Re: [PATCH v8 2/2] net: mana: force full-page RX buffers via ethtool private flag

[Jakub Kicinski <kuba@kernel.org>]

This is an AI-generated review of your patch. The human sending this
email has considered the AI review valid, or at least plausible.
---
net: mana: force full-page RX buffers via ethtool private flag

This patch adds an ethtool private flag "full-page-rx" that forces one
RX buffer per page, bypassing the page_pool fragment path, to address
a 15-20% throughput regression seen on some 4K PAGE_SIZE ARM64
platforms under high connection counts.  Toggling the flag triggers
mana_detach() + mana_attach() to apply the new RX buffer layout.

> diff --git a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> index a28ca461c135..dfd222de11d0 100644
> --- a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c

[ ... ]

> @@ -590,6 +609,88 @@ static int mana_get_link_ksettings(struct net_device *ndev,
>  	return 0;
>  }
>  

[ ... ]

> +static int mana_set_priv_flags(struct net_device *ndev, u32 priv_flags)
> +{
> +	struct mana_port_context *apc = netdev_priv(ndev);
> +	u32 changed = apc->priv_flags ^ priv_flags;
> +	u32 old_priv_flags = apc->priv_flags;
> +	bool schedule_port_reset = false;
> +	int err = 0;
> +
> +	if (!changed)
> +		return 0;
> +
> +	/* Reject unknown bits */
> +	if (priv_flags & ~GENMASK(MANA_PRIV_FLAG_MAX - 1, 0))
> +		return -EINVAL;
> +
> +	if (changed & BIT(MANA_PRIV_FLAG_USE_FULL_PAGE_RXBUF)) {
> +		apc->priv_flags = priv_flags;
> +
> +		if (!apc->port_is_up) {
> +			/* Port is down, flag updated to apply on next up
> +			 * so just return.
> +			 */
> +			return 0;
> +		}
> +
> +		/* Pre-allocate buffers to prevent failure in mana_attach
> +		 * later
> +		 */
> +		err = mana_pre_alloc_rxbufs(apc, ndev->mtu, apc->num_queues);
> +		if (err) {
> +			netdev_err(ndev,
> +				   "Insufficient memory for new allocations\n");
> +			apc->priv_flags = old_priv_flags;
> +			return err;
> +		}
> +
> +		err = mana_detach(ndev, false);
> +		if (err) {
> +			netdev_err(ndev, "mana_detach failed: %d\n", err);
> +			apc->priv_flags = old_priv_flags;
> +
> +			/* Port is in an inconsistent state. Restore
> +			 * 'port_is_up' so that queue reset work handler
> +			 * can properly detach and re-attach.
> +			 */
> +			apc->port_is_up = true;
> +			schedule_port_reset = true;
> +			goto out;
> +		}
> +
> +		err = mana_attach(ndev);
> +		if (err) {
> +			netdev_err(ndev, "mana_attach failed: %d\n", err);
> +			apc->priv_flags = old_priv_flags;
> +
> +			/* Restore 'port_is_up' so the reset work handler
> +			 * can properly detach/attach. Without this,
> +			 * the handler sees port_is_up=false and skips
> +			 * queue allocation, leaving the port dead.
> +			 */
> +			apc->port_is_up = true;
> +			schedule_port_reset = true;
> +		}
> +	}
> +
> +out:
> +	mana_pre_dealloc_rxbufs(apc);
> +
> +	if (schedule_port_reset)
> +		queue_work(apc->ac->per_port_queue_reset_wq,
> +			   &apc->queue_reset_work);
> +
> +	return err;
> +}

[High]
Can this failure path oops inside the reset work handler?

When mana_attach() fails after mana_detach() has already torn the vport
down, apc->tx_qp has been set to NULL by mana_destroy_txq() and apc->rxqs
has been cleared by mana_cleanup_port_context().  This code then forces
apc->port_is_up back to true and queues apc->queue_reset_work:

    apc->port_is_up = true;
    schedule_port_reset = true;

When mana_per_port_queue_reset_work_handler runs it calls
mana_detach(ndev, false), which latches port_st_save = port_is_up = true
and, because port_st_save is true, invokes mana_dealloc_queues().  The TX
drain loop there does:

    txq = &apc->tx_qp[i].txq;
    atomic_read(&txq->pending_sends);

with no NULL check on apc->tx_qp, and mana_destroy_vport further down
iterates apc->rxqs[rxq_idx] while rxqs is NULL.  Would that produce a
NULL pointer dereference in the workqueue worker whenever mana_attach()
fails here (for example on an allocation failure in
mana_init_port_context or mana_create_txq, or a transient HW command
failure in mana_query_vport_cfg / mana_cfg_vport)?

The mana_detach() failure path above (goto out) reaches
mana_dealloc_queues() through the same chain and looks to have the same
exposure.

For comparison, mana_change_mtu() handles a mana_attach() failure by
returning the error without scheduling a reset.  Would a similar
treatment here avoid the asynchronous oops, or is there a reason the
reset must be scheduled in this specific failure case?
-- 
pw-bot: cr