Subject: Re: vhost changes (batched) in linux-next after 12/13 trigger random crashes in KVM guests after reboot

From: Christian Borntraeger <[email-masked]>
Date: Fri, 27 Mar 2020 16:46:39 +0100

On 27.03.20 12:08, Eugenio Pérez wrote:
> Hi Christian.
> 
> Sorry for the late response. Could we try this one over eccb852f1fe6bede630e2e4f1a121a81e34354ab, and see if you still
> can reproduce the bug?

To much time has passed and too many things have changed on that system.
I have trouble reproducing this with either
eccb852f1fe6bede630e2e4f1a121a81e34354ab or 52c36ce7f334.
I will try to reproduce this again :-/

> 
> Apart from that, could you print me the backtrace when qemu calls vhost_kernel_set_vring_base and
> vhost_kernel_get_vring_base functions?
> 
> Thank you very much!
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index e158159671fa..a1a4239512bb 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1505,10 +1505,13 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  
>  	mutex_lock(&n->dev.mutex);
>  	r = vhost_dev_check_owner(&n->dev);
> -	if (r)
> +	if (r) {
> +		pr_debug("vhost_dev_check_owner index=%u fd=%d rc r=%d", index, fd, r);
>  		goto err;
> +	}
>  
>  	if (index >= VHOST_NET_VQ_MAX) {
> +		pr_debug("vhost_dev_check_owner index=%u fd=%d MAX=%d", index, fd, VHOST_NET_VQ_MAX);
>  		r = -ENOBUFS;
>  		goto err;
>  	}
> @@ -1518,22 +1521,26 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  
>  	/* Verify that ring has been setup correctly. */
>  	if (!vhost_vq_access_ok(vq)) {
> +		pr_debug("vhost_net_set_backend index=%u fd=%d !vhost_vq_access_ok", index, fd);
>  		r = -EFAULT;
>  		goto err_vq;
>  	}
>  	sock = get_socket(fd);
>  	if (IS_ERR(sock)) {
>  		r = PTR_ERR(sock);
> +		pr_debug("vhost_net_set_backend index=%u fd=%d get_socket err r=%d", index, fd, r);
>  		goto err_vq;
>  	}
>  
>  	/* start polling new socket */
>  	oldsock = vq->private_data;
>  	if (sock != oldsock) {
> +		pr_debug("sock=%p != oldsock=%p index=%u fd=%d vq=%p", sock, oldsock, index, fd, vq);
>  		ubufs = vhost_net_ubuf_alloc(vq,
>  					     sock && vhost_sock_zcopy(sock));
>  		if (IS_ERR(ubufs)) {
>  			r = PTR_ERR(ubufs);
> +			pr_debug("ubufs index=%u fd=%d err r=%d vq=%p", index, fd, r, vq);
>  			goto err_ubufs;
>  		}
>  
> @@ -1541,11 +1548,15 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  		vq->private_data = sock;
>  		vhost_net_buf_unproduce(nvq);
>  		r = vhost_vq_init_access(vq);
> -		if (r)
> +		if (r) {
> +			pr_debug("init_access index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
>  			goto err_used;
> +		}
>  		r = vhost_net_enable_vq(n, vq);
> -		if (r)
> +		if (r) {
> +			pr_debug("enable_vq index=%u fd=%d r=%d vq=%p", index, fd, r, vq);
>  			goto err_used;
> +		}
>  		if (index == VHOST_NET_VQ_RX)
>  			nvq->rx_ring = get_tap_ptr_ring(fd);
>  
> @@ -1559,6 +1570,8 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  
>  	mutex_unlock(&vq->mutex);
>  
> +	pr_debug("sock=%p", sock);
> +
>  	if (oldubufs) {
>  		vhost_net_ubuf_put_wait_and_free(oldubufs);
>  		mutex_lock(&vq->mutex);
> @@ -1712,6 +1725,9 @@ static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
>  	case VHOST_NET_SET_BACKEND:
>  		if (copy_from_user(&backend, argp, sizeof backend))
>  			return -EFAULT;
> +		pr_debug("VHOST_NET_SET_BACKEND [b.index=%u][b.fd=%d]",
> +			 backend.index, backend.fd);
> +		dump_stack();
>  		return vhost_net_set_backend(n, backend.index, backend.fd);
>  	case VHOST_GET_FEATURES:
>  		features = VHOST_NET_FEATURES;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index b5a51b1f2e79..9dd0bcae0b22 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -372,6 +372,11 @@ static int vhost_worker(void *data)
