Re: [PATCH net-next v6 2/4] vsock/virtio: support to send non-linear skb

[Paolo Abeni]

Hi,

I'm sorry for the long delay here. I was OoO in the past few weeks.

On Tue, 2023-08-15 at 00:27 +0300, Arseniy Krasnov wrote:
> For non-linear skb use its pages from fragment array as buffers in
> virtio tx queue. These pages are already pinned by 'get_user_pages()'
> during such skb creation.
> 
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
>  Changelog:
>  v2 -> v3:
>   * Comment about 'page_to_virt()' is updated. I don't remove R-b,
>     as this change is quiet small I guess.
> 
>  net/vmw_vsock/virtio_transport.c | 41 +++++++++++++++++++++++++++-----
>  1 file changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index e95df847176b..7bbcc8093e51 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -100,7 +100,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>  	vq = vsock->vqs[VSOCK_VQ_TX];
>  
>  	for (;;) {
> -		struct scatterlist hdr, buf, *sgs[2];
> +		/* +1 is for packet header. */
> +		struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
> +		struct scatterlist bufs[MAX_SKB_FRAGS + 1];

Note that MAX_SKB_FRAGS depends on a config knob (CONFIG_MAX_SKB_FRAGS)
and valid/reasonable values are up to 45. The total stack usage can be
pretty large (~700 bytes).

As this is under the vsk tx lock, have you considered moving such data
in the virtio_vsock struct?

>  		int ret, in_sg = 0, out_sg = 0;
>  		struct sk_buff *skb;
>  		bool reply;
> @@ -111,12 +113,39 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>  
>  		virtio_transport_deliver_tap_pkt(skb);
>  		reply = virtio_vsock_skb_reply(skb);
> +		sg_init_one(&bufs[out_sg], virtio_vsock_hdr(skb),
> +			    sizeof(*virtio_vsock_hdr(skb)));
> +		sgs[out_sg] = &bufs[out_sg];
> +		out_sg++;
> +
> +		if (!skb_is_nonlinear(skb)) {
> +			if (skb->len > 0) {
> +				sg_init_one(&bufs[out_sg], skb->data, skb->len);
> +				sgs[out_sg] = &bufs[out_sg];
> +				out_sg++;
> +			}
> +		} else {
> +			struct skb_shared_info *si;
> +			int i;
> +
> +			si = skb_shinfo(skb);

This assumes that the paged skb does not carry any actual data in the
head buffer (only the header). Is that constraint enforced somewhere
else? Otherwise a

	WARN_ON_ONCE(skb_headlen(skb) > sizeof(*virtio_vsock_hdr(skb))

could be helpful to catch early possible bugs.

Thanks!

Paolo

> +
> +			for (i = 0; i < si->nr_frags; i++) {
> +				skb_frag_t *skb_frag = &si->frags[i];
> +				void *va;
>  
> -		sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
> -		sgs[out_sg++] = &hdr;
> -		if (skb->len > 0) {
> -			sg_init_one(&buf, skb->data, skb->len);
> -			sgs[out_sg++] = &buf;
> +				/* We will use 'page_to_virt()' for the userspace page
> +				 * here, because virtio or dma-mapping layers will call
> +				 * 'virt_to_phys()' later to fill the buffer descriptor.
> +				 * We don't touch memory at "virtual" address of this page.
> +				 */
> +				va = page_to_virt(skb_frag->bv_page);
> +				sg_init_one(&bufs[out_sg],
> +					    va + skb_frag->bv_offset,
> +					    skb_frag->bv_len);
> +				sgs[out_sg] = &bufs[out_sg];
> +				out_sg++;
> +			}
>  		}
>  
>  		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);