[PATCH v8 0/6] elf: THP-aware load segment alignment

[WANG Rui]

OK for commit?

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 moves Transparent Huge Page helpers into a new generic hugepages
abstraction, so THP mode detection and default huge page size probing can be
shared between malloc and the dynamic loader. There is no functional change.

Patch 2 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 3 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 4 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 5 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 6 enables this behavior by default on LoongArch64 Linux and defines
the default THP page size (32MB), matching the architecture’s PMD huge page
geometry.

[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):
  hugepages: Move THP helpers to generic hugepages abstraction
  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/dl-load.c                                 |  4 ++
 elf/dl-load.h                                 |  5 +-
 elf/dl-tunables.c                             |  7 +--
 elf/dl-tunables.list                          |  8 +++
 malloc/malloc-internal.h                      |  2 +-
 malloc/malloc.c                               | 27 +++++-----
 manual/tunables.texi                          | 24 +++++++++
 sysdeps/generic/Makefile                      |  4 +-
 sysdeps/generic/dl-map-segment-align.h        | 26 +++++++++
 .../{malloc-hugepages.c => hugepages.c}       | 13 +++--
 .../{malloc-hugepages.h => hugepages.h}       | 32 ++++++-----
 sysdeps/unix/sysv/linux/Makefile              |  1 +
 .../{malloc-hugepages.h => hugepages.h}       |  4 +-
 .../unix/sysv/linux/dl-map-segment-align.c    | 53 +++++++++++++++++++
 .../unix/sysv/linux/dl-map-segment-align.h    | 27 ++++++++++
 .../linux/{malloc-hugepages.c => hugepages.c} | 33 ++++++------
 .../unix/sysv/linux/loongarch/cpu-features.c  |  6 +++
 .../loongarch/lp64/dl-map-segment-align.h     | 22 ++++++++
 18 files changed, 237 insertions(+), 61 deletions(-)
 create mode 100644 sysdeps/generic/dl-map-segment-align.h
 rename sysdeps/generic/{malloc-hugepages.c => hugepages.c} (76%)
 rename sysdeps/generic/{malloc-hugepages.h => hugepages.h} (68%)
 rename sysdeps/unix/sysv/linux/aarch64/{malloc-hugepages.h => hugepages.h} (91%)
 create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.c
 create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.h
 rename sysdeps/unix/sysv/linux/{malloc-hugepages.c => hugepages.c} (89%)
 create mode 100644 sysdeps/unix/sysv/linux/loongarch/lp64/dl-map-segment-align.h

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2.53.0