[PATCH] elf: Align large load segments to PMD huge page size for THP

[Xi Ruoyao]

On Wed, 2026-02-25 at 00:05 +0800, WANG Rui wrote:
> Mapping segments that are at least the size of a PMD huge page to
> huge-page-aligned addresses helps make them eligible for Transparent
> Huge Pages (THP).
> 
> This patch updates the segment mapping logic in the generic ELF loader
> to dynamically calculate the PMD huge page size based on the runtime
> normal page size. On 64-bit systems, the number of page table entries
> is (page_size / sizeof(void *)). Multiplying this by the page size
> yields the PMD huge page size (e.g., a 4K normal page yields a 2M huge
> page, and a 16K normal page yields a 32M huge page).
> 
> For load segments where both the virtual address and file offset are
> already aligned to this huge page size (e.g., zero), and the segment
> size is large enough, we explicitly bump its alignment to the huge
> page size. This optimization significantly reduces TLB pressure and
> improves performance for large objects.
> 
> To prevent excessive address space padding and memory waste on
> architectures with very large normal pages (e.g., 64K pages yielding
> 512M huge pages), the optimization is capped and only applies if the
> calculated huge page size is 32M or less.
> 
> * elf/dl-load.c (_dl_map_object_from_fd): Bump p_align_max to the
> calculated PMD huge page size (capped at 32M) for large load segments
> on 64-bit systems.
> 
> Signed-off-by: WANG Rui <wangrui@loongson.cn>
> ---
>  elf/dl-load.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index 7355eef8e7..a8db4f3abc 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1171,6 +1171,23 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>  
>  	  /* Optimize a common case.  */
>  	  c->prot = pf_to_prot (ph->p_flags);
> +
> +#ifdef __LP64__
> +	  /* Dynamically calculate the PMD huge page size based on the normal
> +	     page size (e.g., 2M for 4K pages, 32M for 16K pages). Aligning
> +	     load segments that are large enough to the PMD size helps improve
> +	     THP eligibility and reduces TLB pressure.
> +	     We cap the huge page size at 32M to avoid over-aligning on systems
> +	     with very large normal pages (like 64K pages with 512M huge pages). */
> +	  size_t page_size = GLRO(dl_pagesize);
> +	  size_t huge_page_size = page_size * (page_size / sizeof (void *));

Is this calculation of huge page size always correct? 
sysdeps/unix/sysv/linux/malloc-hugepages.c parses /proc/meminfo instead.
But I don't know if we can open and parse files in such an early stage
of dynamic linking.  Anyway at the very least the this is Linux-
specific, so IMO the logic modifying p_align_max should be factored out
to sysdeps/unix/sysv/linux/wordsize-64/.

Would this affect ASLR and require LoongArch distros to raise
CONFIG_ARCH_MMAP_RND_BITS to compensate?

Technically we shouldn't be doing this on some circumstances, like if
the kernel lacks THP support, or it is configured without
CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS.  But maybe checking all of the
conditions would be too hard.

-- 
Xi Ruoyao <xry111@xry111.site>