Re: [PATCH net-next 1/8] net: add tx_extra_headroom and LL_ALLOC_SPACE()

[Tianyi Gao <tianyi@cloudflare.com>]

Hi Willem,

On Tue, 10 Nov 2026 11:23:09 -0500, Willem de Bruijn wrote:
> I have some hope that if there is sufficient reason, this backlinking
> to the originating netdev (or socket?) can eventually be addressed as
> well. Rather than work around it.
>
> The skb_orphan and iif issues are well know. But not set in stone.

Thank you for the suggestion. I looked at what backlinking would take.

For the socket, ip_rcv_core() orphans the skb, and after that skb->sk
holds the receiving socket, for tproxy and early demux. Keeping the
sender's socket there would break that assumption.

For the netdev, skb_iif is the last receiving device, not the
allocating one. So keeping the origin needs new state in the skb,
such as an skb extension.

Even with a backlink, the kernel still needs to know when to adjust
tx_extra_headroom of the originating netdev. This brings back
auto-tuning, with the problems from my earlier reply.

Taking a step back, IMHO it is more natural to set the extra headroom
indication on the netdev that the skb is allocated for, than to have
other devices raise it through a backlink, which can get messy.

For v2, I would prefer the narrow VETH_INFO_TX_EXTRA_HEADROOM UAPI,
following what has been done for IFLA_NETKIT_HEADROOM. I will put it
in the last two patches, so the rest of the series can be taken
without it if you prefer.

Thanks,
Tianyi