Re: [PATCHv2 00/14] Eliminate fake head pages from vmemmap optimization

[Kiryl Shutsemau <kas@kernel.org>]

Oopsie. Add the Subject.

On Thu, Dec 18, 2025 at 03:09:31PM +0000, Kiryl Shutsemau wrote:
> 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
> ===================
> 
> Patches 1-2: Preparation - move MAX_FOLIO_ORDER, add alignment check
> 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: Refactor vmemmap_walk for new design
> Patch 9: Eliminate fake heads with shared tail pages
> Patches 10-13: Cleanup - remove fake head infrastructure
> Patch 14: Documentation update
> 
> 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 (14):
>   mm: Move MAX_FOLIO_ORDER definition to mmzone.h
>   mm/sparse: Check memmap alignment
>   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/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 ++--
>  include/linux/mm.h                            |  31 --
>  include/linux/mm_types.h                      |  20 +-
>  include/linux/mmzone.h                        |  47 +++
>  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                          | 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                                   |   3 +
>  mm/util.c                                     |  16 +-
>  18 files changed, 345 insertions(+), 353 deletions(-)
> 
> -- 
> 2.51.2
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov