virtio_net: migrate mergeable rx buffers to page frag allocators

The virtio_net driver's mergeable receive buffer allocator
uses 4KB packet buffers. For MTU-sized traffic, SKB truesize
is > 4KB but only ~1500 bytes of the buffer is used to store
packet data, reducing the effective TCP window size
substantially. This patch addresses the performance concerns
with mergeable receive buffers by allocating MTU-sized packet
buffers using page frag allocators. If more than MAX_SKB_FRAGS
buffers are needed, the SKB frag_list is used.

Signed-off-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Michael Dalton

drivers/net/virtio_net.c

@@ -124,6 +124,11 @@ struct virtnet_info {
 	/* Lock for config space updates */
 	struct mutex config_lock;
 
+	/* Page_frag for GFP_KERNEL packet buffer allocation when we run
+	 * low on memory.
+	 */
+	struct page_frag alloc_frag;
+
 	/* Does the affinity hint is set for virtqueues? */
 	bool affinity_hint_set;
 
@@ -217,33 +222,18 @@ static void skb_xmit_done(struct virtqueue *vq)
 	netif_wake_subqueue(vi->dev, vq2txq(vq));
 }
 
-static void set_skb_frag(struct sk_buff *skb, struct page *page,
-			 unsigned int offset, unsigned int *len)
-{
-	int size = min((unsigned)PAGE_SIZE - offset, *len);
-	int i = skb_shinfo(skb)->nr_frags;
-
-	__skb_fill_page_desc(skb, i, page, offset, size);
-
-	skb->data_len += size;
-	skb->len += size;
-	skb->truesize += PAGE_SIZE;
-	skb_shinfo(skb)->nr_frags++;
-	skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
-	*len -= size;
-}
-
 /* Called from bottom half context */
 static struct sk_buff *page_to_skb(struct receive_queue *rq,
-				   struct page *page, unsigned int len)
+				   struct page *page, unsigned int offset,
+				   unsigned int len, unsigned int truesize)
 {
 	struct virtnet_info *vi = rq->vq->vdev->priv;
 	struct sk_buff *skb;
 	struct skb_vnet_hdr *hdr;
-	unsigned int copy, hdr_len, offset;
+	unsigned int copy, hdr_len, hdr_padded_len;
 	char *p;
 
-	p = page_address(page);
+	p = page_address(page) + offset;
 
 	/* copy small packet so we can reuse these pages for small data */
 	skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
@@ -254,16 +244,17 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
 
 	if (vi->mergeable_rx_bufs) {
 		hdr_len = sizeof hdr->mhdr;
-		offset = hdr_len;
+		hdr_padded_len = sizeof hdr->mhdr;
 	} else {
 		hdr_len = sizeof hdr->hdr;
-		offset = sizeof(struct padded_vnet_hdr);
+		hdr_padded_len = sizeof(struct padded_vnet_hdr);
 	}
 
 	memcpy(hdr, p, hdr_len);
 
 	len -= hdr_len;
-	p += offset;
+	offset += hdr_padded_len;
+	p += hdr_padded_len;
 
 	copy = len;
 	if (copy > skb_tailroom(skb))
@@ -273,6 +264,14 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
 	len -= copy;
 	offset += copy;
 
+	if (vi->mergeable_rx_bufs) {
+		if (len)
+			skb_add_rx_frag(skb, 0, page, offset, len, truesize);
+		else
+			put_page(page);
+		return skb;
+	}
+
 	/*
 	 * Verify that we can indeed put this data into a skb.
 	 * This is here to handle cases when the device erroneously
@@ -284,9 +283,12 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
 		dev_kfree_skb(skb);
 		return NULL;
 	}
-
+	BUG_ON(offset >= PAGE_SIZE);
 	while (len) {
-		set_skb_frag(skb, page, offset, &len);
+		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
+		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
+				frag_size, truesize);
+		len -= frag_size;
 		page = (struct page *)page->private;
 		offset = 0;
 	}
@@ -297,33 +299,52 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
 	return skb;
 }
 
-static int receive_mergeable(struct receive_queue *rq, struct sk_buff *skb)
+static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
 {
-	struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
+	struct skb_vnet_hdr *hdr = skb_vnet_hdr(head_skb);
+	struct sk_buff *curr_skb = head_skb;
+	char *buf;
 	struct page *page;
-	int num_buf, i, len;
+	int num_buf, len;
 
