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

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Date:   Tue, 1 Oct 2019 07:36:49 -0700
From:   Josh Hunt <johunt@...mai.com>
To:     Alexander Duyck <alexander.duyck@...il.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 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