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.
> + 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);
...
