Subject: Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 13 Aug 2025 04:41:09 -0400

On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> transmit data. Unfortunately, these are always linear allocations and
> can therefore result in significant pressure on kmalloc() considering
> that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> allocation for each packet.
> 
> Rework the vsock SKB allocation so that, for sizes with page order
> greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> instead with the packet header in the SKB and the transmit data in the
> fragments. Note that this affects both the vhost and virtio transports.
> 
> Reviewed-by: Stefano Garzarella <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>

So this caused a regression, see syzbot report:

https://lore.kernel.org/all/[email-masked]

I'm inclined to revert unless we have a fix quickly.

> ---
>  net/vmw_vsock/virtio_transport_common.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index c9eb7f7ac00d..fe92e5fa95b4 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -109,7 +109,8 @@ static int virtio_transport_fill_skb(struct sk_buff *skb,
>  		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
>  					       &info->msg->msg_iter, len, NULL);
>  
> -	return memcpy_from_msg(skb_put(skb, len), info->msg, len);
> +	virtio_vsock_skb_put(skb, len);
> +	return skb_copy_datagram_from_iter(skb, 0, &info->msg->msg_iter, len);
>  }
>  
>  static void virtio_transport_init_hdr(struct sk_buff *skb,
> @@ -261,7 +262,7 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
>  	if (!zcopy)
>  		skb_len += payload_len;
>  
> -	skb = virtio_vsock_alloc_linear_skb(skb_len, GFP_KERNEL);
> +	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>  	if (!skb)
>  		return NULL;
>  
> -- 
> 2.50.0.727.gbf7dc18ff4-goog