Memory caches continue to be a critical component to many systems. In recent years, there has been larger amounts of data into main memory, especially in shared environments such as the cloud. The nature of such environments requires resource allocations to provide both performance isolation for multiple users/applications and high utilization for the systems. We study the problem of fair allocation of memory cache for multiple users with shared files. We find that, surprisingly, no memory allocation policy can provide all three desirable properties (isolation-guarantee, strategy-proofness and Pareto-efficiency) that are typically achievable by other types of resources, e.g., CPU or network. We also show that there exist policies that achieve any two of the three properties. We find that the only way to achieve both isolation-guarantee and strategy-proofness is through blocking, which we efficiently adapt in a new policy called FairRide. We implement FairRide in a popular memory-centric storage system using an efficient form of blocking, named as expected delaying, and demonstrate that FairRide can lead to better cache efficiency (2.6× over isolated caches) and fairness in many scenarios.