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);
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