netdev - [PATCH] solos-pci: Increase headroom on received packets

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Date:	Wed, 16 Sep 2015 11:25:45 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	netdev@...r.kernel.org
Cc:	simon@...e.lp0.eu, guy@...verse.com.au,
	linux-atm-general@...ts.sourceforge.net
Subject: [PATCH] solos-pci: Increase headroom on received packets

A comment in include/linux/skbuff.h says that:

 * Various parts of the networking layer expect at least 32 bytes of
 * headroom, you should not reduce this.

This was demonstrated by a panic when handling fragmented IPv6 packets:
http://marc.info/?l=linux-netdev&m=144236093519172&w=2

It's not entirely clear if that comment is still valid — and if it is,
perhaps netif_rx() ought to be enforcing it with a warning.

But either way, it is rather stupid from a performance point of view
for us to be receiving packets into a buffer which doesn't have enough
room to prepend an Ethernet header — it means that *every* incoming
packet is going to be need to be reallocated. So let's fix that.

Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
--- 
Tested in the DMA code path; I don't believe the DMA-capable devices
can still be used in MMIO mode. Simon, Guy, would you be able to test
the MMIO version?

diff --git a/drivers/atm/solos-pci.c b/drivers/atm/solos-pci.c
index 74e18b0..be8225e 100644
--- a/drivers/atm/solos-pci.c
+++ b/drivers/atm/solos-pci.c
@@ -805,13 +805,13 @@ static void solos_bh(unsigned long card_arg)
 					continue;
 				}
 
-				skb = alloc_skb(size + 1, GFP_ATOMIC);
+				skb = alloc_skb(size + NET_SKB_PAD + 1, GFP_ATOMIC);
 				if (!skb) {
 					if (net_ratelimit())
 						dev_warn(&card->dev->dev, "Failed to allocate sk_buff for RX\n");
 					continue;
 				}
-
+				skb_reserve(skb, NET_SKB_PAD);
 				memcpy_fromio(skb_put(skb, size),
 					      RX_BUF(card, port) + sizeof(*header),
 					      size);
@@ -869,8 +869,10 @@ static void solos_bh(unsigned long card_arg)
 		/* Allocate RX skbs for any ports which need them */
 		if (card->using_dma && card->atmdev[port] &&
 		    !card->rx_skb[port]) {
-			struct sk_buff *skb = alloc_skb(RX_DMA_SIZE, GFP_ATOMIC);
+			struct sk_buff *skb = alloc_skb(RX_DMA_SIZE + NET_SKB_PAD,
+							GFP_ATOMIC);
 			if (skb) {
+				skb_reserve(skb, NET_SKB_PAD);
 				SKB_CB(skb)->dma_addr =
 					dma_map_single(&card->dev->dev, skb->data,
 						       RX_DMA_SIZE, DMA_FROM_DEVICE);

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation


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