netdev - Re: smsc75xx & smsc95xx, setting skb->truesize correctly?

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Date:	Tue, 21 Aug 2012 11:46:38 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
Cc:	netdev@...r.kernel.org,
	Steve Glendinning <steve.glendinning@...well.net>
Subject: Re: smsc75xx & smsc95xx, setting skb->truesize correctly?

On Mon, 2012-08-20 at 17:57 +0300, Jussi Kivilinna wrote:
> Hello,
> 
> Is setting skb->truesize in smsc75xx and smsc95xx correct?
> In smsc75xx/smsc95xx_rx_fixup(), input skb containing multiple packets  
> is cloned and truesize for each clone is set to packet-size +  
> sizeof(struct sk_buff), but input skb has minimum allocation size of  
> 9000 bytes (MAX_SINGLE_PACKET_SIZE) and maximum of 18944 bytes  
> (DEFAULT_HS_BURST_CAP_SIZE) (+ NET_IP_ALIGN). Doesn't this cause  
> truesize to be underestimated?

This has been discussed in a "TCP transmit performance regression"
thread some weeks ago.

More generally, skb_clone() is not a good idea in rx path.

I dont have the hardware so cannot send a formal patch.

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index b1112e7..3d9566f 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -1080,30 +1080,17 @@ static int smsc95xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 				return 0;
 			}
 
-			/* last frame in this batch */
-			if (skb->len == size) {
-				if (dev->net->features & NETIF_F_RXCSUM)
-					smsc95xx_rx_csum_offload(skb);
-				skb_trim(skb, skb->len - 4); /* remove fcs */
-				skb->truesize = size + sizeof(struct sk_buff);
-
-				return 1;
-			}
-
-			ax_skb = skb_clone(skb, GFP_ATOMIC);
+			ax_skb = netdev_alloc_skb_ip_align(dev->net, size);
 			if (unlikely(!ax_skb)) {
 				netdev_warn(dev->net, "Error allocating skb\n");
 				return 0;
 			}
 
-			ax_skb->len = size;
-			ax_skb->data = packet;
-			skb_set_tail_pointer(ax_skb, size);
+			memcpy(skb_put(ax_skb, size), packet, size);
 
 			if (dev->net->features & NETIF_F_RXCSUM)
 				smsc95xx_rx_csum_offload(ax_skb);
-			skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
-			ax_skb->truesize = size + sizeof(struct sk_buff);
+			__skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
 
 			usbnet_skb_return(dev, ax_skb);
 		}


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