On Fri, Jun 27, 2025 at 12:50:27PM +0200, Stefano Garzarella wrote:

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.

Sure, I'll do that.

On Wed, Jun 25, 2025 at 02:15:43PM +0100, Will Deacon wrote:

quoted

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.

That would be good. I had a crack at it in the diff I sent in reply to
the earlier patch, so please take a look.

Will