netdev - Re: [PATCH 2/5] vsock/virtio: Resize receive buffers so that each SKB fits in a page

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Message-ID: <rl5x3fw5rgyrptof2h7qc2wgimxd4ldh4tp4yhm52n4utksjdm@zei2wzme65jj>
Date: Fri, 27 Jun 2025 12:41:48 +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 2/5] vsock/virtio: Resize receive buffers so that each
 SKB fits in a page

On Wed, Jun 25, 2025 at 02:15:40PM +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 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 | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>index 36fb3edfa403..67ffb64325ef 100644
>--- a/include/linux/virtio_vsock.h
>+++ b/include/linux/virtio_vsock.h
>@@ -111,7 +111,8 @@ 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_DEFAULT_RX_BUF_SIZE	(SKB_WITH_OVERHEAD(PAGE_SIZE) \
>+						 - VIRTIO_VSOCK_SKB_HEADROOM)

This is only used in net/vmw_vsock/virtio_transport.c :

static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
{
	int total_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM;


What about just remove VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE and use
`SKB_WITH_OVERHEAD(PAGE_SIZE)` there? (maybe with a comment summarizing
the issue we found).

Thanks,
Stefano

> #define VIRTIO_VSOCK_MAX_BUF_SIZE		0xFFFFFFFFUL
> #define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE		(1024 * 64)
>
>-- 
>2.50.0.714.g196bf9f422-goog
>
>