote: >> >> >> >> On 11/9/20 7:28 PM, Willem de Bruijn wrote: >>> On Mon, Nov 9, 2020 at 12:37 PM Eric Dumazet <eric.dumazet@gmail.com> wrote: >>>> >>>> >>>> >>>> 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/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@cp4-web-034.plabs.ch >>>>> >>>>> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.") >>>>> Signed-off-by: Alexander Lobakin <alobakin@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. >>> >>> Oh indeed. This has already been checked before. >>> >>>>> + 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); >>> >>> Isn't that the same as th2 = tcp_hdr(p) in tcp_gro_receive? no frag0 >>> optimization to worry about for packets on the list. >> >> My feeling was that tcp_gro_receive() is terminal in the GRO stack. >> >> While UDP could be encapsulated in UDP :) >> >> I guess we do not support this yet. >> >> Years ago we made sure to propagate the current header offset into GRO stack >> (when we added SIT/IPIP/GRE support to GRO) >> 299603e8370a93dd5d8e8d800f0dff1ce2c53d36 ("net-gro: Prepare GRO stack for the upcoming tunneling support") > > On which note, and Alexander's mention of udp4_lib_lookup_skb(), that > performs a standard ip_hdr on the possibly frag0 optimized incoming > packet: > > struct sock *udp4_lib_lookup_skb(struct sk_buff *skb, > __be16 sport, __be16 dport) > { > const struct iphdr *iph = ip_hdr(skb); > > return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport, > iph->daddr, dport, inet_iif(skb), > inet_sdif(skb), &udp_table, NULL); > } > > This should use skb_gro_header_.. too, then?
Well, both of udp{4,6}_lib_lookup_skb() and a couple of neighbour functions use ip_hdr(skb). I didn't check this path before as my test case was NAT/forwarding with no sk. I suspect they should use skb_gro_network_header() to obtain an IP header. I'll fix them in v3, thanks for pointing this out!
Al
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