net: pktgen: fix pkt_size

The commit 879c7220e828 ("net: pktgen: Observe needed_headroom
of the device") increased the 'pkt_overhead' field value by
LL_RESERVED_SPACE.
As a side effect the generated packet size, computed as:

	/* Eth + IPh + UDPh + mpls */
	datalen = pkt_dev->cur_pkt_size - 14 - 20 - 8 -
		  pkt_dev->pkt_overhead;

is decreased by the same value.
The above changed slightly the behavior of existing pktgen users,
and made the procfs interface somewhat inconsistent.
Fix it by restoring the previous pkt_overhead value and using
LL_RESERVED_SPACE as extralen in skb allocation.
Also, change pktgen_alloc_skb() to only partially reserve
the headroom to allow the caller to prefetch from ll header
start.

v1 -> v2:
 - fixed some typos in the comments

Fixes: 879c7220e828 ("net: pktgen: Observe needed_headroom of the device")
Suggested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -2286,7 +2286,7 @@ out:
 
 static inline void set_pkt_overhead(struct pktgen_dev *pkt_dev)
 {
-	pkt_dev->pkt_overhead = LL_RESERVED_SPACE(pkt_dev->odev);
+	pkt_dev->pkt_overhead = 0;
 	pkt_dev->pkt_overhead += pkt_dev->nr_labels*sizeof(u32);
 	pkt_dev->pkt_overhead += VLAN_TAG_SIZE(pkt_dev);
 	pkt_dev->pkt_overhead += SVLAN_TAG_SIZE(pkt_dev);
@@ -2777,13 +2777,13 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
 }
 
 static struct sk_buff *pktgen_alloc_skb(struct net_device *dev,
-					struct pktgen_dev *pkt_dev,
-					unsigned int extralen)
+					struct pktgen_dev *pkt_dev)
 {
+	unsigned int extralen = LL_RESERVED_SPACE(dev);
 	struct sk_buff *skb = NULL;
-	unsigned int size = pkt_dev->cur_pkt_size + 64 + extralen +
-			    pkt_dev->pkt_overhead;
+	unsigned int size;
 
+	size = pkt_dev->cur_pkt_size + 64 + extralen + pkt_dev->pkt_overhead;
 	if (pkt_dev->flags & F_NODE) {
 		int node = pkt_dev->node >= 0 ? pkt_dev->node : numa_node_id();
 
@@ -2796,8 +2796,9 @@ static struct sk_buff *pktgen_alloc_skb(struct net_device *dev,
 		 skb = __netdev_alloc_skb(dev, size, GFP_NOWAIT);
 	}
 
+	/* the caller pre-fetches from skb->data and reserves for the mac hdr */
 	if (likely(skb))
-		skb_reserve(skb, LL_RESERVED_SPACE(dev));
+		skb_reserve(skb, extralen - 16);
 
 	return skb;
 }
@@ -2830,16 +2831,14 @@ static struct sk_buff *fill_packet_ipv4(struct net_device *odev,
 	mod_cur_headers(pkt_dev);
 	queue_map = pkt_dev->cur_queue_map;
 
-	datalen = (odev->hard_header_len + 16) & ~0xf;
-
-	skb = pktgen_alloc_skb(odev, pkt_dev, datalen);
+	skb = pktgen_alloc_skb(odev, pkt_dev);
 	if (!skb) {
 		sprintf(pkt_dev->result, "No memory");
 		return NULL;
 	}
 
 	prefetchw(skb->data);
-	skb_reserve(skb, datalen);
+	skb_reserve(skb, 16);
 
 	/*  Reserve for ethernet and IP header  */
 	eth = (__u8 *) skb_push(skb, 14);
@@ -2959,7 +2958,7 @@ static struct sk_buff *fill_packet_ipv6(struct net_device *odev,
 	mod_cur_headers(pkt_dev);
 	queue_map = pkt_dev->cur_queue_map;
 
-	skb = pktgen_alloc_skb(odev, pkt_dev, 16);
+	skb = pktgen_alloc_skb(odev, pkt_dev);
 	if (!skb) {
 		sprintf(pkt_dev->result, "No memory");
 		return NULL;