Subject: Re: [PATCH v3 net 1/1] net: fec: correct the counting of XDP sent frames

From: Jakub Kicinski <[email-masked]>
Date: Fri, 5 May 2023 12:23:52 -0700

On Thu,  4 May 2023 10:35:17 -0500 Shenwei Wang wrote:
> In the current xdp_xmit implementation, if any single frame fails to
> transmit due to insufficient buffer descriptors, the function nevertheless
> reports success in sending all frames. This results in erroneously
> indicating that frames were transmitted when in fact they were dropped.
> 
> This patch fixes the issue by ensureing the return value properly
> indicates the actual number of frames successfully transmitted, rather than
> potentially reporting success for all frames when some could not transmit.
> 
> Fixes: 6d6b39f180b8 ("net: fec: add initial XDP support")
> Signed-off-by: Gagandeep Singh <[email-masked]>
> Signed-off-by: Shenwei Wang <[email-masked]>
> ---
>  v3:
>   - resend the v2 fix for "net" as the standalone patch.
> 
>  v2:
>   - only keep the bug fix part of codes according to Horatiu's comments.
>   - restructure the functions to avoid the forward declaration.
> 
>  drivers/net/ethernet/freescale/fec_main.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> index 160c1b3525f5..42ec6ca3bf03 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -3798,7 +3798,8 @@ static int fec_enet_txq_xmit_frame(struct fec_enet_private *fep,
>  	entries_free = fec_enet_get_free_txdesc_num(txq);
>  	if (entries_free < MAX_SKB_FRAGS + 1) {
>  		netdev_err(fep->netdev, "NOT enough BD for SG!\n");

This should really be rate limited :(

> -		return NETDEV_TX_OK;
> +		xdp_return_frame(frame);

Why return this frame? Since error is reported @sent_frames will not be
incremented, and therefore bq_xmit_all() will take care of returning it,
right?

Otherwise the other error return path (see below) needs to be changed
as well.

> +		return NETDEV_TX_BUSY;

On DMA mapping error this function returns FEC_ENET_XDP_CONSUMED,
would be good if the functions return values where from the same
"enum". Are you going to clean that part up in net-next?

>  	}
> 
>  	/* Fill in a Tx ring entry */
> @@ -3856,6 +3857,7 @@ static int fec_enet_xdp_xmit(struct net_device *dev,
>  	struct fec_enet_private *fep = netdev_priv(dev);
>  	struct fec_enet_priv_tx_q *txq;
>  	int cpu = smp_processor_id();
> +	unsigned int sent_frames = 0;
>  	struct netdev_queue *nq;
>  	unsigned int queue;
>  	int i;
> @@ -3866,8 +3868,11 @@ static int fec_enet_xdp_xmit(struct net_device *dev,
> 
>  	__netif_tx_lock(nq, cpu);
> 
> -	for (i = 0; i < num_frames; i++)
> -		fec_enet_txq_xmit_frame(fep, txq, frames[i]);
> +	for (i = 0; i < num_frames; i++) {
> +		if (fec_enet_txq_xmit_frame(fep, txq, frames[i]) != 0)

nit: you can skip the "!= 0", but up to you

> +			break;
> +		sent_frames++;
> +	}

-- 
pw-bot: cr