This currently behind the selinux-fix feature flag. SELinux doesn't allow making parts of the heap executable. In addition it seems that there is an issue in multithreaded environments causing permission issues leading to crashes when using posix_memalign instead of mmap. (see #4980) I don't know why using posix_memalign was chosen initially. Maybe there is some advantage to it that I don't know of. If not, I don't see any reason to stay with posix_memalign as default. @sunfishcode do you remember why you used posix_memalign?