Re: [PATCH] vsock/virtio: use GFP_ATOMIC under RCU read lock

[Stefano Garzarella <sgarzare@xxxxxxxxxx>]

On Wed, Oct 02, 2024 at 09:41:42AM GMT, Michael S. Tsirkin wrote:
> virtio_transport_send_pkt in now called on transport fast path,
> under RCU read lock. In that case, we have a bug: virtio_add_sgs
> is called with GFP_KERNEL, and might sleep.
>
> Pass the gfp flags as an argument, and use GFP_ATOMIC on
> the fast path.
>
> Link: https://lore.kernel.org/all/hfcr2aget2zojmqpr4uhlzvnep4vgskblx5b6xf2ddosbsrke7@nt34bxgp7j2x
> Fixes: efcd71af38be ("vsock/virtio: avoid queuing packets when intermediate queue is empty")
> Reported-by: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Cc: Luigi Leonardi <luigi.leonardi@xxxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>
> Lightly tested. Christian, could you pls confirm this fixes the problem
> for you? Stefano, it's a holiday here - could you pls help test!

Sure, thanks for the quick fix! I was thinking something similar ;-)

> Thanks!
>
>
> net/vmw_vsock/virtio_transport.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index f992f9a216f0..0cd965f24609 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -96,7 +96,7 @@ static u32 virtio_transport_get_local_cid(void)
>
> /* Caller need to hold vsock->tx_lock on vq */
> static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
> -				     struct virtio_vsock *vsock)
> +				     struct virtio_vsock *vsock, gfp_t gfp)
> {
> 	int ret, in_sg = 0, out_sg = 0;
> 	struct scatterlist **sgs;
> @@ -140,7 +140,7 @@ static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
> 		}
> 	}
>
> -	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
> +	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, gfp);
> 	/* Usually this means that there is no more space available in
> 	 * the vq
> 	 */
> @@ -178,7 +178,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>
> 		reply = virtio_vsock_skb_reply(skb);
>
> -		ret = virtio_transport_send_skb(skb, vq, vsock);
> +		ret = virtio_transport_send_skb(skb, vq, vsock, GFP_KERNEL);
> 		if (ret < 0) {
> 			virtio_vsock_skb_queue_head(&vsock->send_pkt_queue, skb);
> 			break;
> @@ -221,7 +221,7 @@ static int virtio_transport_send_skb_fast_path(struct virtio_vsock *vsock, struc
> 	if (unlikely(ret == 0))
> 		return -EBUSY;
>
> -	ret = virtio_transport_send_skb(skb, vq, vsock);

nit: maybe we can add a comment here:
        /* GFP_ATOMIC because we are in RCU section, so we can't sleep */
> +	ret = virtio_transport_send_skb(skb, vq, vsock, GFP_ATOMIC);
> 	if (ret == 0)
> 		virtqueue_kick(vq);
>
> --
> MST

I'll run some tests and come back with R-b when it's done.

Thanks,
Stefano