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

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Date:	Wed, 20 Nov 2013 15:27:29 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Michael Dalton <mwdalton@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>, Shirley Ma <xma@...ibm.com>
Subject: Re: [PATCH net 1/3] virtio-net: drop the rest of buffers when we
 can't allocate skb

On Wed, Nov 20, 2013 at 07:08:02AM -0500, Jason Wang wrote:
> 
> 
> ----- 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 <rusty@...tcorp.com.au>
> > > Cc: Michael S. Tsirkin <mst@...hat.com>
> > > Cc: Michael Dalton <mwdalton@...gle.com>
> > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Shirley Ma <xma@...ibm.com>
> > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > > ---
> > > 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.

I posted a patch showing how :)

> > >  /* 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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> > 
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