[PATCH bpf-next] xsk: build skb by page

[Xuan Zhuo]

This patch is used to construct skb based on page to save memory copy
overhead.

Taking into account the problem of addr unaligned, and the
possibility of frame size greater than page in the future.

Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
---
 net/xdp/xsk.c | 68 ++++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 51 insertions(+), 17 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index ac4a317..7cab40f 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -430,6 +430,55 @@ static void xsk_destruct_skb(struct sk_buff *skb)
 	sock_wfree(skb);
 }
 
+static struct sk_buff *xsk_build_skb_bypage(struct xdp_sock *xs, struct xdp_desc *desc)
+{
+	char *buffer;
+	u64 addr;
+	u32 len, offset, copy, copied;
+	int err, i;
+	struct page *page;
+	struct sk_buff *skb;
+
+	skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
+	if (unlikely(!skb))
+		return NULL;
+
+	addr = desc->addr;
+	len = desc->len;
+
+	buffer = xsk_buff_raw_get_data(xs->pool, addr);
+	offset = offset_in_page(buffer);
+	addr = buffer - (char *)xs->pool->addrs;
+
+	for (copied = 0, i = 0; copied < len; ++i) {
+		page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
+
+		get_page(page);
+
+		copy = min((u32)(PAGE_SIZE - offset), len - copied);
+
+		skb_fill_page_desc(skb, i, page, offset, copy);
+
+		copied += copy;
+		addr += copy;
+		offset = 0;
+	}
+
+	skb->len += len;
+	skb->data_len += len;
+	skb->truesize += len;
+
+	refcount_add(len, &xs->sk.sk_wmem_alloc);
+
+	skb->dev = xs->dev;
+	skb->priority = xs->sk.sk_priority;
+	skb->mark = xs->sk.sk_mark;
+	skb_shinfo(skb)->destructor_arg = (void *)(long)addr;
+	skb->destructor = xsk_destruct_skb;
+
+	return skb;
+}
+
 static int xsk_generic_xmit(struct sock *sk)
 {
 	struct xdp_sock *xs = xdp_sk(sk);
@@ -445,40 +494,25 @@ static int xsk_generic_xmit(struct sock *sk)
 		goto out;
 
 	while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
-		char *buffer;
-		u64 addr;
-		u32 len;
-
 		if (max_batch-- == 0) {
 			err = -EAGAIN;
 			goto out;
 		}
 
-		len = desc.len;
-		skb = sock_alloc_send_skb(sk, len, 1, &err);
+		skb = xsk_build_skb_bypage(xs, &desc);
 		if (unlikely(!skb))
 			goto out;
 
-		skb_put(skb, len);
-		addr = desc.addr;
-		buffer = xsk_buff_raw_get_data(xs->pool, addr);
-		err = skb_store_bits(skb, 0, buffer, len);
 		/* This is the backpressure mechanism for the Tx path.
 		 * Reserve space in the completion queue and only proceed
 		 * if there is space in it. This avoids having to implement
 		 * any buffering in the Tx path.
 		 */
-		if (unlikely(err) || xskq_prod_reserve(xs->pool->cq)) {
+		if (xskq_prod_reserve(xs->pool->cq)) {
 			kfree_skb(skb);
 			goto out;
 		}
 
-		skb->dev = xs->dev;
-		skb->priority = sk->sk_priority;
-		skb->mark = sk->sk_mark;
-		skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
-		skb->destructor = xsk_destruct_skb;
-
 		err = __dev_direct_xmit(skb, xs->queue_id);
 		if  (err == NETDEV_TX_BUSY) {
 			/* Tell user-space to retry the send */
-- 
1.8.3.1