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

From: Simon Horman <[email-masked]>
Date: Sat, 15 Jun 2024 08:35:09 +0100

On Thu, Jun 13, 2024 at 01:05:58PM +0200, Alexander Lobakin wrote:
> 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?
> 
> This is actually a good catch. skb indeed could be NULL and we needed to
> check that in the same condition where !eop is checked.
> Fixed already in my tree, so it will be fixed in v2. Thanks for catching!
> 
> (BTW I fixed that in iavf when submitting the libeth series, but forgot
>  to fix that here lol >_<)
> (Also, it was implicitly fixed in the later commits where I convert skb
>  to xdp_buff here, so I didn't catch this one)

Thanks, much appreciated.
As I mentioned above, I wasn't sure about this one.