 	num_buf = hdr->mhdr.num_buffers;
 	while (--num_buf) {
-		i = skb_shinfo(skb)->nr_frags;
-		if (i >= MAX_SKB_FRAGS) {
-			pr_debug("%s: packet too long\n", skb->dev->name);
-			skb->dev->stats.rx_length_errors++;
-			return -EINVAL;
-		}
-		page = virtqueue_get_buf(rq->vq, &len);
-		if (!page) {
+		int num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
+		buf = virtqueue_get_buf(rq->vq, &len);
+		if (unlikely(!buf)) {
 			pr_debug("%s: rx error: %d buffers missing\n",
-				 skb->dev->name, hdr->mhdr.num_buffers);
-			skb->dev->stats.rx_length_errors++;
+				 head_skb->dev->name, hdr->mhdr.num_buffers);
+			head_skb->dev->stats.rx_length_errors++;
 			return -EINVAL;
 		}
-
-		if (len > PAGE_SIZE)
-			len = PAGE_SIZE;
-
-		set_skb_frag(skb, page, 0, &len);
-
+		if (unlikely(len > MAX_PACKET_LEN)) {
+			pr_debug("%s: rx error: merge buffer too long\n",
+				 head_skb->dev->name);
+			len = MAX_PACKET_LEN;
+		}
+		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
+			struct sk_buff *nskb = alloc_skb(0, GFP_ATOMIC);
+			if (unlikely(!nskb)) {
+				head_skb->dev->stats.rx_dropped++;
+				return -ENOMEM;
+			}
+			if (curr_skb == head_skb)
+				skb_shinfo(curr_skb)->frag_list = nskb;
+			else
+				curr_skb->next = nskb;
+			curr_skb = nskb;
+			head_skb->truesize += nskb->truesize;
+			num_skb_frags = 0;
+		}
+		if (curr_skb != head_skb) {
+			head_skb->data_len += len;
+			head_skb->len += len;
+			head_skb->truesize += MAX_PACKET_LEN;
+		}
+		page = virt_to_head_page(buf);
+		skb_add_rx_frag(curr_skb, num_skb_frags, page,
+				buf - (char *)page_address(page), len,
+				MAX_PACKET_LEN);
 		--rq->num;
 	}
 	return 0;
@@ -341,8 +362,10 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
 	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
 		pr_debug("%s: short packet %i\n", dev->name, len);
 		dev->stats.rx_length_errors++;
-		if (vi->mergeable_rx_bufs || vi->big_packets)
+		if (vi->big_packets)
 			give_pages(rq, buf);
+		else if (vi->mergeable_rx_bufs)
+			put_page(virt_to_head_page(buf));
 		else
 			dev_kfree_skb(buf);
 		return;
@@ -352,19 +375,28 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
 		skb = buf;
 		len -= sizeof(struct virtio_net_hdr);
 		skb_trim(skb, len);
+	} else if (vi->mergeable_rx_bufs) {
+		struct page *page = virt_to_head_page(buf);
+		skb = page_to_skb(rq, page,
+				  (char *)buf - (char *)page_address(page),
+				  len, MAX_PACKET_LEN);
+		if (unlikely(!skb)) {
+			dev->stats.rx_dropped++;
+			put_page(page);
+			return;
+		}
+		if (receive_mergeable(rq, skb)) {
+			dev_kfree_skb(skb);
+			return;
+		}
 	} else {
 		page = buf;
-		skb = page_to_skb(rq, page, len);
+		skb = page_to_skb(rq, page, 0, len, PAGE_SIZE);
 		if (unlikely(!skb)) {
 			dev->stats.rx_dropped++;
 			give_pages(rq, page);
 			return;
 		}
-		if (vi->mergeable_rx_bufs)
-			if (receive_mergeable(rq, skb)) {
-				dev_kfree_skb(skb);
-				return;
-			}
 	}
 
 	hdr = skb_vnet_hdr(skb);
@@ -501,18 +533,28 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp)
 
 static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
 {
-	struct page *page;
+	struct virtnet_info *vi = rq->vq->vdev->priv;
+	char *buf = NULL;
 	int err;
 
-	page = get_a_page(rq, gfp);
-	if (!page)
+	if (gfp & __GFP_WAIT) {
+		if (skb_page_frag_refill(MAX_PACKET_LEN, &vi->alloc_frag,
+					 gfp)) {
+			buf = (char *)page_address(vi->alloc_frag.page) +
+			      vi->alloc_frag.offset;
+			get_page(vi->alloc_frag.page);
+			vi->alloc_frag.offset += MAX_PACKET_LEN;
+		}
+	} else {
+		buf = netdev_alloc_frag(MAX_PACKET_LEN);
+	}
+	if (!buf)
 		return -ENOMEM;
 
-	sg_init_one(rq->sg, page_address(page), PAGE_SIZE);
-
-	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, page, gfp);
+	sg_init_one(rq->sg, buf, MAX_PACKET_LEN);
+	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, buf, gfp);
 	if (err < 0)
-		give_pages(rq, page);
+		put_page(virt_to_head_page(buf));
 
 	return err;
 }
@@ -1343,8 +1385,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
 		struct virtqueue *vq = vi->rq[i].vq;
 
 		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL) {
-			if (vi->mergeable_rx_bufs || vi->big_packets)
+			if (vi->big_packets)
 				give_pages(&vi->rq[i], buf);
+			else if (vi->mergeable_rx_bufs)
+				put_page(virt_to_head_page(buf));
 			else
 				dev_kfree_skb(buf);
 			--vi->rq[i].num;
@@ -1650,6 +1694,8 @@ static int virtnet_probe(struct virtio_device *vdev)
 free_vqs:
 	cancel_delayed_work_sync(&vi->refill);
 	virtnet_del_vqs(vi);
+	if (vi->alloc_frag.page)
+		put_page(vi->alloc_frag.page);
 free_index:
 	free_percpu(vi->vq_index);
 free_stats:
@@ -1685,6 +1731,8 @@ static void virtnet_remove(struct virtio_device *vdev)
 	unregister_netdev(vi->dev);
 
 	remove_vq_common(vi);
+	if (vi->alloc_frag.page)
+		put_page(vi->alloc_frag.page);
 
 	flush_work(&vi->config_work);