[Cake] Re: [PATCH net-next v8 4/4] net/sched: cake: hash before segmenting and reject an overlong split

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Jamal Hadi Salim <jhs@mojatatu.com> writes:

> The Fix:
> with CAKE_FLAG_SPLIT_GSO, cake computes the flow/host hash keys, then
> splits a GSO skb into segments it accounts individually. Key extraction
> must run on the unsegmented skb -- skb_gso_segment() can rewrite an
> encapsulated packet's headers to the inner ones -- and the resolution
> must wait until the packet is admitted, since it commits the
> set-associative tags, host indices and way counters that a later
> rejection would leave mutated (a GSO skb whose segmentation fails was
> dropped with that state already committed).
>
> Split cake_hash() into a side-effect-free cake_hash_keys(), run on the
> unsegmented skb, and cake_hash_resolve(), run only once the packet is
> admitted. Move the split-GSO path ahead of the resolution: sum the
> segment list, test the total against the ceiling before linking any
> segment, and drop the whole list on rejection so no persistent state is
> committed. The list is deferred to to_free so the tail segment's
> sock_wfree() never runs under the qdisc lock.
>
> Tag every deferred segment with QDISC_DROP_OVERLIMIT: __skb_gso_segment()
> keeps SKB_GSO_CB at cb offset 32, aliasing tc_skb_cb.drop_reason, and
> __copy_skb_header() copies it into every segment, so tagging only the
> head left the rest with GSO scratch (a headroom-sized value) as their
> drop reason, which __tcf_kfree_skb_list() would report as an unrelated
> core reason.
>
> Read q->config->flow_mode once with READ_ONCE() in cake_enqueue() and
> pass that value to both cake_hash_keys() and cake_hash_resolve().
> cake_config_change() writes the mode before cake_change() takes
> sch_tree_lock(), and the split introduced two separate loads with the
> segmentation work between them; a concurrent reconfiguration could
> extract keys in one mode and resolve them in another.
>
> Link: https://lore.kernel.org/netdev/179231933068.3135.80340792633077986@kernel.org/
> Reviewed-by: Victor Nogueira <victor@mojatatu.com>
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>

Thank you for sticking with the series! :)

Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>