[PATCH v7 0/6] elf: THP-aware load segment alignment

[H.J. Lu]

On Tue, Mar 10, 2026 at 6:21 AM WANG Rui <wangrui@loongson.cn> wrote:
>
> 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.
>
> [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.hugetlb 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.list                          |  8 +++
>  malloc/arena.c                                | 12 ++---
>  malloc/malloc-internal.h                      |  2 +-
>  malloc/malloc.c                               | 17 +++---
>  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, 234 insertions(+), 59 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
>
> --
> 2.53.0

It was proposed to make -z noseparate-code as default for x86:

https://sourceware.org/pipermail/binutils/2026-March/148583.html

Should ld.so map read-only data segment plus code segment into the same huge
page so that the first code huge page starts at file offset 0?  This should have
the similar effect as -z noseparate-code.

-- 
H.J.