In datacenters, lossless network is very attractive as it can achieve ultra-low latency. In commodity Ethernet, lossless forwarding is achieved by hop-by-hop Priority-based Flow Control (PFC). To avoid buffer overflow, PFC-enabled switches need to reserve some buffer asheadroom, absorbing in-flight packets during the delay for backpressure messages to take effect. However, with the growing link speed in production networks, the buffer becomes increasingly insufficient, and the headroom can occupy a considerable fraction of buffer. As a result, the remaining buffer for absorbing normal traffic bursts is significantly squeezed, leading to frequent PFC messages that degrade the network performance. Worse yet, we find that the current static and queue-independent headroom allocation scheme is quite inefficient, resulting in significant buffer wastage. In light of this, we propose Dynamic and Shared Headroom allocation scheme (DSH), which dynamically allocates headroom to congested queues and enables sharing of allocated headroom among different queues. To achieve this, DSH first introduces port-level flow control, which performs flow control at the granularity of individual ports, guaranteeing lossless forwarding with a small fraction of per-port headroom. With this lossless guarantee, the switch is liberated for dynamic headroom adjustment. DSH dynamically allocates per-queue headroom based on the congestion status of each queue. Meanwhile, DSH preserves the queue-level flow control to protect the non-congested queues from being paused by congested queues, ensuring performance isolation on buffer sharing. Extensive experiments show that DSH can reduce the flow completion time by up to ~78.8%.