Messages in this thread | | | From | Willem de Bruijn <> | Date | Mon, 9 Nov 2020 13:28:54 -0500 | Subject | Re: [PATCH v2 net] net: udp: fix Fast/frag0 UDP GRO |
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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.
> ... >
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