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

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Message-ID: <3s4lvbnzdj72dcvvh2nnx4s7skyco4pbpwuyycccqv3iudqhnn@5szfvvgxojkb>
Date: Wed, 2 Jul 2025 15:16:19 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Will Deacon <will@...nel.org>, 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 v2 4/8] vsock/virtio: Resize receive buffers so that each
 SKB fits in a page

On Tue, Jul 01, 2025 at 08:14:00PM +0100, David Laight wrote:
>On Tue,  1 Jul 2025 17:45:03 +0100
>Will Deacon <will@...nel.org> 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     | 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);
>
>Should that be an explicit 4096?
>Otherwise it is very wasteful of memory on systems with large pages.

This is a good point!

What about SKB_WITH_OVERHEAD(VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE) ?

Thanks,
Stefano

>
>	David
>
>>  	struct scatterlist pkt, *p;
>>  	struct virtqueue *vq;
>>  	struct sk_buff *skb;
>
>