The new boot flow when it comes to initialization of gigantic pages is as
follows:

- At boot time, for a gigantic page during __alloc_bootmem_hugepage, the
  region after the first struct page is marked as noinit.

- This results in only the first struct page to be initialized in
  reserve_bootmem_region.  As the tail struct pages are not initialized at
  this point, there can be a significant saving in boot time if HVO
  succeeds later on.

- Later on in the boot, the head page is prepped and the first
  HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
  are initialized.

- HVO is attempted.  If it is not successful, then the rest of the tail
  struct pages are initialized.  If it is successful, no more tail struct
  pages need to be initialized saving significant boot time.

The WARN_ON for increased ref count in gather_bootmem_prealloc was changed
to a VM_BUG_ON.  This is OK as there should be no speculative references
this early in boot process.  The VM_BUG_ON's are there just in case such
code is introduced.

[akpm@linux-foundation.org: make it nicer for 80 cols]
Link: https://lkml.kernel.org/r/20230913105401.519709-5-usama.arif@bytedance.comSigned-off-by: Usama Arif <usama.arif@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Fam Zheng <fam.zheng@bytedance.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>