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Date: Tue, 10 Nov 2020 13:49:56 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Alexander Lobakin <alobakin@...me>
Cc: "David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
Paolo Abeni <pabeni@...hat.com>,
Steffen Klassert <steffen.klassert@...unet.com>,
Eric Dumazet <edumazet@...gle.com>,
Network Development <netdev@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 net] net: udp: fix Fast/frag0 UDP GRO
On Mon, Nov 9, 2020 at 7:29 PM Alexander Lobakin <alobakin@...me> wrote:
>
> From: Alexander Lobakin <alobakin@...me>
> Date: Tue, 10 Nov 2020 00:17:18 +0000
>
> > 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() or 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 v3 [1]:
> > - restore the original {,__}udp{4,6}_lib_lookup_skb() and use
> > private versions of them inside GRO code (Willem).
>
> Note: this doesn't cover a support for nested tunnels as it's out of
> the subject and requires more invasive changes. It will be handled
> separately in net-next series.
Thanks for looking into that.
In that case, should the p->data + off change be deferred to that,
too? It adds some risk unrelated to the bug fix.
> > Since v2 [2]:
> > - dropped redundant check introduced in v2 as it's performed right
> > before (thanks to Eric);
> > - udp_hdr() switched to data + off for skbs from list (also Eric);
> > - fixed possible malfunction of {,__}udp{4,6}_lib_lookup_skb() with
> > Fast/frag0 due to ip{,v6}_hdr() usage (Willem).
> >
> > Since v1 [3]:
> > - added a NULL pointer check for "uh" as suggested by Willem.
> >
> > [1] https://lore.kernel.org/netdev/MgZce9htmEtCtHg7pmWxXXfdhmQ6AHrnltXC41zOoo@cp7-web-042.plabs.ch
> > [2] https://lore.kernel.org/netdev/0eaG8xtbtKY1dEKCTKUBubGiC9QawGgB3tVZtNqVdY@cp4-web-030.plabs.ch
> > [3] https://lore.kernel.org/netdev/YazU6GEzBdpyZMDMwJirxDX7B4sualpDG68ADZYvJI@cp4-web-034.plabs.ch
> >
> > Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Cc: Willem de Bruijn <willemb@...gle.com>
> > Signed-off-by: Alexander Lobakin <alobakin@...me>
> > ---
> > net/ipv4/udp_offload.c | 23 +++++++++++++++++++----
> > net/ipv6/udp_offload.c | 14 +++++++++++++-
> > 2 files changed, 32 insertions(+), 5 deletions(-)
> >
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index e67a66fbf27b..6064efe17cdb 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -366,11 +366,11 @@ 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;
> > + u32 ulen, off;
a specific reason for changing type here?
> > int ret = 0;
> >
> > /* requires non zero csum, for symmetry with GSO */
> > @@ -385,6 +385,9 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> > NAPI_GRO_CB(skb)->flush = 1;
> > return NULL;
> > }
> > +
> > + off = skb_gro_offset(skb);
> > +
> > /* pull encapsulating udp header */
> > skb_gro_pull(skb, sizeof(struct udphdr));
> >
> > @@ -392,7 +395,7 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
> > if (!NAPI_GRO_CB(p)->same_flow)
> > continue;
> >
> > - uh2 = udp_hdr(p);
> > + uh2 = (void *)p->data + off;
> >
> > /* Match ports only, as csum is always non zero */
> > if ((*(u32 *)&uh->source != *(u32 *)&uh2->source)) {
> > @@ -500,6 +503,16 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
> > }
> > EXPORT_SYMBOL(udp_gro_receive);
> >
> > +static struct sock *udp4_gro_lookup_skb(struct sk_buff *skb, __be16 sport,
> > + __be16 dport)
> > +{
> > + const struct iphdr *iph = skb_gro_network_header(skb);
> > +
> > + return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
> > + iph->daddr, dport, inet_iif(skb),
> > + inet_sdif(skb), &udp_table, NULL);
> > +}
> > +
> > INDIRECT_CALLABLE_SCOPE
> > struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
> > {
> > @@ -523,7 +536,9 @@ struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
> > skip:
> > NAPI_GRO_CB(skb)->is_ipv6 = 0;
> > rcu_read_lock();
> > - sk = static_branch_unlikely(&udp_encap_needed_key) ? udp4_lib_lookup_skb(skb, uh->source, uh->dest) : NULL;
> > + sk = static_branch_unlikely(&udp_encap_needed_key) ?
> > + udp4_gro_lookup_skb(skb, uh->source, uh->dest) :
> > + NULL;
Does this indentation pass checkpatch?
Else, the line limit is no longer strict,a and this only shortens the
line, so a single line is fine.
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