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

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Message-ID: <CAKgT0UfXYHDiz7uf51araHXTizRQpQgi8tDqNp6nX2YzeOoZ3A@mail.gmail.com>
Date:   Mon, 30 Sep 2019 16:56:56 -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 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