Take a look at vm.max_map_count. A lot of systems have that set artificially low and that will force a memory allocator to fail and return NULL. I can confirm this is the case for jemalloc.

Also, on Linux, only 48 of 64 bits are available for VM addressing. That leaves you with about 250TB of address space. Seems like a lot until you start using VM for disk mmaps. I have actually seen this limit hit.