Re: [PATCH net v2 2/2] vsock/test: add test for small packets under pressure

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 08, 2026 at 12:29:04PM +0200, Stefano Garzarella wrote:
> From: Stefano Garzarella <sgarzare@redhat.com>
> 
> Add a test that sends 2 MB of data using randomly sized small packets
> (129-512 bytes) over a SOCK_STREAM connection. Packets above
> GOOD_COPY_LEN (128) bypass the in-place coalescing in recv_enqueue(),
> forcing each one into its own skb.
> 
> Without receive queue collapsing, the per-skb overhead eventually
> exceeds buf_alloc and the connection is reset. The test verifies
> that all data arrives and that content integrity is preserved.
> 
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

maybe cut down SO_VM_SOCKETS_BUFFER_SIZE? will make it easier to
trigger?

anyway

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  tools/testing/vsock/vsock_test.c | 87 ++++++++++++++++++++++++++++++++
>  1 file changed, 87 insertions(+)
> 
> diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
> index 76be0e4a7f0e..b4ff9f946565 100644
> --- a/tools/testing/vsock/vsock_test.c
> +++ b/tools/testing/vsock/vsock_test.c
> @@ -2347,6 +2347,88 @@ static void test_stream_tx_credit_bounds_server(const struct test_opts *opts)
>  	close(fd);
>  }
>  
> +/* Test that many small packets don't cause a connection reset under pressure
> + * and that data integrity is preserved.  Packet sizes vary randomly between
> + * 129 and 512 bytes, above GOOD_COPY_LEN (128) to bypass in-place coalescing
> + * in recv_enqueue, forcing each one into its own skb.  Without receive queue
> + * collapsing, the per-skb overhead eventually exceeds buf_alloc and the
> + * connection is reset.
> + */
> +#define COLLAPSE_PKT_MIN 129
> +#define COLLAPSE_PKT_MAX 512
> +#define COLLAPSE_TOTAL (2 * 1024 * 1024)
> +
> +static void test_stream_collapse_client(const struct test_opts *opts)
> +{
> +	unsigned char *data;
> +	unsigned long hash;
> +	size_t offset = 0;
> +	int i, fd;
> +
> +	data = malloc(COLLAPSE_TOTAL);
> +	if (!data) {
> +		perror("malloc");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	for (i = 0; i < COLLAPSE_TOTAL; i++)
> +		data[i] = rand() & 0xff;
> +
> +	fd = vsock_stream_connect(opts->peer_cid, opts->peer_port);
> +	if (fd < 0) {
> +		perror("connect");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	while (offset < COLLAPSE_TOTAL) {
> +		size_t pkt_size = COLLAPSE_PKT_MIN +
> +			rand() % (COLLAPSE_PKT_MAX - COLLAPSE_PKT_MIN + 1);
> +
> +		pkt_size = min(pkt_size, COLLAPSE_TOTAL - offset);
> +
> +		send_buf(fd, data + offset, pkt_size, 0, pkt_size);
> +		offset += pkt_size;
> +	}
> +
> +	hash = hash_djb2(data, COLLAPSE_TOTAL);
> +	control_writeulong(hash);
> +
> +	free(data);
> +	close(fd);
> +}
> +
> +static void test_stream_collapse_server(const struct test_opts *opts)
> +{
> +	unsigned long hash, remote_hash;
> +	unsigned char *data;
> +	int fd;
> +
> +	data = malloc(COLLAPSE_TOTAL);
> +	if (!data) {
> +		perror("malloc");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	fd = vsock_stream_accept(VMADDR_CID_ANY, opts->peer_port, NULL);
> +	if (fd < 0) {
> +		perror("accept");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	recv_buf(fd, data, COLLAPSE_TOTAL, 0, COLLAPSE_TOTAL);
> +
> +	hash = hash_djb2(data, COLLAPSE_TOTAL);
> +	remote_hash = control_readulong();
> +	if (hash != remote_hash) {
> +		fprintf(stderr, "hash mismatch: local %lu remote %lu\n",
> +			hash, remote_hash);
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	free(data);
> +	close(fd);
> +}
> +
>  static struct test_case test_cases[] = {
>  	{
>  		.name = "SOCK_STREAM connection reset",
> @@ -2546,6 +2628,11 @@ static struct test_case test_cases[] = {
>  		.run_client = test_stream_msg_peek_client,
>  		.run_server = test_stream_peek_after_recv_server,
>  	},
> +	{
> +		.name = "SOCK_STREAM small packets backpressure",
> +		.run_client = test_stream_collapse_client,
> +		.run_server = test_stream_collapse_server,
> +	},
>  	{},
>  };
>  
> -- 
> 2.55.0