netdev - [PATCH v2 4/8] vsock/virtio: Resize receive buffers so that each SKB fits in a page

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Message-Id: <20250701164507.14883-5-will@kernel.org>
Date: Tue,  1 Jul 2025 17:45:03 +0100
From: Will Deacon <will@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: Will Deacon <will@...nel.org>,
	Keir Fraser <keirf@...gle.com>,
	Steven Moreland <smoreland@...gle.com>,
	Frederick Mayle <fmayle@...gle.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Stefano Garzarella <sgarzare@...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: [PATCH v2 4/8] vsock/virtio: Resize receive buffers so that each SKB fits in a page

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 for the sake of a few hundred bytes of
packet data.

Limit the vsock virtio RX buffers to a page 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     | 1 -
 net/vmw_vsock/virtio_transport.c | 7 ++++++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index eb6980aa19fd..1b5731186095 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -109,7 +109,6 @@ 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)
 #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 488e6ddc6ffa..3daba06ed499 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -307,7 +307,12 @@ 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;
+	/* Dimension the SKB so that the entire thing fits exactly into
+	 * a single 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.
+	 */
+	int total_len = SKB_WITH_OVERHEAD(PAGE_SIZE);
 	struct scatterlist pkt, *p;
 	struct virtqueue *vq;
 	struct sk_buff *skb;
-- 
2.50.0.727.gbf7dc18ff4-goog