Re: [PATCH 2/2] udp: only do GSO if # of segs > 1

[Josh Hunt <johunt@akamai.com>]

On 10/1/19 5:22 AM, Willem de Bruijn wrote:
> On Mon, Sep 30, 2019 at 7:57 PM Alexander Duyck
> <alexander.duyck@gmail.com> wrote:
>>
>> On Mon, Sep 30, 2019 at 3:13 PM Josh Hunt <johunt@akamai.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@akamai.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?
> 
> Segmentation drops packets in this boundary case (not sure why).
> 
>> 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
> 
> The original choice was made to match GSO behavior of other protocols.
> The drop occurs in protocol-independent skb_segment.
> 
> But I had not anticipated HW GSO to behave differently. With that,

Right and for HW GSO we don't call skb_segment().

> aligning the two makes sense. Especially as UDP GSO is exposed to
> userspace. Having to explicitly code a branch whether or not to pass
> UDP_SEGMENT on each send based on size is confusing.
> 
> gso_size is supplied by the user. That value need not be smaller than
> or equal to MTU minus headers. Some of the tests inside the branch,
> especially
> 
>        if (hlen + cork->gso_size > cork->fragsize) {
>                kfree_skb(skb);
>                return -EINVAL;
>        }
> 
> still need to be checked.
> 

Thanks for the review Willem. I will look at those checks and send a v2.

Thanks!
Josh