Can MemoryAllocator set max capacity?

[Yohahaha]

For MallocAllocator, its constructor has no params for max capacity, it seems every allocate interface only check capacity by reservationCB. How to check capacity when reservationCB is nullptr?
For MmapAllocator, its capacity_ has min capacity limit with 64 * 256. Can we set smaller capacity?

Is any design docs to describe the relations for MemoryAllocator, MemoryPool, MemoryUsageTracker and MemoryManager? They all has maxMemory variable or similar.

cc @xiaoxmeng @mbasmanova

[xiaoxmeng]

The memory reservation callback will always be set to enforce memory limit by MemoryPool. Unlike MmapAllocator, MallocAllocator doesn't need to check capacity as it delegates the memory allocations to std::malloc. MmapAllocator manages the mmaped memory space by itself so needs the capacity enforcement.

There is no updated design as for now but we will publish one soon.