Re: [PATCH v4 0/2] binfmt_elf: Align eligible read-only PT_LOAD segments to PMD_SIZE for THP

[Baolin Wang <baolin.wang@linux.alibaba.com>]

CC Usama

On 3/10/26 11:11 AM, WANG Rui wrote:
> Changes since [v3]:
> * Fixed compilation failure under !CONFIG_TRANSPARENT_HUGEPAGE.
> * No functional changes otherwise.
> 
> Changes since [v2]:
> * Renamed align_to_pmd() to should_align_to_pmd().
> * Added benchmark results to the commit message.
> 
> Changes since [v1]:
> * Dropped the Kconfig option CONFIG_ELF_RO_LOAD_THP_ALIGNMENT.
> * Moved the alignment logic into a helper align_to_pmd() for clarity.
> * Improved the comment explaining why we skip the optimization
>    when PMD_SIZE > 32MB.
> 
> When Transparent Huge Pages (THP) are enabled in "always" mode,
> file-backed read-only mappings can be backed by PMD-sized huge pages
> if they meet the alignment and size requirements.
> 
> For ELF executables loaded by the kernel ELF binary loader, PT_LOAD
> segments are normally aligned according to p_align, which is often
> only page-sized. As a result, large read-only segments that are
> otherwise eligible may fail to be mapped using PMD-sized THP.
> 
> A segment is considered eligible if:
> 
> * THP is in "always" mode,
> * it is not writable,
> * both p_vaddr and p_offset are PMD-aligned,
> * its file size is at least PMD_SIZE, and
> * its existing p_align is smaller than PMD_SIZE.
> 
> To avoid excessive address space padding on systems with very large
> PMD_SIZE values, this optimization is applied only when PMD_SIZE <= 32MB,
> since requiring larger alignments would be unreasonable, especially on
> 32-bit systems with a much more limited virtual address space.
> 
> This increases the likelihood that large text segments of ELF
> executables are backed by PMD-sized THP, reducing TLB pressure and
> improving performance for large binaries.
> 
> This only affects ELF executables loaded directly by the kernel
> binary loader. Shared libraries loaded by user space (e.g. via the
> dynamic linker) are not affected.

Usama posted a similar patchset[1], and I think using exec_folio_order() 
for exec-segment alignment is reasonable. In your case, you can override 
exec_folio_order() to return a PMD‑sized order.

[1] 
https://lore.kernel.org/all/20260310145406.3073394-1-usama.arif@linux.dev/

> Benchmark
> 
> Machine: AMD Ryzen 9 7950X (x86_64)
> Binutils: 2.46
> GCC: 15.2.1 (built with -z,noseparate-code + --enable-host-pie)
> 
> Workload: building Linux v7.0-rc1 vmlinux with x86_64_defconfig.
> 
>                  Without patch        With patch
> instructions    8,246,133,611,932    8,246,025,137,750
> cpu-cycles      8,001,028,142,928    7,565,925,107,502
> itlb-misses     3,672,158,331        26,821,242
> time elapsed    64.66 s              61.97 s
> 
> Instructions are basically unchanged. iTLB misses drop from ~3.67B to
> ~26M (~99.27% reduction), which results in about a ~5.44% reduction in
> cycles and ~4.18% shorter wall time for this workload.
> 
> [v3]: https://lore.kernel.org/linux-fsdevel/20260310013958.103636-1-r@hev.cc
> [v2]: https://lore.kernel.org/linux-fsdevel/20260304114727.384416-1-r@hev.cc
> [v1]: https://lore.kernel.org/linux-fsdevel/20260302155046.286650-1-r@hev.cc
> 
> WANG Rui (2):
>    huge_mm: add stubs for THP-disabled configs
>    binfmt_elf: Align eligible read-only PT_LOAD segments to PMD_SIZE for
>      THP
> 
>   fs/binfmt_elf.c         | 29 +++++++++++++++++++++++++++++
>   include/linux/huge_mm.h | 10 ++++++++++
>   2 files changed, 39 insertions(+)
>