[PATCH] malloc: Show hugetlb tunable default in --list-tunables

[Dimitri John Ledkov]

Hi,

On Tue, 10 Mar 2026 at 19:32, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
>
> Explicitly set the hugetlb tunable value in aarch64/cpu-features.c so it
> is shown as 1 with /lib/ld-linux-aarch64.so.1 --list-tunables.
> Move the intitialization of thp_mode/thp_pagesize to do_set_hugetlb() and
> avoid accessing /sys/kernel/mm if DEFAULT_THP_PAGESIZE > 0.
>
> Passes regress, OK for commit?
>

Performed test build and executed things interactively on aarch64.
This does what it says on the tin, and helps me as an end user.
But do note the review comments from Adhemerval Zanella Netto for an
alternative implementation that might be easier to maintain.

Ideally when the next architecture adds this behaviour (RISC-V patches
are in review) this should update automatically for that platform too.

Tested-By: Dimitri John Ledkov <dimitri.ledkov@surgut.co.uk>

> ---
>
> diff --git a/malloc/arena.c b/malloc/arena.c
> index 75f2f32b5f61a33a06dc285ca9899a708c78cf10..d2ed6279226856d630cfdf734303c4b936512f58 100644
> --- a/malloc/arena.c
> +++ b/malloc/arena.c
> @@ -275,15 +275,6 @@ __ptmalloc_init (void)
>      __always_fail_morecore = true;
>  #endif
>
> -  /* Enable THP if DEFAULT_THP_PAGESIZE is non-zero.  Avoid quering the THP
> -     page size or mode since accessing /sys/kernel/mm is relatively slow and
> -     might not be accessible in containers.  */
> -  if (DEFAULT_THP_PAGESIZE > 0)
> -    {
> -      mp_.thp_mode = malloc_thp_mode_madvise;
> -      mp_.thp_pagesize = DEFAULT_THP_PAGESIZE;
> -    }
> -
>    thread_arena = &main_arena;
>
>    malloc_init_state (&main_arena);
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 4aac196533d896353e3a32e34fa143d03eb82627..7241a79a936999ab5c3ac989133ad4c7919eec31 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -5026,13 +5026,25 @@ do_set_mxfast (size_t value)
>  static __always_inline int
>  do_set_hugetlb (size_t value)
>  {
> +  /* Enable THP if DEFAULT_THP_PAGESIZE is non-zero.  */
> +  if (DEFAULT_THP_PAGESIZE > 0)
> +    {
> +      mp_.thp_mode = malloc_thp_mode_madvise;
> +      mp_.thp_pagesize = DEFAULT_THP_PAGESIZE;
> +    }
> +
>    if (value == 0)
>      mp_.thp_mode = malloc_thp_mode_never;
>    else if (value == 1)
>      {
> +      /* Avoid querying the THP page size/mode since accessing /sys/kernel/mm
> +        is relatively slow and might not be accessible in containers.  */
> +      if (DEFAULT_THP_PAGESIZE > 0)
> +       return 0;
> +
>        mp_.thp_mode = __malloc_thp_mode ();
>        if (mp_.thp_mode == malloc_thp_mode_madvise
> -          || mp_.thp_mode == malloc_thp_mode_always)
> +         || mp_.thp_mode == malloc_thp_mode_always)
>         mp_.thp_pagesize = __malloc_default_thp_pagesize ();
>      }
>    else if (value >= 2)
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index 70d3272772f5f36a3435d0ae8919d781a1e2412e..af502eff82e05efbe0a2e9418a71473e5c7ec8b9 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -179,4 +179,8 @@ init_cpu_features (struct cpu_features *cpu_features)
>    if (GLRO (dl_hwcap) & HWCAP_GCS)
>      /* GCS status may be updated later by binary compatibility checks.  */
>      GL (dl_aarch64_gcs) = TUNABLE_GET (glibc, cpu, aarch64_gcs, uint64_t, 0);
> +
> +  /* Default to glibc.malloc.hugetlb=1 if the tunable is not set.  */
> +  if (!TUNABLE_IS_INITIALIZED (glibc, malloc, hugetlb))
> +    TUNABLE_SET (glibc, malloc, hugetlb, 1);
>  }
>


-- 
Regards,

Dimitri.