Re: [PATCH v2 4/4] Defer skb allocation -- change allocation & receiving in recv path

Michael S. Tsirkin
2009-12-13T11:08:22Z

On Fri, Dec 11, 2009 at 04:49:53AM -0800, Shirley Ma wrote:
> Signed-off-by: Shirley Ma <xma@us.ibm.com>
> -------------

Comments about splitting up this patch apply here.

> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index dde8060..b919169 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -270,99 +270,44 @@ static struct sk_buff *receive_mergeable(struct virtnet_info *vi,
> return skb;
> }
>
> -static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> - unsigned len)
> +static void receive_buf(struct net_device *dev, void *buf, unsigned int len)
> {
> struct virtnet_info *vi = netdev_priv(dev);
> - struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
> - int err;
> - int i;
> + struct sk_buff *skb = (struct sk_buff *)buf;
> + struct page *page = (struct page *)buf;
> + struct skb_vnet_hdr *hdr;

Do not cast away void*.
This initialization above looks very strange: in
fact only one of skb, page makes sense.
So I think you should either get rid of both page and
skb variables (routines such as give_pages get page *
so they will accept void* just as happily) or
move initialization or even use of these variables to
where they are used.

>
> 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++;
> - goto drop;
> - }
> -
> - if (vi->mergeable_rx_bufs) {
> - unsigned int copy;
> - char *p = page_address(skb_shinfo(skb)->frags[0].page);
> -
> - if (len > PAGE_SIZE)
> - len = PAGE_SIZE;
> - len -= sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -
> - memcpy(&hdr->mhdr, p, sizeof(hdr->mhdr));
> - p += sizeof(hdr->mhdr);
> -
> - copy = len;
> - if (copy > skb_tailroom(skb))
> - copy = skb_tailroom(skb);
> -
> - memcpy(skb_put(skb, copy), p, copy);
> -
> - len -= copy;
> -
> - if (!len) {
> - give_pages(vi, skb_shinfo(skb)->frags[0].page);
> - skb_shinfo(skb)->nr_frags--;
> + if (vi->mergeable_rx_bufs || vi->big_packets) {
> + give_pages(vi, page);
> + return;
> } else {
> - skb_shinfo(skb)->frags[0].page_offset +=
> - sizeof(hdr->mhdr) + copy;
> - skb_shinfo(skb)->frags[0].size = len;
> - skb->data_len += len;
> - skb->len += len;
> - }
> -
> - while (--hdr->mhdr.num_buffers) {
> - struct sk_buff *nskb;
> -
> - i = skb_shinfo(skb)->nr_frags;
> - if (i >= MAX_SKB_FRAGS) {
> - pr_debug("%s: packet too long %d\n", dev->name,
> - len);
> - dev->stats.rx_length_errors++;
> - goto drop;
> - }
> -
> - nskb = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> - if (!nskb) {
> - pr_debug("%s: rx error: %d buffers missing\n",
> - dev->name, hdr->mhdr.num_buffers);
> - dev->stats.rx_length_errors++;
> - goto drop;
> - }
> -
> - __skb_unlink(nskb, &vi->recv);
> - vi->num--;
> -
> - skb_shinfo(skb)->frags[i] = skb_shinfo(nskb)->frags[0];
> - skb_shinfo(nskb)->nr_frags = 0;
> - kfree_skb(nskb);
> -
> - if (len > PAGE_SIZE)
> - len = PAGE_SIZE;
> -
> - skb_shinfo(skb)->frags[i].size = len;
> - skb_shinfo(skb)->nr_frags++;
> - skb->data_len += len;
> - skb->len += len;
> + skb_unlink(skb, &vi->recv);
> + goto drop;
> }
> - } else {
> - len -= sizeof(hdr->hdr);
> + }
>
> - if (len <= MAX_PACKET_LEN)
> - trim_pages(vi, skb);
> + if (vi->mergeable_rx_bufs)
> + skb = receive_mergeable(vi, page, len);
> + else if (vi->big_packets)
> + skb = receive_big(vi, page, len);
> + else {
> + __skb_unlink(skb, &vi->recv);
> + len -= sizeof(struct virtio_net_hdr);
> + skb_trim(skb, len);

So above you override skb that you initialized
at the start of function. It would be better
to do in the 3'd case:
skb = buf;
as well. And then it would be clear why
" Only for mergeable and big" comment below
is true.

> + }
>
> - err = pskb_trim(skb, len);
> - if (err) {
> - pr_debug("%s: pskb_trim failed %i %d\n", dev->name,
> - len, err);
> - dev->stats.rx_dropped++;
> - goto drop;
> - }
> + if (unlikely(!skb)) {
> + dev->stats.rx_dropped++;
> + /* only for mergeable buf and big packets */
> + give_pages(vi, page);
> + return;

Did you remove drop: label? If no, is it unused now?

