netdev - [net-next PATCH v2 5/8] udp: Partially unroll handling of first segment and last segment

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Date:   Fri, 04 May 2018 11:30:32 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     netdev@...r.kernel.org, willemb@...gle.com, davem@...emloft.net
Subject: [net-next PATCH v2 5/8] udp: Partially unroll handling of first
 segment and last segment

From: Alexander Duyck <alexander.h.duyck@...el.com>

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@...el.com>
---

v2: New break-out patch based on one patch from earlier series

 net/ipv4/udp_offload.c |   35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 5c4bb8b9e83e..946d06d2aa0c 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -193,7 +193,6 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	struct sk_buff *seg, *segs = ERR_PTR(-EINVAL);
 	struct sock *sk = gso_skb->sk;
 	unsigned int sum_truesize = 0;
-	unsigned int hdrlen;
 	struct udphdr *uh;
 	unsigned int mss;
 	__sum16 check;
@@ -206,7 +205,6 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	if (!pskb_may_pull(gso_skb, sizeof(*uh)))
 		goto out;
 
-	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 */
@@ -219,7 +217,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		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);
@@ -227,25 +226,31 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 					    ((__force u32)uh->len ^ 0xFFFF) +
 					    (__force u32)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 = ~csum_fold((__force __wsum)((__force u32)uh->check +
-							    ((__force u32)uh->len ^ 0xFFFF) +
-							    (__force u32)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 = ~csum_fold((__force __wsum)((__force u32)uh->check +
+					    ((__force u32)uh->len ^ 0xFFFF)  +
+					    (__force u32)newlen));
+	uh->len = newlen;
+	uh->check = check;
+
+	/* update refcount for the packet */
 	refcount_add(sum_truesize - gso_skb->truesize, &sk->sk_wmem_alloc);
 out:
 	return segs;