[PATCH v9 0/6] elf: THP-aware load segment alignment

[WANG Rui]

Changes since [v8]:
  * Add a new test case.
  * Minor formatting fixes.
  * Mark _dl_map_segment_align with attribute_hidden.

Changes since [v7]:
  * Rename tunable glibc.elf.hugetlb to glibc.elf.thp.
  * Rebase on current master.

Changes since [v6]:
  * Move MAX_THP_PAGESIZE to hugepages.
  * Skip THP mode probing when DL_MAP_DEFAULT_THP_PAGESIZE is non-zero.

Changes since [v5]:
  * No functional changes.
  * Add benchmark results to the commit message.

Changes since [v4]:
  * Merge malloc-hugepages into hugepages.
  * Limit the glibc.elf.hugetlb tunable maximum value to 1.

Changes since [v3]:
  * Rebased on current master.
  * Resolved conflicts with recently merged changes.
  * No functional changes intended.

Changes since [v2]:
  * Refactor THP detection into a new generic hugepages abstraction,
    moving helpers out of malloc-hugepages.
  * Add a new tunable, `glibc.elf.hugetlb`, to control THP-aware ELF
    segment alignment.
  * Move the Linux implementation of `_dl_map_segment_align` to
    sysdeps/unix/sysv/linux/, making it generic for Linux (including
    32-bit) and avoiding wordsize-64-specific overrides.
  * Use `static inline` instead of `static __always_inline`.

Changes since [v1]:
  * Fix CI build failure (-Wunused-function).

This patch series introduces a small extension point in the ELF loader to allow
architecture-specific adjustment of load segment alignment, and uses it to
improve Transparent Huge Page (THP) usage on Linux.

Patch 1 removes a redundant declaration of `_dl_map_segments` from
dl-load.h, avoiding `-Wunused-function` build failures and keeping the
prototype colocated with its definition.

Patch 2 adds a new helper, `_dl_map_segment_align`, which is called when
determining the maximum alignment for ELF load segments. The generic
implementation is a no-op and preserves existing behavior.

Patch 3 introduces a new tunable, `glibc.elf.hugetlb`, which controls
THP-aware alignment of ELF loadable segments. By default, the value is 0
and existing behavior is preserved.

Patch 4 provides a Linux implementation of `_dl_map_segment_align` that
opportunistically aligns large, suitably aligned, non-writable `PT_LOAD`
segments to the system’s default THP page size when THP is configured to
be used unconditionally and the tunable is enabled.

Patch 5 enables this behavior by default on LoongArch64 Linux and defines
the default THP page size (32MB), matching the architecture’s PMD huge page
geometry.

Patch 6 adds a new test for THP alignment of large load segments.

[v8]: https://sourceware.org/pipermail/libc-alpha/2026-April/176440.html
[v7]: https://sourceware.org/pipermail/libc-alpha/2026-March/175776.html
[v6]: https://sourceware.org/pipermail/libc-alpha/2026-March/175737.html
[v5]: https://sourceware.org/pipermail/libc-alpha/2026-March/175694.html
[v4]: https://sourceware.org/pipermail/libc-alpha/2026-March/175644.html
[v3]: https://sourceware.org/pipermail/libc-alpha/2026-February/175464.html
[v2]: https://sourceware.org/pipermail/libc-alpha/2026-February/175394.html
[v1]: https://sourceware.org/pipermail/libc-alpha/2026-February/175359.html

WANG Rui (6):
  elf: Remove redundant _dl_map_segments declaration from dl-load.h
  elf: Introduce _dl_map_segment_align hook for segment alignment tuning
  tunables: Add glibc.elf.thp tunable for THP-aware segment alignment
  elf: Align large load segments to PMD huge page size for THP
  loongarch: Enable THP-aligned load segments by default on 64-bit
  elf: Add test for THP alignment of large load segments

 elf/dl-load.c                                 |   4 +
 elf/dl-load.h                                 |   5 +-
 elf/dl-tunables.list                          |   8 +
 manual/tunables.texi                          |  24 +++
 sysdeps/generic/dl-map-segment-align.h        |  26 +++
 sysdeps/unix/sysv/linux/Makefile              |   8 +-
 .../unix/sysv/linux/dl-map-segment-align.c    |  55 +++++++
 .../unix/sysv/linux/dl-map-segment-align.h    |  27 ++++
 .../unix/sysv/linux/loongarch/cpu-features.c  |   6 +
 .../loongarch/lp64/dl-map-segment-align.h     |  22 +++
 sysdeps/unix/sysv/linux/tst-thp-align.c       | 149 ++++++++++++++++++
 sysdeps/unix/sysv/linux/tst-thp-size-mod.S    |  22 +++
 12 files changed, 352 insertions(+), 4 deletions(-)
 create mode 100644 sysdeps/generic/dl-map-segment-align.h
 create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.c
 create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.h
 create mode 100644 sysdeps/unix/sysv/linux/loongarch/lp64/dl-map-segment-align.h
 create mode 100644 sysdeps/unix/sysv/linux/tst-thp-align.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-thp-size-mod.S

-- 
2.53.0