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

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Message-ID: <aGKL7F18knOCQVVS@willie-the-truck>
Date: Mon, 30 Jun 2025 14:06:52 +0100
From: Will Deacon <will@...nel.org>
To: Stefano Garzarella <sgarzare@...hat.com>
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 Fri, Jun 27, 2025 at 12:41:48PM +0200, Stefano Garzarella wrote:
> 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).

Sure, works for me. That gets rid of the funny +- VIRTIO_VSOCK_SKB_HEADROOM
too.

Will