Subject: Re: [PATCH net] virtio_net: fix missing dma unmap for resize

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 10 Nov 2023 00:37:32 -0500

On Fri, Nov 10, 2023 at 09:58:45AM +0800, Xuan Zhuo wrote:
> On Thu, 9 Nov 2023 07:06:16 -0500, "Michael S. Tsirkin" <[email-masked]> wrote:
> > On Mon, Nov 06, 2023 at 04:18:32PM +0800, Xuan Zhuo wrote:
> > > For rq, we have three cases getting buffers from virtio core:
> > >
> > > 1. virtqueue_get_buf{,_ctx}
> > > 2. virtqueue_detach_unused_buf
> > > 3. callback for virtqueue_resize
> > >
> > > But in commit 295525e29a5b("virtio_net: merge dma operations when
> > > filling mergeable buffers"), I missed the dma unmap for the #3 case.
> > >
> > > That will leak some memory, because I did not release the pages referred
> > > by the unused buffers.
> > >
> > > If we do such script, we will make the system OOM.
> > >
> > >     while true
> > >     do
> > >             ethtool -G ens4 rx 128
> > >             ethtool -G ens4 rx 256
> > >             free -m
> > >     done
> > >
> > > Fixes: 295525e29a5b ("virtio_net: merge dma operations when filling mergeable buffers")
> > > Signed-off-by: Xuan Zhuo <[email-masked]>
> > > ---
> > >  drivers/net/virtio_net.c | 43 ++++++++++++++++++++--------------------
> > >  1 file changed, 22 insertions(+), 21 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index d16f592c2061..6423a3a007ce 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -408,6 +408,17 @@ static struct page *get_a_page(struct receive_queue *rq, gfp_t gfp_mask)
> > >  	return p;
> > >  }
> > >
> > > +static void virtnet_rq_free_buf(struct virtnet_info *vi,
> > > +				struct receive_queue *rq, void *buf)
> > > +{
> > > +	if (vi->mergeable_rx_bufs)
> > > +		put_page(virt_to_head_page(buf));
> > > +	else if (vi->big_packets)
> > > +		give_pages(rq, buf);
> > > +	else
> > > +		put_page(virt_to_head_page(buf));
> > > +}
> > > +
> >
> > >  static void enable_delayed_refill(struct virtnet_info *vi)
> > >  {
> > >  	spin_lock_bh(&vi->refill_lock);
> > > @@ -634,17 +645,6 @@ static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 *len, void **ctx)
> > >  	return buf;
> > >  }
> > >
> > > -static void *virtnet_rq_detach_unused_buf(struct receive_queue *rq)
> > > -{
> > > -	void *buf;
> > > -
> > > -	buf = virtqueue_detach_unused_buf(rq->vq);
> > > -	if (buf && rq->do_dma)
> > > -		virtnet_rq_unmap(rq, buf, 0);
> > > -
> > > -	return buf;
> > > -}
> > > -
> > >  static void virtnet_rq_init_one_sg(struct receive_queue *rq, void *buf, u32 len)
> > >  {
> > >  	struct virtnet_rq_dma *dma;
> > > @@ -1764,7 +1764,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >  	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> > >  		pr_debug("%s: short packet %i\n", dev->name, len);
> > >  		DEV_STATS_INC(dev, rx_length_errors);
> > > -		virtnet_rq_free_unused_buf(rq->vq, buf);
> > > +		virtnet_rq_free_buf(vi, rq, buf);
> > >  		return;
> > >  	}
> > >
> > > @@ -4034,14 +4034,15 @@ static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf)
> > >  static void virtnet_rq_free_unused_buf(struct virtqueue *vq, void *buf)
> > >  {
> > >  	struct virtnet_info *vi = vq->vdev->priv;
> > > +	struct receive_queue *rq;
> > >  	int i = vq2rxq(vq);
> > >
> > > -	if (vi->mergeable_rx_bufs)
> > > -		put_page(virt_to_head_page(buf));
> > > -	else if (vi->big_packets)
> > > -		give_pages(&vi->rq[i], buf);
> > > -	else
> > > -		put_page(virt_to_head_page(buf));
> > > +	rq = &vi->rq[i];
> > > +
> > > +	if (rq->do_dma)
> > > +		virtnet_rq_unmap(rq, buf, 0);
> > > +
> > > +	virtnet_rq_free_buf(vi, rq, buf);
> > >  }
> > >
> >
> > So we have virtnet_rq_free_buf which sounds like it should free any
> > buf, and we have virtnet_rq_free_unused_buf which is only for unused.
> > Or so it would seem from names but this is not true.
> > Better function names?
> 
> Sorry. not get it.
> 
> virtnet_rq_free_buf() that free the buf passed in. That is called by
> virtnet_rq_free_unused_buf or receive_buf to free the buffer. I think
> the name is right.
> 
> virtnet_rq_free_unused_buf is called by free_unused_bufs() and the
> virtqueue_resize() to free the unused bufs. I think this name is right also.
> 
> So I do not get your mean.
> Are there any details I've overlooked?
> 
> Thanks.

Bad function names - they are too similar. Function name should
say what it does not where it's called from.
What is the difference? That virtnet_rq_free_unused_buf unmaps
and frees and virtnet_rq_free_buf just frees memory?

> >
> > >  static void free_unused_bufs(struct virtnet_info *vi)
> > > @@ -4057,10 +4058,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
> > >  	}
> > >
> > >  	for (i = 0; i < vi->max_queue_pairs; i++) {
> > > -		struct receive_queue *rq = &vi->rq[i];
> > > +		struct virtqueue *vq = vi->rq[i].vq;
> > >
> > > -		while ((buf = virtnet_rq_detach_unused_buf(rq)) != NULL)
> > > -			virtnet_rq_free_unused_buf(rq->vq, buf);
> > > +		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
> > > +			virtnet_rq_free_unused_buf(vq, buf);
> > >  		cond_resched();
> > >  	}
> > >  }
> > > --
> > > 2.32.0.3.g01195cf9f
> >