mm: introduce ANON_VMA_LAZY for deferred anon_vma creation

TL;DR
-----

This series introduces ANON_VMA_LAZY, which defers anon_vma creation
until it is actually required.

- anon_vma memory reduced by ~92-97%, anon_vma_chain reduced by ~50-57%  
- rmap operations on ANON_VMA_LAZY VMAs do not require anon_vma locking

Background
----------

Currently anon_vma structures are created eagerly when anonymous VMAs
are initialized. However, many VMAs never participate in fork or rmap
operations that require anon_vma chains, so the allocated anon_vma and
anon_vma_chain objects are often unnecessary.

Design overview
---------------

ANON_VMA_LAZY defers anon_vma allocation until it is actually needed
(for example during fork). VMAs that never participate in sharing can
avoid creating anon_vma structures entirely.

Before an anon_vma exists, rmap operations rely directly on VMA
information, so no anon_vma locking is required. An anon_vma is created
and linked only when sharing semantics are required.

This series introduces anon_rmap helpers to make rmap less dependent on
direct anon_vma access. It also introduces anon_vma_tree_t as a container
to support both the lazy and the existing anon_vma layouts.

Once a VMA becomes associated with an anon_vma, the normal behavior
remains unchanged.

Memory impact
-------------

Preliminary measurements show significant reductions in anon_vma-related
slab allocations.

After boot:

Object            | Before (active KB) | After (active KB) | Change
vm_area_struct    | 117035             | 118176            | +1.0%
anon_vma_chain    | 18865.8            | 8112.06           | -57.0%
anon_vma          | 20426.4            | 613.75            | -97.0%

After launching 24 apps:

Object            | Before (active KB) | After (active KB) | Change
vm_area_struct    | 196873             | 197345            | +0.2%
anon_vma_chain    | 31477.1            | 15576.8           | -50.5%
anon_vma          | 33280              | 2648.12           | -92.0%

Simple fork microbenchmarks also show a slight improvement in fork
performance, since child VMAs do not need to allocate anon_vma
structures during fork.

Feedback and suggestions are welcome.


tao (15):
  mm/rmap: introduce anon_rmap APIs for anonymous folios
  mm: convert anon_vma rmap APIs to anon_rmap
  mm: introduce anon_vma_tree_t for multiple anon_vma topologies
  mm: switch to anon_vma_tree_t APIs in preparation for ANON_VMA_LAZY
  mm: add CONFIG_ANON_VMA_LAZY and folio helpers
  mm: add CONFIG_VMA_REF and VMA helpers
  mm: replace direct FOLIO_MAPPING_ANON usage with helpers
  mm: prepare rmap infrastructure for ANON_VMA_LAZY
  mm: implement ANON_VMA_LAZY rmap semantics
  mm: defer anon_vma creation with ANON_VMA_LAZY
  mm: handle ANON_VMA_LAZY in huge page operations
  mm: handle ANON_VMA_LAZY during migration
  mm: support setup and upgrade of ANON_VMA_LAZY folios
  mm: support merging of ANON_VMA_LAZY VMAs
  mm: enable CONFIG_ANON_VMA_LAZY on arm64 and x86_64

 arch/arm64/Kconfig         |   1 +
 arch/x86/Kconfig           |   1 +
 fs/proc/page.c             |   6 +-
 include/linux/mm.h         |  38 ++
 include/linux/mm_types.h   |   9 +-
 include/linux/page-flags.h |  34 +-
 include/linux/pagemap.h    |   2 +-
 include/linux/rmap.h       | 165 ++++++++-
 mm/Kconfig                 |  22 ++
 mm/damon/ops-common.c      |   4 +-
 mm/debug.c                 |   2 +-
 mm/debug_vm_pgtable.c      |   2 +-
 mm/gup.c                   |   6 +-
 mm/huge_memory.c           |  16 +-
 mm/internal.h              | 171 +++++++++
 mm/khugepaged.c            |  13 +-
 mm/ksm.c                   |  43 ++-
 mm/memory-failure.c        |  11 +-
 mm/memory.c                |  19 +-
 mm/migrate.c               | 126 ++++---
 mm/mmap.c                  |  15 +-
 mm/mremap.c                |   4 +-
 mm/page_idle.c             |   2 +-
 mm/rmap.c                  | 690 ++++++++++++++++++++++++++++++++++---
 mm/vma.c                   |  76 ++--
 mm/vma.h                   |   4 +-
 mm/vma_exec.c              |   2 +-
 mm/vma_init.c              |   1 +
 28 files changed, 1279 insertions(+), 206 deletions(-)

-- 
2.17.1