[$] Fixing page-cache side channels, second attempt

The kernel's page cache, which holds copies of data stored in filesystems,is crucial to the performance of the system as a whole. But, as hasrecently been demonstrated, it can also be exploited to learn about what other usersin the system are doing and extract information that should be keptsecret. In January, the behavior of the mincore()system call waschanged in an attempt to close this vulnerability, but that solution was shown to break existing applications while notfully solving the problem. A better solution will have to wait for the5.1 development cycle, but the shape of the proposed changes has started tocome into focus.