I propose that we change the paramter order from:
os::attempt_reserve_memory_at(size, addr, <extra params>)

to:
os::attempt_reserve_memory_at(addr, size, <extra params>)

The motivation is that it's simply awkward to have attempt_reserve_memory_at use the opposite order of most other memory range APIs.

Examples of APIs using the order addr, size:
1) void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
2) PVOID VirtualAlloc2(..., BaseAddress, Size, ...)
3) ReservedMemoryRegion(address base, size_t size)
4) MemRegion(HeapWord* start, size_t word_size)
5) ReservedSpace(char* base, size_t size
6) commit_memory(char* addr, size_t bytes, bool executable)