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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Wang Zhan wrote:
> 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.

Precomputing in the caller, as the current series does, is fine too,
if some caller-specific context is needed.

I'm mostly concerned about duplicating logic and the number of
functions touched in this series. But skb_segment itself is too
complex already, so preferable to minimize complication there.
(The reuse of partial for this purpose is very neat.)

If only validate_xmit_skb allows this, because all other segmentation
callers do want full segmentation (not checked, but I can believe
that), the current approach is fine. If we can clean up the repeated
tests and simplify the code in general.