Messages in this thread | | | Subject | Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO | From | Eric Dumazet <> | Date | Mon, 9 Nov 2020 19:54:02 +0100 |
| |
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")
udp_hdr() is using transport header, which is unique for skb "on the list"
| |