>  	return 0;
>  }
>  
> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> +{
> +	return vq->max_descs - UIO_MAXIOV;
> +}
> +
>  static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
>  {
>  	kfree(vq->descs);
> @@ -394,7 +399,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>  	for (i = 0; i < dev->nvqs; ++i) {
>  		vq = dev->vqs[i];
>  		vq->max_descs = dev->iov_limit;
> -		vq->batch_descs = dev->iov_limit - UIO_MAXIOV;
> +		if (vhost_vq_num_batch_descs(vq) < 0) {
> +			return -EINVAL;
> +		}
>  		vq->descs = kmalloc_array(vq->max_descs,
>  					  sizeof(*vq->descs),
>  					  GFP_KERNEL);
> @@ -1642,15 +1649,27 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
>  			r = -EINVAL;
>  			break;
>  		}
> +
> +		pr_debug(
> +			"VHOST_SET_VRING_BASE [vq=%p][s.index=%u][s.num=%u][vq->avail_idx=%d][vq->last_avail_idx=%d][vq-
>> ndescs=%d][vq->first_desc=%d]",
> +			vq, s.index, s.num, vq->avail_idx, vq->last_avail_idx,
> +			vq->ndescs, vq->first_desc);
> +		dump_stack();
>  		vq->last_avail_idx = s.num;
>  		/* Forget the cached index value. */
>  		vq->avail_idx = vq->last_avail_idx;
> +		vq->ndescs = vq->first_desc = 0;
>  		break;
>  	case VHOST_GET_VRING_BASE:
>  		s.index = idx;
>  		s.num = vq->last_avail_idx;
>  		if (copy_to_user(argp, &s, sizeof s))
>  			r = -EFAULT;
> +		pr_debug(
> +			"VHOST_GET_VRING_BASE [vq=%p][s.index=%u][s.num=%u][vq->avail_idx=%d][vq->last_avail_idx=%d][vq-
>> ndescs=%d][vq->first_desc=%d]",
> +			vq, s.index, s.num, vq->avail_idx, vq->last_avail_idx,
> +			vq->ndescs, vq->first_desc);
> +		dump_stack();
>  		break;
>  	case VHOST_SET_VRING_KICK:
>  		if (copy_from_user(&f, argp, sizeof f)) {
> @@ -2239,8 +2258,8 @@ static int fetch_buf(struct vhost_virtqueue *vq)
>  		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>  
>  		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> -			vq_err(vq, "Guest moved used index from %u to %u",
> -				last_avail_idx, vq->avail_idx);
> +			vq_err(vq, "Guest moved vq %p used index from %u to %u",
> +				vq, last_avail_idx, vq->avail_idx);
>  			return -EFAULT;
>  		}
>  
> @@ -2316,6 +2335,9 @@ static int fetch_buf(struct vhost_virtqueue *vq)
>  	BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
>  
>  	/* On success, increment avail index. */
> +	pr_debug(
> +		"[vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][vq->ndescs=%d][vq->first_desc=%d]",
> +		vq, vq->last_avail_idx, vq->avail_idx, vq->ndescs, vq->first_desc);
>  	vq->last_avail_idx++;
>  
>  	return 0;
> @@ -2333,7 +2355,7 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>  	if (vq->ndescs)
>  		return 0;
>  
> -	while (!ret && vq->ndescs <= vq->batch_descs)
> +	while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
>  		ret = fetch_buf(vq);
>  
>  	return vq->ndescs ? 0 : ret;
> @@ -2432,6 +2454,9 @@ EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
>  /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
>  void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
>  {
> +	pr_debug(
> +		"DISCARD [vq=%p][vq->last_avail_idx=%u][vq->avail_idx=%u][n=%d]",
> +		vq, vq->last_avail_idx, vq->avail_idx, n);
>  	vq->last_avail_idx -= n;
>  }
>  EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 661088ae6dc7..e648b9b997d4 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -102,7 +102,6 @@ struct vhost_virtqueue {
>  	int ndescs;
>  	int first_desc;
>  	int max_descs;
> -	int batch_descs;
>  
>  	const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
>  	struct file *kick;
>