This isn't true for either of the common high performance mallocs. tcmalloc doesn't support mremap at all, and jemalloc disables it in its recommended build con

This isn't true for either of the common high performance mallocs. tcmalloc doesn't support mremap at all, and jemalloc disables it in its recommended build configuration. glibc will remap, but any performance gains that get realized from realloc tricks would probably be small compared to the benefits of switching to a better malloc implementation.