Re: [PATCH net-next v2 2/4] net: gso: support bounded TCP segmentation

[Wang Zhan <wang.zhan@smartx.com>]

On Mon, 21 Sep 2026 16:36:37 -0400 Willem de Bruijn wrote:
> Wang Zhan wrote:
>> I tried it: https://github.com/zwtop/linux/pull/3
>>
>> It does read better, but whether to resegment is the caller's choice: the
>> qdiscs strip the GSO bits to get one packet per segment (sch_netem.c:443),
>> and a device-derived limit groups that output instead - which sch_netem then
>> drops, because skb_checksum_help() on the first segment rejects a GSO skb
>> (sch_netem.c:538, net/core/dev.c:3626).
>
> So this is a rare netem edge case we need to handle.

That is only one example.  I have not checked every caller, but at least tc
sched tbf/cake and the OVS upcall are as well.

> In the hot path, we should be able to defer the decision whether to
> segment entirely or segment to the capabilities of the device to
> skb_segment itself.

Deciding it entirely inside skb_segment(), without passing extra
information, is difficult: the oversize case overlaps with the cases above.
And the features we pass in may also have had NETIF_F_GSO_MASK cleared, so
it would have to take the device from skb->dev (that should not be much of a
problem if we only handle the TX path).

>> The features cannot tell the two
>> cases apart either: gso_features_check() clears the same bits for an
>> over-limit skb (net/core/dev.c:3843).
>
> I wonder if we can refine this instead.

Maybe we can add a bool to skb_gso_cb saying whether GSO output is allowed,
and leave it to skb_segment() to decide whether a GSO packet is actually
emitted, and the max-segs.