Subject: Re: [PATCH iwl-next 12/12] idpf: use libeth Rx buffer management for payload buffer

From: Simon Horman <[email-masked]>
Date: Sat, 1 Jun 2024 10:08:42 +0100

+ Dan Carpenter

On Tue, May 28, 2024 at 03:48:46PM +0200, Alexander Lobakin wrote:
> idpf uses Page Pool for data buffers with hardcoded buffer lengths of
> 4k for "classic" buffers and 2k for "short" ones. This is not flexible
> and does not ensure optimal memory usage. Why would you need 4k buffers
> when the MTU is 1500?
> Use libeth for the data buffers and don't hardcode any buffer sizes. Let
> them be calculated from the MTU for "classics" and then divide the
> truesize by 2 for "short" ones. The memory usage is now greatly reduced
> and 2 buffer queues starts make sense: on frames <= 1024, you'll recycle
> (and resync) a page only after 4 HW writes rather than two.
> 
> Signed-off-by: Alexander Lobakin <[email-masked]>

...

> diff --git a/drivers/net/ethernet/intel/idpf/idpf_txrx.c b/drivers/net/ethernet/intel/idpf/idpf_txrx.c

...

Hi Alexander,

The code above the hunk below, starting at line 3321, is:

		if (unlikely(!hdr_len && !skb)) {
			hdr_len = idpf_rx_hsplit_wa(hdr, rx_buf, pkt_len);
			pkt_len -= hdr_len;
			u64_stats_update_begin(&rxq->stats_sync);
			u64_stats_inc(&rxq->q_stats.hsplit_buf_ovf);
			u64_stats_update_end(&rxq->stats_sync);
		}
		if (libeth_rx_sync_for_cpu(hdr, hdr_len)) {
			skb = idpf_rx_build_skb(hdr, hdr_len);
			if (!skb)
				break;
			u64_stats_update_begin(&rxq->stats_sync);
			u64_stats_inc(&rxq->q_stats.hsplit_pkts);
			u64_stats_update_end(&rxq->stats_sync);
		}

> @@ -3413,24 +3340,24 @@ static int idpf_rx_splitq_clean(struct idpf_rx_queue *rxq, int budget)
>  		hdr->page = NULL;
>  
>  payload:
> -		if (pkt_len) {
> -			idpf_rx_sync_for_cpu(rx_buf, pkt_len);
> -			if (skb)
> -				idpf_rx_add_frag(rx_buf, skb, pkt_len);
> -			else
> -				skb = idpf_rx_construct_skb(rxq, rx_buf,
> -							    pkt_len);
> -		} else {
> -			idpf_rx_put_page(rx_buf);
> -		}
> +		if (!libeth_rx_sync_for_cpu(rx_buf, pkt_len))
> +			goto skip_data;
> +
> +		if (skb)
> +			idpf_rx_add_frag(rx_buf, skb, pkt_len);
> +		else
> +			skb = idpf_rx_build_skb(rx_buf, pkt_len);
>  
>  		/* exit if we failed to retrieve a buffer */
>  		if (!skb)
>  			break;
>  
> -		idpf_rx_post_buf_refill(refillq, buf_id);
> +skip_data:
> +		rx_buf->page = NULL;
>  
> +		idpf_rx_post_buf_refill(refillq, buf_id);
>  		IDPF_RX_BUMP_NTC(rxq, ntc);
> +
>  		/* skip if it is non EOP desc */
>  		if (!idpf_rx_splitq_is_eop(rx_desc))
>  			continue;

The code following this hunk, ending at line 3372, looks like this:

		/* pad skb if needed (to make valid ethernet frame) */
		if (eth_skb_pad(skb)) {
			skb = NULL;
			continue;
		}
		/* probably a little skewed due to removing CRC */
		total_rx_bytes += skb->len;

Smatch warns that:
.../idpf_txrx.c:3372 idpf_rx_splitq_clean() error: we previously assumed 'skb' could be null (see line 3321)

I think, but am not sure, this is because it thinks skb might
be NULL at the point where "goto skip_data;" is now called above.

Could you look into this?

...