As a pre-requisite to off-heap caching of blocks, we need a mechanism to prevent pages / blocks from being evicted while they are being read. With on-heap objects, evicting a block while it is being read merely leads to memory-accounting problems (because we assume that an evicted block is a candidate for garbage-collection, which will not be true during a read), but with off-heap memory this will lead to either data corruption or segmentation faults.
To address this, we should add a reference-counting mechanism to track which blocks/pages are being read in order to prevent them from being evicted prematurely. I propose to do this in two phases: first, add a safe, conservative approach in which all BlockManager.get*() calls implicitly increment the reference count of blocks and where tasks' references are automatically freed upon task completion. This will be correct but may have adverse performance impacts because it will prevent legitimate block evictions. In phase two, we should incrementally add release() calls in order to fix the eviction of unreferenced blocks. The latter change may need to touch many different components, which is why I propose to do it separately in order to make the changes easier to reason about and review.