> }
>
> + hdr = skb_vnet_hdr(skb);
> +
> skb->truesize += skb->data_len;
> dev->stats.rx_bytes += skb->len;
> dev->stats.rx_packets++;
> @@ -520,105 +465,22 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi, gfp_t gfp, bool *oom)
> return err;
> }
>
> -static bool try_fill_recv_maxbufs(struct virtnet_info *vi, gfp_t gfp)
> -{
> - struct sk_buff *skb;
> - struct scatterlist sg[2+MAX_SKB_FRAGS];
> - int num, err, i;
> - bool oom = false;
> -
> - sg_init_table(sg, 2+MAX_SKB_FRAGS);
> - do {
> - struct skb_vnet_hdr *hdr;
> -
> - skb = netdev_alloc_skb_ip_align(vi->dev, MAX_PACKET_LEN);
> - if (unlikely(!skb)) {
> - oom = true;
> - break;
> - }
> -
> - skb_put(skb, MAX_PACKET_LEN);
> -
> - hdr = skb_vnet_hdr(skb);
> - sg_set_buf(sg, &hdr->hdr, sizeof(hdr->hdr));
> -
> - if (vi->big_packets) {
> - for (i = 0; i < MAX_SKB_FRAGS; i++) {
> - skb_frag_t *f = &skb_shinfo(skb)->frags[i];
> - f->page = get_a_page(vi, gfp);
> - if (!f->page)
> - break;
> -
> - f->page_offset = 0;
> - f->size = PAGE_SIZE;
> -
> - skb->data_len += PAGE_SIZE;
> - skb->len += PAGE_SIZE;
> -
> - skb_shinfo(skb)->nr_frags++;
> - }
> - }
> -
> - num = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
> - skb_queue_head(&vi->recv, skb);
> -
> - err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, num, skb);
> - if (err < 0) {
> - skb_unlink(skb, &vi->recv);
> - trim_pages(vi, skb);
> - kfree_skb(skb);
> - break;
> - }
> - vi->num++;
> - } while (err >= num);
> - if (unlikely(vi->num > vi->max))
> - vi->max = vi->num;
> - vi->rvq->vq_ops->kick(vi->rvq);
> - return !oom;
> -}
> -
> /* Returns false if we couldn't fill entirely (OOM). */
> static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
> {
> - struct sk_buff *skb;
> - struct scatterlist sg[1];
> int err;
> bool oom = false;
>
> - if (!vi->mergeable_rx_bufs)
> - return try_fill_recv_maxbufs(vi, gfp);
> -
> do {
> - skb_frag_t *f;
> -
> - skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
> - if (unlikely(!skb)) {
> - oom = true;
> - break;
> - }
> -
> - f = &skb_shinfo(skb)->frags[0];
> - f->page = get_a_page(vi, gfp);
> - if (!f->page) {
> - oom = true;
> - kfree_skb(skb);
> - break;
> - }
> -
> - f->page_offset = 0;
> - f->size = PAGE_SIZE;
> -
> - skb_shinfo(skb)->nr_frags++;
> -
> - sg_init_one(sg, page_address(f->page), PAGE_SIZE);
> - skb_queue_head(&vi->recv, skb);
> + if (vi->mergeable_rx_bufs)
> + err = add_recvbuf_mergeable(vi, gfp, &oom);
> + else if (vi->big_packets)
> + err = add_recvbuf_big(vi, gfp, &oom);
> + else
> + err = add_recvbuf_small(vi, gfp, &oom);
>
> - err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, skb);
> - if (err < 0) {
> - skb_unlink(skb, &vi->recv);
> - kfree_skb(skb);
> + if (oom || err < 0)
> break;
> - }
> vi->num++;
> } while (err > 0);
> if (unlikely(vi->num > vi->max))
> @@ -657,14 +519,13 @@ static void refill_work(struct work_struct *work)
> static int virtnet_poll(struct napi_struct *napi, int budget)
> {
> struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
> - struct sk_buff *skb = NULL;
> + void *buf = NULL;

Does this have to be initialized? If not (as it seems) better not do it.

> unsigned int len, received = 0;
>
> again:
> while (received < budget &&
> - (skb = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> - __skb_unlink(skb, &vi->recv);
> - receive_skb(vi->dev, skb, len);
> + (buf = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> + receive_buf(vi->dev, buf, len);
> vi->num--;
> received++;
> }
> @@ -1205,6 +1066,7 @@ static void __devexit virtnet_remove(struct virtio_device *vdev)
> {
> struct virtnet_info *vi = vdev->priv;
> struct sk_buff *skb;
> + int freed;
>
> /* Stop all the virtqueues. */
> vdev->config->reset(vdev);
> @@ -1216,11 +1078,14 @@ static void __devexit virtnet_remove(struct virtio_device *vdev)
> }
> __skb_queue_purge(&vi->send);
>
> - BUG_ON(vi->num != 0);
> -
> unregister_netdev(vi->dev);
> cancel_delayed_work_sync(&vi->refill);
>
> + freed = vi->rvq->vq_ops->destroy_bufs(vi->rvq, virtio_net_free_pages);

This looks like double free to me: should not you remove code that does
__skb_dequeue on recv above?

> + vi->num -= freed;
> +
> + BUG_ON(vi->num != 0);
> +
> vdev->config->del_vqs(vi->vdev);
>
> while (vi->pages)
>