Subject: Re: [PATCH net] virtio-net: fix page refcnt leaking when fail to allocate frag skb

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 19 Nov 2013 20:44:11 +0200

On Tue, Nov 19, 2013 at 06:03:48AM -0800, Eric Dumazet wrote:
> On Tue, 2013-11-19 at 16:05 +0800, Jason Wang wrote:
> > We need to drop the refcnt of page when we fail to allocate an skb for frag
> > list, otherwise it will be leaked. The bug was introduced by commit
> > 2613af0ed18a11d5c566a81f9a6510b73180660a ("virtio_net: migrate mergeable rx
> > buffers to page frag allocators").
> > 
> > Cc: Michael Dalton <[email-masked]>
> > Cc: Eric Dumazet <[email-masked]>
> > Cc: Rusty Russell <[email-masked]>
> > Cc: Michael S. Tsirkin <[email-masked]>
> > Signed-off-by: Jason Wang <[email-masked]>
> > ---
> > The patch was needed for 3.12 stable.
> 
> Good catch, but if we return from receive_mergeable() in the 'middle'
> of the frags we would need for the current skb, who will
> call the virtqueue_get_buf() to flush the remaining frags ?
> 
> Don't we also need to call virtqueue_get_buf() like 
> 
> while (--num_buf) {
>     buf = virtqueue_get_buf(rq->vq, &len);
>     if (!buf)
>         break;
>     put_page(virt_to_head_page(buf));
> }
> 
> ?
> 
> 
> 

virtqueue_get_buf only gives you back a buffer that has been DMA-ed
to by hardware. ATM there's no way to get back a buffer once you
gave it to hardware without doing a NIC reset.