Re: [PATCH net 1/2] veth: wake a stopped peer txq beyond real_num_tx_queues

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] veth: Fixes trailer cites dc82a33297fc instead of 08f566e8f83b
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commit 4511ff49371a3a9f93211f5b51b7a6404ecb1f22
Author: Simon Schippers <simon.schippers@tu-dortmund.de>

> veth: wake a stopped peer txq beyond real_num_tx_queues
>
> Since commit dc82a33297fc ("veth: apply qdisc backpressure on full
> ptr_ring to reduce TX drops"), only the peer's veth_poll() wakes a txq
> that veth_xmit() stopped on a full ptr_ring. veth_poll() skips txqs at
> or above the peer's real_num_tx_queues.
>
> If the peer shrinks its TX channels while such a txq is stopped, nothing
> wakes it. netif_set_real_num_tx_queues() keeps its state, so once the
> channels are grown back, its first skb is requeued forever. With a single
> root qdisc this blocks all transmission of the device.
>
> Look the txq up below num_tx_queues instead. Waking a txq that is not in
> use is harmless.
>
> Reproduced in virtme-ng with a UDP flood from veth0 into veth1, which has
> GRO, while switching veth0 between one and two TX channels: without this
> patch veth0 stopped transmitting within 11 cycles in 3 of 3 runs, with
> it veth0 kept transmitting through 30 cycles in 3 of 3 runs.
>
> Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>

[Severity: Low]
Should the Fixes: tag reference 08f566e8f83b ("veth: fix OOB txq access
in veth_poll() with asymmetric queue counts") instead?

Commit dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to
reduce TX drops") looked up peer_txq unconditionally via
netdev_get_tx_queue(peer_dev, queue_idx).  The queue_idx <
peer_dev->real_num_tx_queues check that skips txqs above
real_num_tx_queues in veth_poll() was introduced in 08f566e8f83b ("veth:
fix OOB txq access in veth_poll() with asymmetric queue counts").

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Sashiko AI review · https://sashiko.dev/#/patchset/20261113104648.225983-1-simon.schippers@tu-dortmund.de?part=1