Date: Mon, 3 Jun 2024 09:02:51 +0100
From: Paul Barker <paul.barker.ct@...renesas.com>
To: Simon Horman <horms@...nel.org>,
 Paul Barker <paul.barker.ct@...renesas.com>,
 Sergey Shtylyov <s.shtylyov@....ru>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Niklas Söderlund <niklas.soderlund+renesas@...natech.se>,
 Biju Das <biju.das.jz@...renesas.com>,
 Claudiu Beznea <claudiu.beznea.uj@...renesas.com>,
 Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
 netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [net-next PATCH v4 7/7] net: ravb: Allocate RX buffers via page
 pool

On 01/06/2024 11:13, Simon Horman wrote:
> On Tue, May 28, 2024 at 04:03:39PM +0100, Paul Barker wrote:
>> This patch makes multiple changes that can't be separated:
>>
>>   1) Allocate plain RX buffers via a page pool instead of allocating
>>      SKBs, then use build_skb() when a packet is received.
>>   2) For GbEth IP, reduce the RX buffer size to 2kB.
>>   3) For GbEth IP, merge packets which span more than one RX descriptor
>>      as SKB fragments instead of copying data.
>>
>> Implementing (1) without (2) would require the use of an order-1 page
>> pool (instead of an order-0 page pool split into page fragments) for
>> GbEth.
>>
>> Implementing (2) without (3) would leave us no space to re-assemble
>> packets which span more than one RX descriptor.
>>
>> Implementing (3) without (1) would not be possible as the network stack
>> expects to use put_page() or page_pool_put_page() to free SKB fragments
>> after an SKB is consumed.
>>
>> RX checksum offload support is adjusted to handle both linear and
>> nonlinear (fragmented) packets.
>>
>> This patch gives the following improvements during testing with iperf3.
>>
>>   * RZ/G2L:
>>     * TCP RX: same bandwidth at -43% CPU load (70% -> 40%)
>>     * UDP RX: same bandwidth at -17% CPU load (88% -> 74%)
>>
>>   * RZ/G2UL:
>>     * TCP RX: +30% bandwidth (726Mbps -> 941Mbps)
>>     * UDP RX: +417% bandwidth (108Mbps -> 558Mbps)
>>
>>   * RZ/G3S:
>>     * TCP RX: +64% bandwidth (562Mbps -> 920Mbps)
>>     * UDP RX: +420% bandwidth (90Mbps -> 468Mbps)
>>
>>   * RZ/Five:
>>     * TCP RX: +217% bandwidth (145Mbps -> 459Mbps)
>>     * UDP RX: +470% bandwidth (20Mbps -> 114Mbps)
>>
>> There is no significant impact on bandwidth or CPU load in testing on
>> RZ/G2H or R-Car M3N.
>>
>> Signed-off-by: Paul Barker <paul.barker.ct@...renesas.com>
> 
> Hi Paul,
> 
> Some minor feedback from my side.
> 
> ...
> 
>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> 
> ...
> 
>> @@ -298,13 +269,14 @@ static void ravb_ring_free(struct net_device *ndev, int q)
>>  		priv->tx_ring[q] = NULL;
>>  	}
>>  
>> -	/* Free RX skb ringbuffer */
>> -	if (priv->rx_skb[q]) {
>> -		for (i = 0; i < priv->num_rx_ring[q]; i++)
>> -			dev_kfree_skb(priv->rx_skb[q][i]);
>> +	/* Free RX buffers */
>> +	for (i = 0; i < priv->num_rx_ring[q]; i++) {
>> +		if (priv->rx_buffers[q][i].page)
>> +			page_pool_put_page(priv->rx_pool[q], priv->rx_buffers[q][i].page, 0, true);
> 
> nit: Networking still prefers code to be 80 columns wide or less.
>      It looks like that can be trivially achieved here.
> 
>      Flagged by checkpatch.pl --max-line-length=80

Sergey has asked me to wrap to 100 cols [1]. I can only find a reference
to 80 in the docs though [2], so I guess you may be right.

