linux-kernel - Re: [PATCH v3 4/9] vsock/virtio: Resize receive buffers so that each SKB fits in a 4K page

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Message-ID: <i5gsr2exiuhxdupdpbypr5mph3mbd2rfwdwxbg77kcclih2ffd@k5o7dwo3q2pw>
Date: Tue, 15 Jul 2025 11:56:10 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org, Keir Fraser <keirf@...gle.com>, 
	Steven Moreland <smoreland@...gle.com>, Frederick Mayle <fmayle@...gle.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Jason Wang <jasowang@...hat.com>, Eugenio Pérez <eperezma@...hat.com>, 
	netdev@...r.kernel.org, virtualization@...ts.linux.dev
Subject: Re: [PATCH v3 4/9] vsock/virtio: Resize receive buffers so that each
 SKB fits in a 4K page

On Mon, Jul 14, 2025 at 04:20:58PM +0100, Will Deacon wrote:
>When allocating receive buffers for the vsock virtio RX virtqueue, an
>SKB is allocated with a 4140 data payload (the 44-byte packet header +
>VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE). Even when factoring in the SKB
>overhead, the resulting 8KiB allocation thanks to the rounding in
>kmalloc_reserve() is wasteful (~3700 unusable bytes) and results in a
>higher-order page allocation on systems with 4KiB pages just for the
>sake of a few hundred bytes of packet data.
>
>Limit the vsock virtio RX buffers to 4KiB per SKB, resulting in much
>better memory utilisation and removing the need to allocate higher-order
>pages entirely.
>
>Signed-off-by: Will Deacon <will@...nel.org>
>---
> include/linux/virtio_vsock.h     | 7 ++++++-
> net/vmw_vsock/virtio_transport.c | 2 +-
> 2 files changed, 7 insertions(+), 2 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

>
>diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>index 97465f378ade..879f1dfa7d3a 100644
>--- a/include/linux/virtio_vsock.h
>+++ b/include/linux/virtio_vsock.h
>@@ -106,7 +106,12 @@ static inline size_t virtio_vsock_skb_len(struct sk_buff *skb)
> 	return (size_t)(skb_end_pointer(skb) - skb->head);
> }
>
>-#define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE	(1024 * 4)
>+/* Dimension the RX SKB so that the entire thing fits exactly into
>+ * a single 4KiB page. This avoids wasting memory due to alloc_skb()
>+ * rounding up to the next page order and also means that we
>+ * don't leave higher-order pages sitting around in the RX queue.
>+ */
>+#define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE	SKB_WITH_OVERHEAD(1024 * 4)
> #define VIRTIO_VSOCK_MAX_BUF_SIZE		0xFFFFFFFFUL
> #define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE		(1024 * 64)
>
>diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>index 1af7723669cb..5416214ae666 100644
>--- a/net/vmw_vsock/virtio_transport.c
>+++ b/net/vmw_vsock/virtio_transport.c
>@@ -307,7 +307,7 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>
> static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
> {
>-	int total_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM;
>+	int total_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE;
> 	struct scatterlist pkt, *p;
> 	struct virtqueue *vq;
> 	struct sk_buff *skb;
>-- 
>2.50.0.727.gbf7dc18ff4-goog
>