[PATCH v2 0/2] mm: optimize HugeTLB bootmem hugepage init

["Li Zhe" <lizhe.67@bytedance.com>]

This series speeds up the fallback initialization path for HugeTLB tail
struct pages during boot-time hugepage allocation, when HugeTLB vmemmap
optimization (HVO) is unavailable or fails.

Patch 1 factors the existing ZONE_DEVICE template-copy operation into
memmap_init_page_from_template().

Patch 2 reuses that helper in hugetlb_folio_init_tail_vmemmap():
initialize the first tail page normally, copy it into a local template,
then initialize the remaining tail pages by refreshing only PFN-dependent
fields before copying the template.

In the earlier discussion, Mike asked whether the same kind of
template-based initialization also helps the ordinary struct page
initialization path:

https://lore.kernel.org/all/akfB6Qe-jKDEMUxz@kernel.org/

I tested that direction as well, but did not see a measurable benefit
for ordinary struct page initialization. The measurable improvement I
could find is in the HugeTLB bootmem hugepage tail path, where gigantic
pages can require initializing a large number of tail struct pages when
HVO does not cover the allocation.

Tested on an arm64 QEMU/KVM guest with 4K base pages and 64 boot-time 1G
hugepages. HVO is unavailable on arm64 in v7.4-rc1, so the fallback tail
initialization path is exercised.

Results averaged over five boots, measured with a temporary
jiffies-based printk:

  base (v7.4-rc1):  318.4 ms
  with this patch:  202.4 ms

This is a 36.43% reduction in the measured fallback tail initialization
time. The temporary measurement instrumentation is not part of this
series.

Li Zhe (2):
  mm/mm_init: factor out memmap_init_page_from_template()
  mm/hugetlb: optimize bootmem hugepage tail vmemmap init

 mm/hugetlb.c | 20 +++++++++++++++-----
 mm/mm_init.c | 34 ++++++++++++++++++++--------------
 mm/mm_init.h |  2 ++
 3 files changed, 37 insertions(+), 19 deletions(-)

---
v1: https://lore.kernel.org/all/20261105120615.16817-1-lizhe.67@bytedance.com/#t

Changes in v2:
- Add a comment for the template-before-copy ordering, as suggested by
  Muchun Song.
- Drop the empty-range guard in hugetlb_folio_init_tail_vmemmap()
  as suggested by Balbir Singh and Mike Rapoport; current callers are
  expected to pass a non-empty range.

-- 
2.20.1