This article presents LegoSwap, a cross-device memory swapping mechanism for mobile devices. It exploits the unbalanced utilization of memory resources across devices. With LegoSwap, remote memory is utilized in a seamless plug-and-play manner. It achieves comparable-to-local swapping performance based on existing network infrastructure. In addition, LegoSwap frees from the effect of remote I/O disconnection and minimizes the effect on remote devices. This is realized by three novel approaches: resource-dedicated swapping for fast swapping among devices, app-aware swapping for network connectivity considerations, and elastic swap area management for inter-device interference relieving. LegoSwap is implemented on real-life mobile devices. Experimental results show that LegoSwap can enhance app caching capability by 2x compared with no swapping, and improve performance by 2.3x compared with state-of-the-art remote swapping. More importantly, local swapping induced read-write conflicts are largely removed.