Re: [PATCH 5/5] vhost/vsock: Allocate nonlinear SKBs for handling large transmit buffers

Stefano Garzarella
2025-06-27T10:50:43Z

nit: I'd use `vsock/virtio: ` prefix since we are touching the virtio
transport common code. Maybe we can mention that this will affect both
virtio and vhost transports.

On Wed, Jun 25, 2025 at 02:15:43PM +0100, Will Deacon wrote:
>When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
>virtio_transport_alloc_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.
>
>Signed-off-by: Will Deacon <will@kernel.org>
>---
> net/vmw_vsock/virtio_transport_common.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 1b5d9896edae..424eb69e84f9 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);
>+ 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 +262,11 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
> if (!zcopy)
> skb_len += payload_len;
>
>- skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>+ if (skb_len > SKB_WITH_OVERHEAD(PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
>+ skb = virtio_vsock_alloc_skb_with_frags(skb_len, GFP_KERNEL);
>+ else
>+ skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>+

As I mentioned in the other patch, we may avoid this code duplication
hiding this in virtio_vsock_alloc_skb() or adding a new function that
we can use when we want to allocate frags or not.

Thanks,
Stefano

> if (!skb)
> return NULL;
>
>--
>2.50.0.714.g196bf9f422-goog
>
>