From: Alexander Duyck <alexander.h.duyck@intel.com>
Date: Mon, 7 May 2018 11:08:40 -0700
Subject: udp: Partially unroll handling of first segment and last segment
Patch-mainline: v4.18-rc1
Git-commit: 0ad6509571e06b302d519f2f05e616ac8c1a10d7
References: bsc#1109837

This patch allows us to take care of unrolling the first segment and the
last segment of the loop for processing the segmented skb. Part of the
motivation for this is that it makes it easier to process the fact that the
first fame and all of the frames in between should be mostly identical
in terms of header data, and the last frame has differences in the length
and partial checksum.

In addition I am dropping the header length calculation since we don't
really need it for anything but the last frame and it can be easily
obtained by just pulling the data_len and offset of tail from the transport
header.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/ipv4/udp_offload.c |   33 +++++++++++++++++++--------------
 1 file changed, 19 insertions(+), 14 deletions(-)

--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -193,7 +193,6 @@ struct sk_buff *__udp_gso_segment(struct
 	struct sock *sk = gso_skb->sk;
 	unsigned int sum_truesize = 0;
 	struct sk_buff *segs, *seg;
-	unsigned int hdrlen;
 	struct udphdr *uh;
 	unsigned int mss;
 	__sum16 check;
@@ -203,7 +202,6 @@ struct sk_buff *__udp_gso_segment(struct
 	if (gso_skb->len <= sizeof(*uh) + mss)
 		return ERR_PTR(-EINVAL);
 
-	hdrlen = gso_skb->data - skb_mac_header(gso_skb);
 	skb_pull(gso_skb, sizeof(*uh));
 
 	/* clear destructor to avoid skb_segment assigning it to tail */
@@ -216,30 +214,37 @@ struct sk_buff *__udp_gso_segment(struct
 		return segs;
 	}
 
-	uh = udp_hdr(segs);
+	seg = segs;
+	uh = udp_hdr(seg);
 
 	/* compute checksum adjustment based on old length versus new */
 	newlen = htons(sizeof(*uh) + mss);
 	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
 
-	for (seg = segs; seg; seg = seg->next) {
-		uh = udp_hdr(seg);
+	for (;;) {
+		seg->destructor = sock_wfree;
+		seg->sk = sk;
+		sum_truesize += seg->truesize;
 
-		/* last packet can be partial gso_size */
-		if (!seg->next) {
-			newlen = htons(seg->len - hdrlen);
-			check = csum16_add(csum16_sub(uh->check, uh->len),
-					   newlen);
-		}
+		if (!seg->next)
+			break;
 
 		uh->len = newlen;
 		uh->check = check;
 
-		seg->destructor = sock_wfree;
-		seg->sk = sk;
-		sum_truesize += seg->truesize;
+		seg = seg->next;
+		uh = udp_hdr(seg);
 	}
 
+	/* last packet can be partial gso_size, account for that in checksum */
+	newlen = htons(skb_tail_pointer(seg) - skb_transport_header(seg) +
+		       seg->data_len);
+	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
+
+	uh->len = newlen;
+	uh->check = check;
+
+	/* update refcount for the packet */
 	atomic_add(sum_truesize - gso_skb->truesize, &sk->sk_wmem_alloc);
 
 	return segs;