On Dec 6, 2025, at 03:43, Kiryl Shutsemau [off-list ref] 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.

Besides, the code simplification also looks good.

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.

A very interesting approach.

The prerequisite is that the starting address of vmemmap must be aligned to
16MB boundaries (for 1GB huge pages). Right? We should add some checks
somewhere to guarantee this (not compile time but at runtime like for KASLR).

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.

And it is also a common approach as well for DAX to eliminate an
additional tail page.

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 have no idea about the feature of memdesc, but regarding HVO, it is
a nice improvement. I'll look into the details later.

Muchun,
Thanks.

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