netdev - Re: [PATCH 2/2] udp: only do GSO if # of segs

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Date:   Tue, 1 Oct 2019 08:05:20 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Josh Hunt <johunt@...mai.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>
Subject: Re: [PATCH 2/2] udp: only do GSO if # of segs > 1

On Tue, Oct 1, 2019 at 7:36 AM Josh Hunt <johunt@...mai.com> wrote:
>
>
>
> On 9/30/19 4:56 PM, Alexander Duyck wrote:
> > On Mon, Sep 30, 2019 at 3:13 PM Josh Hunt <johunt@...mai.com> wrote:
> >>
> >> Prior to this change an application sending <= 1MSS worth of data and
> >> enabling UDP GSO would fail if the system had SW GSO enabled, but the
> >> same send would succeed if HW GSO offload is enabled. In addition to this
> >> inconsistency the error in the SW GSO case does not get back to the
> >> application if sending out of a real device so the user is unaware of this
> >> failure.
> >>
> >> With this change we only perform GSO if the # of segments is > 1 even
> >> if the application has enabled segmentation. I've also updated the
> >> relevant udpgso selftests.
> >>
> >> Fixes: bec1f6f69736 ("udp: generate gso with UDP_SEGMENT")
> >> Signed-off-by: Josh Hunt <johunt@...mai.com>
> >> ---
> >>   net/ipv4/udp.c                       |  5 +++--
> >>   net/ipv6/udp.c                       |  5 +++--
> >>   tools/testing/selftests/net/udpgso.c | 16 ++++------------
> >>   3 files changed, 10 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> >> index be98d0b8f014..ac0baf947560 100644
> >> --- a/net/ipv4/udp.c
> >> +++ b/net/ipv4/udp.c
> >> @@ -821,6 +821,7 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
> >>          int is_udplite = IS_UDPLITE(sk);
> >>          int offset = skb_transport_offset(skb);
> >>          int len = skb->len - offset;
> >> +       int datalen = len - sizeof(*uh);
> >>          __wsum csum = 0;
> >>
> >>          /*
> >> @@ -832,7 +833,7 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
> >>          uh->len = htons(len);
> >>          uh->check = 0;
> >>
> >> -       if (cork->gso_size) {
> >> +       if (cork->gso_size && datalen > cork->gso_size) {
> >>                  const int hlen = skb_network_header_len(skb) +
> >>                                   sizeof(struct udphdr);
> >>
> >
> > So what about the datalen == cork->gso_size case? That would only
> > generate one segment wouldn't it?
> >
> > Shouldn't the test really be "datalen < cork->gso_size"? That should
> > be the only check you need since if gso_size is 0 this statement would
> > always fail anyway.
> >
> > Thanks.
> >
> > - Alex
> >
>
> Alex thanks for the review. The intent of the patch is to only use GSO
> when the # of segs > 1. The two cases you've mentioned are when the # of
> segs == 1. In those cases we don't want to set gso_size and treat this
> as a non-GSO case, skipping the if block. Let me know if I misunderstood
> your points or you'd like further clarification.
>
> Thanks!
> Josh

Your right. Somehow I got the logic backwards in my head.

Thanks.

- Alex