On Sat, Nov 7, 2020 at 8:11 PM Alexander Lobakin <aloba...@pm.me> 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.
>
> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
> Signed-off-by: Alexander Lobakin <aloba...@pm.me>
> ---
> net/ipv4/udp_offload.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index e67a66fbf27b..13740e9fe6ec 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -366,7 +366,7 @@ 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;
Good catch. skb_gro_header_slow may fail and return NULL. Need to
check that before dereferencing uh below in
/* requires non zero csum, for symmetry with GSO */
if (!uh->check) {
NAPI_GRO_CB(skb)->flush = 1;
return NULL;
}
