Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[David Ahern <dsahern@kernel.org>]

On 9/30/26 7:58 AM, Eric Dumazet wrote:
> On Wed, Sep 30, 2026 at 2:29 PM David Ahern <dsahern@kernel.org> wrote:
>>
>> On 9/29/26 8:12 AM, Eric Dumazet wrote:
>>> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>>>       iph->ttl = ttl;
>>>       iph->protocol = sk->sk_protocol;
>>>       ip_copy_addrs(iph, fl4);
>>> -     ip_select_ident(net, skb, sk);
>>> +
>>> +     /* UDP GSO packets are segmented later (see udp_send_skb()):
>>> +      * reserve one IP ID per segment.
>>> +      */
>>> +     segs = 1;
>>> +     if (cork->gso_size) {
>>> +             int datalen = skb->len - skb_transport_offset(skb) -
>>> +                           sizeof(struct udphdr);
>>
>> hard coding a specific transport header in L3 code is crossing layers.
>>
> 
> Only UDP sets cork->gso_size, so this is UDP specific code in
> __ip_make_skb(), like the existing sk_is_tcp() case in the same
> function.

I went looking for other transport references in L3 code and missed this
one.

> 
> I can make this explicit in v2 with:
> 
> if (cork->gso_size && sk_is_udp(sk)) {
> 
> Selecting the IP ID from udp_send_skb() instead would either burn
> one extra IP ID per packet (50 % more for 2-segment GSO packets,
> making wraparound faster), or require __ip_make_skb() to leave
> iph->id unset for UDP GSO packets, with a simlar kayer violation concern.
> 
> Would this be acceptable, or do you have another suggestion?

sure. I went down the path of adding an input arg to __ip_make_skb, but
this is simpler.