Unified Memory Architecture (UMA) enables the CPU and GPU to share a single physical memory. However, its complexity and opacity often drive applications back to inefficient copying or to incorrect direct access that sacrifices performance or even correctness. This paper presents a comprehensive study of CPU-GPU memory sharing on UMA, covering correctness and performance across data-plane transmission and control-plane coordination on four platforms. It yields four findings that guide libumsh, a portable library that delivers correct and efficient memory sharing across platforms and extends to emerging ones (e.g., NVIDIA DGX Spark). Applied to nine real-world systems, libumsh reduces data-plane time by 16-75% and control-plane overhead by 7-90%.

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