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

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

Tianyi Gao wrote:
> 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.

As said, the problem with that is that an admin has to do it, each
time. Whereas the kernel has this information.

> 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.

Ok. I don't expect this to be solved in this series.