asix: asix_rx_fixup surgery to reduce skb truesizes

asix_rx_fixup() is complex, and does some unnecessary memory copies (at
least on x86 where NET_IP_ALIGN is 0)

Also, it tends to provide skbs with a big truesize (4096+256 with
MTU=1500) to upper stack, so incoming trafic consume a lot of memory and
I noticed early packet drops because we hit socket rcvbuf too fast.

Switch to a different strategy, using copybreak so that we provide nice
skbs to upper stack (including the NET_SKB_PAD to avoid future head
reallocations in some paths)

With this patch, I no longer see packets drops or tcp collapses on
various tcp workload with a AX88772 adapter.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Aurelien Jacobs <aurel@gnuage.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Trond Wuellner <trond@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Paul Stewart <pstew@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/asix.c

@@ -305,88 +305,40 @@ asix_write_cmd_async(struct usbnet *dev, u8 cmd, u16 value, u16 index,
 
 static int asix_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 {
-	u8  *head;
-	u32  header;
-	char *packet;
+	int offset = 0;
+
+	while (offset + sizeof(u32) < skb->len) {
 		struct sk_buff *ax_skb;
 		u16 size;
+		u32 header = get_unaligned_le32(skb->data + offset);
 
-	head = (u8 *) skb->data;
-	memcpy(&header, head, sizeof(header));
-	le32_to_cpus(&header);
-	packet = head + sizeof(header);
-
-	skb_pull(skb, 4);
-
-	while (skb->len > 0) {
-		if ((header & 0x07ff) != ((~header >> 16) & 0x07ff))
-			netdev_err(dev->net, "asix_rx_fixup() Bad Header Length\n");
+		offset += sizeof(u32);
 
 		/* get the packet length */
-		size = (u16) (header & 0x000007ff);
-
-		if ((skb->len) - ((size + 1) & 0xfffe) == 0) {
-			u8 alignment = (unsigned long)skb->data & 0x3;
-			if (alignment != 0x2) {
-				/*
-				 * not 16bit aligned so use the room provided by
-				 * the 32 bit header to align the data
-				 *
-				 * note we want 16bit alignment as MAC header is
-				 * 14bytes thus ip header will be aligned on
-				 * 32bit boundary so accessing ipheader elements
-				 * using a cast to struct ip header wont cause
-				 * an unaligned accesses.
-				 */
-				u8 realignment = (alignment + 2) & 0x3;
-				memmove(skb->data - realignment,
-					skb->data,
-					size);
-				skb->data -= realignment;
-				skb_set_tail_pointer(skb, size);
-			}
-			return 2;
+		size = (u16) (header & 0x7ff);
+		if (size != ((~header >> 16) & 0x07ff)) {
+			netdev_err(dev->net, "asix_rx_fixup() Bad Header Length\n");
+			return 0;
 		}
 
-		if (size > dev->net->mtu + ETH_HLEN) {
+		if ((size > dev->net->mtu + ETH_HLEN) ||
+		    (size + offset > skb->len)) {
 			netdev_err(dev->net, "asix_rx_fixup() Bad RX Length %d\n",
 				   size);
 			return 0;
 		}
-		ax_skb = skb_clone(skb, GFP_ATOMIC);
-		if (ax_skb) {
-			u8 alignment = (unsigned long)packet & 0x3;
-			ax_skb->len = size;
-
-			if (alignment != 0x2) {
-				/*
-				 * not 16bit aligned use the room provided by
-				 * the 32 bit header to align the data
-				 */
-				u8 realignment = (alignment + 2) & 0x3;
-				memmove(packet - realignment, packet, size);
-				packet -= realignment;
-			}
-			ax_skb->data = packet;
-			skb_set_tail_pointer(ax_skb, size);
-			usbnet_skb_return(dev, ax_skb);
-		} else {
+		ax_skb = netdev_alloc_skb_ip_align(dev->net, size);
+		if (!ax_skb)
 			return 0;
-		}
 
-		skb_pull(skb, (size + 1) & 0xfffe);
-
-		if (skb->len < sizeof(header))
-			break;
+		skb_put(ax_skb, size);
+		memcpy(ax_skb->data, skb->data + offset, size);
+		usbnet_skb_return(dev, ax_skb);
 
-		head = (u8 *) skb->data;
-		memcpy(&header, head, sizeof(header));
-		le32_to_cpus(&header);
-		packet = head + sizeof(header);
-		skb_pull(skb, 4);
+		offset += (size + 1) & 0xfffe;
 	}
 
-	if (skb->len < 0) {
+	if (skb->len != offset) {
 		netdev_err(dev->net, "asix_rx_fixup() Bad SKB Length %d\n",
 			   skb->len);
 		return 0;
@@ -1541,7 +1493,7 @@ static const struct driver_info ax88772_info = {
 	.status = asix_status,
 	.link_reset = ax88772_link_reset,
 	.reset = ax88772_reset,
-	.flags = FLAG_ETHER | FLAG_FRAMING_AX | FLAG_LINK_INTR,
+	.flags = FLAG_ETHER | FLAG_FRAMING_AX | FLAG_LINK_INTR | FLAG_MULTI_PACKET,
 	.rx_fixup = asix_rx_fixup,
 	.tx_fixup = asix_tx_fixup,
 };