[PATCHv3 00/15] mm: 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.

The key insight is that all tail pages of the same order now have
identical compound_info values, regardless of which compound page they
belong to. This allows a single page of tail struct pages to be shared
across all huge pages of the same order on a NUMA node.

Benefits
========

1. Simplified compound_head(): No fake head detection needed, can be
   implemented in a branchless manner.

2. Simplified page_ref_add_unless(): RCU protection removed since there's
   no race with fake head remapping.

3. Cleaner architecture: The shared tail pages are truly read-only and
   contain valid tail page metadata.

If sizeof(struct page) is not power-of-2, there are no functional changes.
HVO is not supported in this configuration.

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
===================

Patch 1-2: Preparation - move MAX_FOLIO_ORDER to mmzone.h and build fix
	   for x86/vdso32
Patches 3-5: Refactoring - interface changes, field rename, code movement
Patch 6: Core change - new mask-based compound_head() encoding
Patch 7: Correctness fix - page_zonenum() must use head page
Patch 8: Add memmap alignment check for compound_info_has_mask()
Patch 9: Refactor vmemmap_walk for new design
Patch 10: Eliminate fake heads with shared tail pages
Patches 11-14: Cleanup - remove fake head infrastructure
Patch 15: Documentation update

Changes in v3:
==============
  - Fixed error recovery path in vmemmap_remap_free() to pass correct start
    address for TLB flush. (Muchun)

  - Wrapped the mask-based compound_info encoding within CONFIG_SPARSEMEM_VMEMMAP
    check via compound_info_has_mask(). For other memory models, alignment
    guarantees are harder to verify. (Muchun)

  - Updated vmemmap_dedup.rst documentation wording: changed "vmemmap_tail
    shared for the struct hstate" to "A single, per-node page frame shared
    among all hugepages of the same size". (Muchun)

  - Fixed build error with MAX_FOLIO_ORDER expanding to undefined PUD_ORDER
    in certain configurations. (kernel test robot)

Changes in v2:
==============

- Handle boot-allocated huge pages correctly. (Frank)

- Changed from per-hstate vmemmap_tail to per-node vmemmap_tails[] array
  in pglist_data. (Muchun)

- Added spin_lock(&hugetlb_lock) protection in vmemmap_get_tail() to fix
  a race condition where two threads could both allocate tail pages.
  The losing thread now properly frees its allocated page. (Usama)

- Add warning if memmap is not aligned to MAX_FOLIO_SIZE, which is
  required for the mask approach. (Muchun)

- Make page_zonenum() use head page - correctness fix since shared
  tail pages cannot have valid zone information. (Muchun)

- Added 'const' qualifier to head parameter in set_compound_head() and
  prep_compound_tail(). (Usama)

- Updated commit messages.

Kiryl Shutsemau (15):
  x86/vdso32: Prepare for <linux/pgtable.h> inclusion
  mm: Move MAX_FOLIO_ORDER definition to mmzone.h
  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() next to compound_head()
  mm: Rework compound_head() for power-of-2 sizeof(struct page)
  mm: Make page_zonenum() use head page
  mm/sparse: Check memmap alignment for compound_info_has_mask()
  mm/hugetlb: Refactor code around vmemmap_walk
  mm/hugetlb: Remove fake head pages
  mm: Drop fake head checks
  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 ++--
 arch/x86/entry/vdso/vdso32/fake_32bit_build.h |   2 +
 include/linux/mm.h                            |  31 --
 include/linux/mm_types.h                      |  20 +-
 include/linux/mmzone.h                        |  48 ++++
 include/linux/page-flags.h                    | 167 +++++------
 include/linux/page_ref.h                      |   8 +-
 include/linux/types.h                         |   2 +-
 kernel/vmcore_info.c                          |   2 +-
 mm/hugetlb.c                                  |   8 +-
 mm/hugetlb_vmemmap.c                          | 270 +++++++++---------
 mm/internal.h                                 |  12 +-
 mm/mm_init.c                                  |   2 +-
 mm/page_alloc.c                               |   4 +-
 mm/slab.h                                     |   2 +-
 mm/sparse-vmemmap.c                           |  44 ++-
 mm/sparse.c                                   |   5 +
 mm/util.c                                     |  16 +-
 19 files changed, 363 insertions(+), 344 deletions(-)

-- 
2.51.2