Re: [PATCH net] amt: do not store tunnel pointer in skb control block

[Taehee Yoo]

On Wed, Aug 26, 2026 at 1:15 PM Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx> wrote:
>
> Thanks for the feedback. Just getting back to this thread.
>
> On Thu, Aug 20, 2026 at 10:18:43PM +0900, Taehee Yoo wrote:
> > This looks like it adds a per-packet linear scan over all tunnels,
> > so I'm concerned about performance when there are many tunnels.
> > Do you have any idea how to avoid this regression?
>
> I see two possible approaches:
>
> 1. Keep the v1 implementation. It is the simplest fix, but changes the
>    Query dequeue path from an O(1) pointer access to an O(n) lookup. I am
>    not sure whether that cost is significant in practice for two reasons:
>    this handles IGMP/MLD Query control packets rather than multicast data
>    packets [1], while the hotter multicast data path already walks
>    tunnel_list for each skb [2]; and max_tunnels defaults to 128. However,
>    max_tunnels is configurable, so users can set it much higher.
>
> 2. Add a small per-device bucket hash keyed by (ip4, source_port) to [1],
>    while keeping tunnel_list for multicast fan-out in [2]. This avoids
>    a new linear lookup cost, but adds a second index that must be
>    maintained during tunnel creation, expiry, and device teardown.
>    I am not sure whether that is overkill for this fix.
>
> I would appreciate your further feedback here and will prepare v2 after
> settling on the approach.

Thanks for the investigation.

How about protecting amt_tunnel_list with a refcount?
Currently amt_tunnel_expire() frees the tunnel immediately, but with a
refcount we could just drop the reference there and let the memory be
freed once it reaches 0.

What do you think?

>
> > I think this comment is not necessary. Please remove it.
>
> Agreed. I will remove it in v2.
>
> [1] https://github.com/torvalds/linux/blob/0f23d56f17fdfc7db69d51f64c8b91bbab947aa9/drivers/net/amt.c#L1261
> [2] https://github.com/torvalds/linux/blob/0f23d56f17fdfc7db69d51f64c8b91bbab947aa9/drivers/net/amt.c#L1276