Linux Virtualization - Nov 2013 - [PATCH net 2/3] virtio-net: fix num calculation on frag skb allocation failure

On Wed, Nov 20, 2013 at 05:07:26PM +0800, Jason Wang
wrote:
> We need decrease the rq->num after we can get a buf through
> virtqueue_get_buf() even if we could not allocate frag skb. Otherwise, the
> refill routine won't be triggered under heavy memory stress since the
driver may
> still think there's enough room.
> 
> This bug was introduced by commit 2613af0ed18a11d5c566a81f9a6510b73180660a
> (virtio_net: migrate mergeable rx buffers to page frag allocators).
> 
> Cc: Rusty Russell <rusty at rustcorp.com.au>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: Michael Dalton <mwdalton at google.com>
> Cc: Eric Dumazet <edumazet at google.com>
> Signed-off-by: Jason Wang <jasowang at redhat.com>
So let's wrap virtqueue_get_buf to make sure we get it right?
> ---
> The patch was needed for 3.12 stable.
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 24fd502..de1d6ca 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -333,6 +333,7 @@ static int receive_mergeable(struct receive_queue *rq,
struct sk_buff *head_skb)
>  			head_skb->dev->stats.rx_length_errors++;
>  			return -EINVAL;
>  		}
> +		--rq->num;
>  		if (unlikely(len > MERGE_BUFFER_LEN)) {
>  			pr_debug("%s: rx error: merge buffer too long\n",
>  				 head_skb->dev->name);
> @@ -367,7 +368,6 @@ static int receive_mergeable(struct receive_queue *rq,
struct sk_buff *head_skb)
>  			skb_add_rx_frag(curr_skb, num_skb_frags, page,
>  					offset, len, MERGE_BUFFER_LEN);
>  		}
> -		--rq->num;
>  	}
>  	return 0;
>  }
> -- 
> 1.8.3.2

----- ???? -----
> On Wed, Nov 20, 2013 at 05:07:26PM +0800, Jason Wang wrote:
> > We need decrease the rq->num after we can get a buf through
> > virtqueue_get_buf() even if we could not allocate frag skb. Otherwise,
the
> > refill routine won't be triggered under heavy memory stress since
the
> > driver may
> > still think there's enough room.
> > 
> > This bug was introduced by commit
2613af0ed18a11d5c566a81f9a6510b73180660a
> > (virtio_net: migrate mergeable rx buffers to page frag allocators).
> > 
> > Cc: Rusty Russell <rusty at rustcorp.com.au>
> > Cc: Michael S. Tsirkin <mst at redhat.com>
> > Cc: Michael Dalton <mwdalton at google.com>
> > Cc: Eric Dumazet <edumazet at google.com>
> > Signed-off-by: Jason Wang <jasowang at redhat.com>
> 
> So let's wrap virtqueue_get_buf to make sure we get it right?
> 
Ok. good idea.
> > ---
> > The patch was needed for 3.12 stable.
> > ---
> >  drivers/net/virtio_net.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 24fd502..de1d6ca 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -333,6 +333,7 @@ static int receive_mergeable(struct receive_queue
*rq,
> > struct sk_buff *head_skb)
> >  			head_skb->dev->stats.rx_length_errors++;
> >  			return -EINVAL;
> >  		}
> > +		--rq->num;
> >  		if (unlikely(len > MERGE_BUFFER_LEN)) {
> >  			pr_debug("%s: rx error: merge buffer too long\n",
> >  				 head_skb->dev->name);
> > @@ -367,7 +368,6 @@ static int receive_mergeable(struct receive_queue
*rq,
> > struct sk_buff *head_skb)
> >  			skb_add_rx_frag(curr_skb, num_skb_frags, page,
> >  					offset, len, MERGE_BUFFER_LEN);
> >  		}
> > -		--rq->num;
> >  	}
> >  	return 0;
> >  }
> > --
> > 1.8.3.2
>

On Wed, Nov 20, 2013 at 07:08:50AM -0500, Jason Wang
wrote:
> 
> 
> ----- ???? -----
> > On Wed, Nov 20, 2013 at 05:07:26PM +0800, Jason Wang wrote:
> > > We need decrease the rq->num after we can get a buf through
> > > virtqueue_get_buf() even if we could not allocate frag skb.
Otherwise, the
> > > refill routine won't be triggered under heavy memory stress
since the
> > > driver may
> > > still think there's enough room.
> > > 
> > > This bug was introduced by commit
2613af0ed18a11d5c566a81f9a6510b73180660a
> > > (virtio_net: migrate mergeable rx buffers to page frag
allocators).
> > > 
> > > Cc: Rusty Russell <rusty at rustcorp.com.au>
> > > Cc: Michael S. Tsirkin <mst at redhat.com>
> > > Cc: Michael Dalton <mwdalton at google.com>
> > > Cc: Eric Dumazet <edumazet at google.com>
> > > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > 
> > So let's wrap virtqueue_get_buf to make sure we get it right?
> > 
> 
> Ok. good idea.
So I did this (below) but the compiler is not smart enough to
avoid two branches on data path.
So I'm not sure anymore: with my patch it's pretty clear how
the code works.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>

but I don't think we need to apply this.

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 11d9cc9..1cc2e43 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -296,6 +296,14 @@ static struct sk_buff *page_to_skb(struct receive_queue
*rq,
 	return skb;
 }
 
+static void *rq_get_buf(struct receive_queue *rq, unsigned int *len)
+{
+	void *buf = virtqueue_get_buf(rq->vq, len);
+	if (buf)
+		rq->num--;
+	return buf;
+}
+
 static struct sk_buff *receive_mergeable(struct net_device *dev,
 					 struct receive_queue *rq,
 					 void *buf,
@@ -315,7 +323,7 @@ static struct sk_buff *receive_mergeable(struct net_device
*dev,
 	while (--num_buf) {
 		int num_skb_frags;
 
-		buf = virtqueue_get_buf(rq->vq, &len);
+		buf = rq_get_buf(rq, &len);
 		if (unlikely(!buf)) {
 			pr_debug("%s: rx error: %d buffers out of %d missing\n",
 				 dev->name, num_buf, hdr->mhdr.num_buffers);
@@ -329,7 +337,6 @@ static struct sk_buff *receive_mergeable(struct net_device
*dev,
 		}
 
 		page = virt_to_head_page(buf);
-		--rq->num;
 
 		num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
 		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
@@ -370,7 +377,7 @@ err_buf:
 	dev->stats.rx_dropped++;
 	dev_kfree_skb(head_skb);
 	while (--num_buf) {
-		buf = virtqueue_get_buf(rq->vq, &len);
+		buf = rq_get_buf(rq, &len);
 		if (unlikely(!buf)) {
 			pr_debug("%s: rx error: %d buffers missing\n",
 				 dev->name, num_buf);
@@ -379,7 +386,6 @@ err_buf:
 		}
 		page = virt_to_head_page(buf);
 		put_page(page);
-		--rq->num;
 	}
 	return NULL;
 }
@@ -675,9 +681,8 @@ static int virtnet_poll(struct napi_struct *napi, int
budget)
 
 again:
 	while (received < budget &&
-	       (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
+	       (buf = rq_get_buf(rq, &len)) != NULL) {
 		receive_buf(rq, buf, len);
-		--rq->num;
 		received++;
 	}