Tony Finch's blog

Note that upper bits of pointers will also be used for memory-tagging on Arm CPUs with that feature (it's a v8.5 thing), which you might encounter even if your application isn't explicitly using it itself -- eg there's an llvm memory sanitizer option which works using memory tagging. It's also possible for Arm systems to have 52-bit virtual addresses, but Linux won't give your application those unless it explicitly opts in via the hint addr argument to mmap().