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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 9 Nov 2023 07:06:16 -0500

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?

>  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