[1]: https://lore.kernel.org/all/611a49b8-ecdb-6b91-9d3e-262bf3851f5b@omp.ru/
[2]: https://www.kernel.org/doc/html/latest/process/coding-style.html

> 
>>  	}
>> -	kfree(priv->rx_skb[q]);
>> -	priv->rx_skb[q] = NULL;
>> +	kfree(priv->rx_buffers[q]);
>> +	priv->rx_buffers[q] = NULL;
>> +	page_pool_destroy(priv->rx_pool[q]);
>>  
>>  	/* Free aligned TX buffers */
>>  	kfree(priv->tx_align[q]);
>> @@ -317,35 +289,56 @@ static void ravb_ring_free(struct net_device *ndev, int q)
>>  	priv->tx_skb[q] = NULL;
>>  }
>>  
>> +static int
>> +ravb_alloc_rx_buffer(struct net_device *ndev, int q, u32 entry, gfp_t gfp_mask,
>> +		     struct ravb_rx_desc *rx_desc)
>> +{
>> +	struct ravb_private *priv = netdev_priv(ndev);
>> +	const struct ravb_hw_info *info = priv->info;
>> +	struct ravb_rx_buffer *rx_buff = &priv->rx_buffers[q][entry];
>> +	dma_addr_t dma_addr;
>> +	unsigned int size;
> 
> nit: I would appreciate it if some consideration could be given to
>      moving this driver towards rather than away from reverse xmas
>      tree - longest line to shortest - for local variable declarations.
> 
>      I'm not suggesting a clean-up patch. Rather, that in cases
>      like this where new code is added, and also in cases where
>      code is modified, reverse xmas tree is preferred.
> 
>      Here I would suggest separating the assinment of rx_buf from
>      it's declaration (completely untested!):
> 
> 	struct ravb_private *priv = netdev_priv(ndev);
> 	const struct ravb_hw_info *info = priv->info;
> 	struct ravb_rx_buffer *rx_buff;
> 	dma_addr_t dma_addr;
> 	unsigned int size;
> 
> 	rx_buff = &priv->rx_buffers[q][entry];
> 
>      Edward Cree's xmastree tool can be helpful here:
>      https://github.com/ecree-solarflare/xmastree

Ack.

