[PATCH 00/11] mm/hugetlb: Eliminate fake head pages from vmemmap optimization

[Kiryl Shutsemau <kas@kernel.org>]

This series removes "fake head pages" from the HugeTLB vmemmap
optimization (HVO) by changing how tail pages encode their relationship
to the head page.

It simplifies compound_head() and page_ref_add_unless(). Both are in the
hot path.

Background
==========

HVO reduces memory overhead by freeing vmemmap pages for HugeTLB pages
and remapping the freed virtual addresses to a single physical page.
Previously, all tail page vmemmap entries were remapped to the first
vmemmap page (containing the head struct page), creating "fake heads" -
tail pages that appear to have PG_head set when accessed through the
deduplicated vmemmap.

This required special handling in compound_head() to detect and work
around fake heads, adding complexity and overhead to a very hot path.

New Approach
============

For architectures/configs where sizeof(struct page) is a power of 2 (the
common case), this series changes how position of the head page is encoded
in the tail pages.

Instead of storing a pointer to the head page, the ->compound_info
(renamed from ->compound_head) now stores a mask.

The mask can be applied to any tail page's virtual address to compute
the head page address. Critically, all tail pages of the same order now
have identical compound_info values, regardless of which compound page
they belong to.

This enables a key optimization: instead of remapping tail vmemmap
entries to the head page (creating fake heads), we remap them to a
shared, pre-initialized vmemmap_tail page per hstate. The head page
gets its own dedicated vmemmap page, eliminating fake heads entirely.

Benefits
========

1. Smaller generated code. On defconfig, I see ~15K reduction of text
   in vmlinux:

   add/remove: 6/33 grow/shrink: 54/262 up/down: 6130/-21922 (-15792)

2. Simplified compound_head(): No fake head detection needed. The
   function is now branchless for power-of-2 struct page sizes.

3. Eliminated race condition: The old scheme required synchronize_rcu()
   to coordinate between HVO remapping and speculative PFN walkers that
   might write to fake heads. With the head page always in writable
   memory, this synchronization is unnecessary.

4. Removed static key: hugetlb_optimize_vmemmap_key is no longer needed
   since compound_head() no longer has HVO-specific branches.

5. Cleaner architecture: The vmemmap layout is now straightforward -
   head page has its own vmemmap, tails share a read-only template.

I had hoped to see performance improvement, but my testing thus far has
shown either no change or only a slight improvement within the noise.

Series Organization
===================

Patches 1-3: Preparatory refactoring
  - Change prep_compound_tail() interface to take order
  - Rename compound_head field to compound_info
  - Move set/clear_compound_head() near compound_head()

Patch 4: Core encoding change
  - Implement mask-based encoding for power-of-2 struct page

Patches 5-6: HVO restructuring
  - Refactor vmemmap_walk to support separate head/tail pages
  - Introduce per-hstate vmemmap_tail, eliminate fake heads

Patches 7-9: Cleanup
  - Remove fake head checks from compound_head(), PageTail(), etc.
  - Remove VMEMMAP_SYNCHRONIZE_RCU and synchronize_rcu() calls
  - Remove hugetlb_optimize_vmemmap_key static key

Patch 10: Optimization
  - Implement branchless compound_head() for power-of-2 case

Patch 11: Documentation
  - Update vmemmap_dedup.rst to reflect new architecture

Kiryl Shutsemau (11):
  mm: Change the interface of prep_compound_tail()
  mm: Rename the 'compound_head' field in the 'struct page' to
    'compound_info'
  mm: Move set/clear_compound_head() to compound_head()
  mm: Rework compound_head() for power-of-2 sizeof(struct page)
  mm/hugetlb: Refactor code around vmemmap_walk
  mm/hugetlb: Remove fake head pages
  mm: Drop fake head checks and fix a race condition
  hugetlb: Remove VMEMMAP_SYNCHRONIZE_RCU
  mm/hugetlb: Remove hugetlb_optimize_vmemmap_key static key
  mm: Remove the branch from compound_head()
  hugetlb: Update vmemmap_dedup.rst

 .../admin-guide/kdump/vmcoreinfo.rst          |   2 +-
 Documentation/mm/vmemmap_dedup.rst            |  62 ++---
 include/linux/hugetlb.h                       |   3 +
 include/linux/mm_types.h                      |  20 +-
 include/linux/page-flags.h                    | 163 +++++-------
 include/linux/page_ref.h                      |   8 +-
 include/linux/types.h                         |   2 +-
 kernel/vmcore_info.c                          |   2 +-
 mm/hugetlb.c                                  |   8 +-
 mm/hugetlb_vmemmap.c                          | 245 ++++++++----------
 mm/hugetlb_vmemmap.h                          |   4 +-
 mm/internal.h                                 |  11 +-
 mm/mm_init.c                                  |   2 +-
 mm/page_alloc.c                               |   4 +-
 mm/slab.h                                     |   2 +-
 mm/util.c                                     |  15 +-
 16 files changed, 242 insertions(+), 311 deletions(-)

-- 
2.51.2