Subject: Re: [PATCH net 1/3] virtio-net: drop the rest of buffers when we can't allocate skb

From: Jason Wang <[email-masked]>
Date: Wed, 20 Nov 2013 07:08:02 -0500 (EST)

----- 原始邮件 -----
> On Wed, Nov 20, 2013 at 05:07:25PM +0800, Jason Wang wrote:
> > When mergeable buffer were used, we only put the first page buf leave the
> > rest
> > of buffers in the virt queue. This will cause the driver could not get the
> > correct head buffer any more. Fix this by dropping the rest of buffers for
> > this
> > packet.
> > 
> > The bug was introduced by commit 9ab86bbcf8be755256f0a5e994e0b38af6b4d399
> > (virtio_net: Defer skb allocation in receive path).
> > 
> > Cc: Rusty Russell <[email-masked]>
> > Cc: Michael S. Tsirkin <[email-masked]>
> > Cc: Michael Dalton <[email-masked]>
> > Cc: Eric Dumazet <[email-masked]>
> > Cc: Shirley Ma <[email-masked]>
> > Signed-off-by: Jason Wang <[email-masked]>
> > ---
> > This patch was needed for stable
> > ---
> >  drivers/net/virtio_net.c | 18 +++++++++++++++++-
> >  1 file changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 7bab4de..24fd502 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -222,6 +222,17 @@ static void skb_xmit_done(struct virtqueue *vq)
> >  	netif_wake_subqueue(vi->dev, vq2txq(vq));
> >  }
> >  
> > +static void drop_mergeable_buffer(struct receive_queue *rq, int num_buf)
> > +{
> > +	char *buf;
> > +	int len;
> > +
> > +	while (--num_buf && (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
> > +		--rq->num;
> > +		put_page(virt_to_head_page(buf));
> > +	}
> > +}
> > +
> 
> This is the same code we have in receive_mergeable anyway.
> So let's reuse that.
> 
> 

receive_mergeable() was called after page_to_skb() was called and 
there's lots of conditions check there. I'm not sure how could we 
reuse them.
> >  /* Called from bottom half context */
> >  static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >  				   struct page *page, unsigned int offset,
> > @@ -237,8 +248,13 @@ static struct sk_buff *page_to_skb(struct
> > receive_queue *rq,
> >  
> >  	/* copy small packet so we can reuse these pages for small data */
> >  	skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
> > -	if (unlikely(!skb))
> > +	if (unlikely(!skb)) {
> > +		if (vi->mergeable_rx_bufs) {
> > +			hdr = (struct skb_vnet_hdr *)p;
> > +			drop_mergeable_buffer(rq, hdr->mhdr.num_buffers);
> > +		}
> >  		return NULL;
> > +	}
> >  
> >  	hdr = skb_vnet_hdr(skb);
> >  
> > --
> > 1.8.3.2
> --
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