> 
>> +
>> +	size = info->rx_buffer_size;
>> +	rx_buff->page = page_pool_alloc(priv->rx_pool[q], &rx_buff->offset, &size,
>> +					gfp_mask);
>> +	if (unlikely(!rx_buff->page)) {
>> +		/* We just set the data size to 0 for a failed mapping
>> +		 * which should prevent DMA from happening...
>> +		 */
>> +		rx_desc->ds_cc = cpu_to_le16(0);
>> +		return -ENOMEM;
>> +	}
>> +
>> +	dma_addr = page_pool_get_dma_addr(rx_buff->page) + rx_buff->offset;
>> +	dma_sync_single_for_device(ndev->dev.parent, dma_addr,
>> +				   info->rx_buffer_size, DMA_FROM_DEVICE);
>> +	rx_desc->dptr = cpu_to_le32(dma_addr);
>> +
>> +	/* The end of the RX buffer is used to store skb shared data, so we need
>> +	 * to ensure that the hardware leaves enough space for this.
>> +	 */
>> +	rx_desc->ds_cc = cpu_to_le16(info->rx_buffer_size
>> +				     - SKB_DATA_ALIGN(sizeof(struct skb_shared_info))
>> +				     - ETH_FCS_LEN + sizeof(__sum16));
>> +	return 0;
>> +}
> 
> ...
> 
>> @@ -816,14 +824,26 @@ static int ravb_rx_gbeth(struct net_device *ndev, int budget, int q)
>>  			if (desc_status & MSC_CEEF)
>>  				stats->rx_missed_errors++;
>>  		} else {
>> +			struct ravb_rx_buffer *rx_buff = &priv->rx_buffers[q][entry];
>> +			void *rx_addr = page_address(rx_buff->page) + rx_buff->offset;
>>  			die_dt = desc->die_dt & 0xF0;
>> -			skb = ravb_get_skb_gbeth(ndev, entry, desc);
>> +			dma_sync_single_for_cpu(ndev->dev.parent, le32_to_cpu(desc->dptr),
>> +						desc_len, DMA_FROM_DEVICE);
>> +
>>  			switch (die_dt) {
>>  			case DT_FSINGLE:
>>  			case DT_FSTART:
>>  				/* Start of packet:
>> -				 * Set initial data length.
>> +				 * Prepare an SKB and add initial data.
>>  				 */
>> +				skb = napi_build_skb(rx_addr, info->rx_buffer_size);
>> +				if (unlikely(!skb)) {
>> +					stats->rx_errors++;
>> +					page_pool_put_page(priv->rx_pool[q],
>> +							   rx_buff->page, 0, true);
> 
> Here skb is NULL.
> 
>> +					break;
>> +				}
>> +				skb_mark_for_recycle(skb);
>>  				skb_put(skb, desc_len);
>>  
>>  				/* Save this SKB if the packet spans multiple
>> @@ -836,14 +856,23 @@ static int ravb_rx_gbeth(struct net_device *ndev, int budget, int q)
>>  			case DT_FMID:
>>  			case DT_FEND:
>>  				/* Continuing a packet:
>> -				 * Move data into the saved SKB.
>> +				 * Add this buffer as an RX frag.
>>  				 */
>> -				skb_copy_to_linear_data_offset(priv->rx_1st_skb,
>> -							       priv->rx_1st_skb->len,
>> -							       skb->data,
>> -							       desc_len);
>> -				skb_put(priv->rx_1st_skb, desc_len);
>> -				dev_kfree_skb(skb);
>> +
>> +				/* rx_1st_skb will be NULL if napi_build_skb()
>> +				 * failed for the first descriptor of a
>> +				 * multi-descriptor packet.
>> +				 */
>> +				if (unlikely(!priv->rx_1st_skb)) {
>> +					stats->rx_errors++;
>> +					page_pool_put_page(priv->rx_pool[q],
>> +							   rx_buff->page, 0, true);
> 
> And here skb seems to be uninitialised.
> 
>> +					break;
>> +				}
>> +				skb_add_rx_frag(priv->rx_1st_skb,
>> +						skb_shinfo(priv->rx_1st_skb)->nr_frags,
>> +						rx_buff->page, rx_buff->offset,
>> +						desc_len, info->rx_buffer_size);
>>  
>>  				/* Set skb to point at the whole packet so that
>>  				 * we only need one code path for finishing a
> 
> The code between the hunk above and the hunk below is:
> 
> 				/* Set skb to point at the whole packet so that
> 				 * we only need one code path for finishing a
> 				 * packet.
> 				 */
> 				skb = priv->rx_1st_skb;
> 			}
> 			switch (die_dt) {
> 			case DT_FSINGLE:
> 			case DT_FEND:
> 				/* Finishing a packet:
> 				 * Determine protocol & checksum, hand off to
> 				 * NAPI and update our stats.
> 				 */
> 				skb->protocol = eth_type_trans(skb, ndev);
> 				if (ndev->features & NETIF_F_RXCSUM)
> 					ravb_rx_csum_gbeth(skb);
> 				stats->rx_bytes += skb->len;
> 				napi_gro_receive(&priv->napi[q], skb);
> 				rx_packets++;
> 
> It seems that the inter-hunk code above may dereference skb when it is NULL
> or uninitialised.
> 
> Flagged by Smatch.

I see what has happened here. I wrote this using if statements first,
then changed to switch statements in response to Sergey's review. So the
break statements were intended to break out of the outer for loop, not
the switch statement. I'll need to replace them with goto statements.

Thanks for your review!

-- 
Paul Barker
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