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

[Stefano Garzarella]

On Tue, Jul 01, 2025 at 08:14:00PM +0100, David Laight wrote:

On Tue,  1 Jul 2025 17:45:03 +0100
Will Deacon <w...@kernel.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 <w...@kernel.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;