direct memory always increases when adding custom filter to get request body size

[sweat123]

I 've built a spring cloud gateway application, gateway version 2.2.9.RELEASE.
And I have created a filter to get request body size.
The filter 'RequestBodySizeFilter' to get request body size:

@Component
public class RequestBodySizeFilter implements GlobalFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        NettyDataBufferFactory factory = (NettyDataBufferFactory) exchange.getResponse().bufferFactory();
        return DataBufferUtils.join(exchange.getRequest().getBody())
                .defaultIfEmpty(factory.wrap(new EmptyByteBuf(factory.getByteBufAllocator())))
                .flatMap(dataBuffer -> {
                    int size = dataBuffer.readableByteCount();
                    log.info("request body size: {}", size);
                    if (size == 0) {
                        DataBufferUtils.release(dataBuffer);
                        return chain.filter(exchange);
                    }
                    ServerHttpRequest mutatedReq = new ServerHttpRequestDecorator(exchange.getRequest()) {
                        @Override
                        public Flux<DataBuffer> getBody() {
                            return Flux.just(dataBuffer);
                        }
                    };
                    return chain.filter(exchange.mutate().request(mutatedReq).build());
                });
    }
}

I monitor the direct memory by getting value from PlatformDependent#DIRECT_MEMORY_COUNTER.
I have sent some request with file or large body, the direct memory is always increase.
But the direct memory in Gateway without RequestBodySizeFilter will not increase when sending large body request(picture left part).

We add -Dio.netty.allocator.type=unpooled and direct memory will release with RequestBodySizeFilter.

I'm not sure if this is the right way to get request body size, can anyone give me some advice. Thank you!