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vsock/virtio: Resize receive buffers so that each SKB fits in a 4K page

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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 on systems with 4KiB pages just for the
sake of a few hundred bytes of packet data.

Limit the vsock virtio RX buffers to 4KiB per SKB, resulting in much
better memory utilisation and removing the need to allocate higher-order
pages entirely.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-5-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Will Deacon 2025-07-17 10:01:11 +01:00 • committed by Michael S. Tsirkin
parent 87dbae5e36
commit 03a92f036a
2 changed files with 7 additions and 2 deletions
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7
include/linux/virtio_vsock.h
View file

@ -106,7 +106,12 @@ 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)
/* Dimension the RX SKB so that the entire thing fits exactly into
* a single 4KiB 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.
*/
#define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE SKB_WITH_OVERHEAD(1024 * 4)
#define VIRTIO_VSOCK_MAX_BUF_SIZE 0xFFFFFFFFUL
#define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE (1024 * 64)

2
net/vmw_vsock/virtio_transport.c
View file

@ -307,7 +307,7 @@ out_rcu:
static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
{
int total_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE + VIRTIO_VSOCK_SKB_HEADROOM;
int total_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE;
struct scatterlist pkt, *p;
struct virtqueue *vq;
struct sk_buff *skb;