virtio_net: optimize mergeable_xdp_get_buf()

The previous patch, in order to facilitate review, I do not do any
modification. This patch has made some optimization on the top.

* remove some repeated logics in this function.
* add fast check for passing without any alloc.
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/virtio_net.c

@@ -1196,6 +1196,11 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
 	 */
 	*frame_sz = truesize;
 
+	if (likely(headroom >= virtnet_get_headroom(vi) &&
+		   (*num_buf == 1 || xdp_prog->aux->xdp_has_frags))) {
+		return page_address(*page) + offset;
+	}
+
 	/* This happens when headroom is not enough because
 	 * of the buffer was prefilled before XDP is set.
 	 * This should only happen for the first several packets.
@@ -1204,22 +1209,15 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
 	 * support it, and we don't want to bother users who are
 	 * using xdp normally.
 	 */
-	if (!xdp_prog->aux->xdp_has_frags &&
-	    (*num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
+	if (!xdp_prog->aux->xdp_has_frags) {
 		/* linearize data for XDP */
 		xdp_page = xdp_linearize_page(rq, num_buf,
 					      *page, offset,
 					      VIRTIO_XDP_HEADROOM,
 					      len);
-		*frame_sz = PAGE_SIZE;
-
 		if (!xdp_page)
 			return NULL;
-		offset = VIRTIO_XDP_HEADROOM;
-
-		put_page(*page);
-		*page = xdp_page;
-	} else if (unlikely(headroom < virtnet_get_headroom(vi))) {
+	} else {
 		xdp_room = SKB_DATA_ALIGN(VIRTIO_XDP_HEADROOM +
 					  sizeof(struct skb_shared_info));
 		if (*len + xdp_room > PAGE_SIZE)
@@ -1231,14 +1229,15 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
 
 		memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
 		       page_address(*page) + offset, *len);
-		*frame_sz = PAGE_SIZE;
-		offset = VIRTIO_XDP_HEADROOM;
-
-		put_page(*page);
-		*page = xdp_page;
 	}
 
-	return page_address(*page) + offset;
+	*frame_sz = PAGE_SIZE;
+
+	put_page(*page);
+
+	*page = xdp_page;
+
+	return page_address(*page) + VIRTIO_XDP_HEADROOM;
 }
 
 static struct sk_buff *receive_mergeable(struct net_device *dev,