- When mlockall() is invoked with the MCL_FUTURE flag, the VM_LOCKED
  flag is set in the task's mm_struct (current->mm->def_flags)
- In the brk() system call, if current->mm->def_flags & VM_LOCKED then
  mm_populate() is invoked
  + mm_populate() -> mm/gup.c::__mm_populate() ("gup" stands for get user pages)
    + __mm_populate() finds the vma (virtual memory region) for each address
      to populate, and then invokes populate_vma_page_range()
      + populate_vma_page_range() -> __get_user_pages()
        + __get_user_pages() ends up doing "page = follow_page_mask()" to
          search for a physical page in the (hierarchical!) page table
        + If the page is not mapped yet, follow_page_mask() will return NULL
          and faultin_page() is invoked
        + If faultin_page() does not fail (if it returns 0), then
          follow_page_mask() is tried again, and this time it will
          return a physical page!
- faultin_page() -> handle_mm_fault() which allocates a page (see the
  page fault callchain)