Re: [PATCH 3/5] vhost/vsock: Allocate nonlinear SKBs for handling large receive buffers

Stefano Garzarella
2025-06-27T10:46:00Z

On Wed, Jun 25, 2025 at 02:15:41PM +0100, Will Deacon wrote:
>When receiving a packet from a guest, vhost_vsock_handle_tx_kick()
>calls vhost_vsock_alloc_skb() to allocate and fill an SKB with the
>receive 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 receive data in the
>fragments.
>
>Signed-off-by: Will Deacon <will@kernel.org>
>---
> drivers/vhost/vsock.c | 15 +++++++++------
> include/linux/virtio_vsock.h | 31 +++++++++++++++++++++++++------
> 2 files changed, 34 insertions(+), 12 deletions(-)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index 66a0f060770e..cfa4e1bcf367 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -344,11 +344,16 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
>
> len = iov_length(vq->iov, out);
>
>- if (len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM)
>+ if (len < VIRTIO_VSOCK_SKB_HEADROOM ||

Why moving this check here?

>+ len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM)
> return NULL;
>
> /* len contains both payload and hdr */
>- skb = virtio_vsock_alloc_skb(len, GFP_KERNEL);
>+ if (len > SKB_WITH_OVERHEAD(PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
>+ skb = virtio_vsock_alloc_skb_with_frags(len, GFP_KERNEL);
>+ else
>+ skb = virtio_vsock_alloc_skb(len, GFP_KERNEL);

Can we do this directly in virtio_vsock_alloc_skb() so we don't need
to duplicate code on virtio/vhost code?

>+
> if (!skb)
> return NULL;
>
>@@ -377,10 +382,8 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq,
>
> virtio_vsock_skb_rx_put(skb);
>
>- nbytes = copy_from_iter(skb->data, payload_len, &iov_iter);
>- if (nbytes != payload_len) {
>- vq_err(vq, "Expected %zu byte payload, got %zu bytes\n",
>- payload_len, nbytes);
>+ if (skb_copy_datagram_from_iter(skb, 0, &iov_iter, payload_len)) {
>+ vq_err(vq, "Failed to copy %zu byte payload\n", payload_len);
> kfree_skb(skb);
> return NULL;
> }
>diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>index 67ffb64325ef..8f9fa1cab32a 100644
>--- a/include/linux/virtio_vsock.h
>+++ b/include/linux/virtio_vsock.h
>@@ -51,27 +51,46 @@ static inline void virtio_vsock_skb_rx_put(struct sk_buff *skb)
> {
> u32 len;
>
>+ DEBUG_NET_WARN_ON_ONCE(skb->len);

Should we mention in the commit message?

> len = le32_to_cpu(virtio_vsock_hdr(skb)->len);
>
>- if (len > 0)

Why removing this check?

Thanks,
Stefano

>+ if (skb_is_nonlinear(skb))
>+ skb->len = len;
>+ else
> skb_put(skb, len);
> }
>
>-static inline struct sk_buff *virtio_vsock_alloc_skb(unsigned int size, gfp_t mask)
>+static inline struct sk_buff *
>+__virtio_vsock_alloc_skb_with_frags(unsigned int header_len,
>+ unsigned int data_len,
>+ gfp_t mask)
> {
> struct sk_buff *skb;
>+ int err;
>
>- if (size < VIRTIO_VSOCK_SKB_HEADROOM)
>- return NULL;
>-
>- skb = alloc_skb(size, mask);
>+ skb = alloc_skb_with_frags(header_len, data_len,
>+ PAGE_ALLOC_COSTLY_ORDER, &err, mask);
> if (!skb)
> return NULL;
>
> skb_reserve(skb, VIRTIO_VSOCK_SKB_HEADROOM);
>+ skb->data_len = data_len;
> return skb;
> }
>
>+static inline struct sk_buff *
>+virtio_vsock_alloc_skb_with_frags(unsigned int size, gfp_t mask)
>+{
>+ size -= VIRTIO_VSOCK_SKB_HEADROOM;
>+ return __virtio_vsock_alloc_skb_with_frags(VIRTIO_VSOCK_SKB_HEADROOM,
>+ size, mask);
>+}
>+
>+static inline struct sk_buff *virtio_vsock_alloc_skb(unsigned int size, gfp_t mask)
>+{
>+ return __virtio_vsock_alloc_skb_with_frags(size, 0, mask);
>+}
>+
> static inline void
> virtio_vsock_skb_queue_head(struct sk_buff_head *list, struct sk_buff *skb)
> {
>--
>2.50.0.714.g196bf9f422-goog
>
>