[PATCH v2] malloc: Don't call __get_thp_mode/__get_thp_size twice

[Adhemerval Zanella Netto]

On 22/06/26 10:15, H.J. Lu wrote:
> This is identical to the first version.   Now it can be applied
> to master branch since its prerequisite has been merged into
> master branch.
> 
> From ad568336b63148195af93cf7bd4adf6efa282deb Mon Sep 17 00:00:00 2001
> From: "H.J. Lu" <hjl.tools@gmail.com>
> Date: Sat, 18 Apr 2026 11:36:41 +0800
> Subject: [PATCH v2] malloc: Don't call __get_thp_mode/__get_thp_size twice
> 
> Both ld.so and malloc track kernel THP mode and THP page size when THP
> in ld.so is enabled by
> 
> GLIBC_TUNABLES=glibc.elf.thp=1
> 
> and THP in malloc is enabled by
> 
> GLIBC_TUNABLES=glibc.malloc.hugetlb=1
> 
> But DL_MAP_DEFAULT_THP_PAGESIZE and MALLOC_DEFAULT_THP_PAGESIZE may be
> different when they are defined in <hugepages.h>.
> 
> If THP in ld.so is enabled, change malloc to use kernel THP mode from
> ld.so, instead of calling __get_thp_mode, and use THP page size from
> ld.so if it came from __get_thp_size.  This avoids calling __get_thp_mode
> and __get_thp_size again, which are quite expensive:
> 
> 1. Initialize mp_.thp_mode and GL(dl_thp_mode) to thp_mode_unknown.
> 2. Set mp_.thp_mode to GL(dl_thp_mode) if GL(dl_thp_mode) isn't
> thp_mode_unknown.  Otherwise call __get_thp_mode to set mp_.thp_mode.
> 3. GL(dl_elf_thp_pagesize) is set to DL_MAP_DEFAULT_THP_PAGESIZE without
> calling __get_thp_size and THP page size for malloc may be different from
> THP page size for ld.so.  Set mp_.thp_pagesize to GL(dl_elf_thp_pagesize)
> if DL_MAP_DEFAULT_THP_PAGESIZE is defined.  Otherwise call __get_thp_size
> to set mp_.thp_pagesize.
> 
> This fixes BZ #34083.
> 
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>

Looks good, some comments below.  I think Andreas will need to ack it now that
we are in soft-freeze.

> ---
>  malloc/malloc.c             | 56 +++++++++++++++++++++++++++++++++----
>  sysdeps/generic/hugepages.h |  3 +-
>  2 files changed, 53 insertions(+), 6 deletions(-)
> 
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index a354576aca..f0ed95213f 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1529,7 +1529,7 @@ static struct malloc_par mp_ =
>    .mmap_threshold = DEFAULT_MMAP_THRESHOLD,
>    .trim_threshold = DEFAULT_TRIM_THRESHOLD,
>    .arena_test = sizeof (long) == 4 ? 2 : 8,
> -  .thp_mode = thp_mode_not_supported
> +  .thp_mode = thp_mode_unknown
>  #if USE_TCACHE
>    ,
>    .tcache_count = TCACHE_FILL_COUNT,
> @@ -4657,13 +4657,41 @@ do_set_mxfast (size_t value)
>    return 1;
>  }
>  
> +#ifdef HAVE_THP
> +static __always_inline enum thp_mode_t
> +get_dl_thp_mode (void)
> +{
> +  return GL(dl_thp_mode);
> +}
> +
> +static __always_inline unsigned long int
> +get_dl_elf_thp_pagesize (void)
> +{
> +  return GL(dl_elf_thp_pagesize);
> +}
> +#else
> +# define get_dl_thp_mode()		__get_thp_mode ()
> +# define get_dl_elf_thp_pagesize()	__get_thp_size ()
> +#endif
> +
>  static __always_inline int
>  do_set_hugetlb (size_t value)
>  {
> +  /* If __get_thp_mode and __get_thp_size have been called during
> +     startup, don't call them again here.  */
> +  enum thp_mode_t thp_mode = get_dl_thp_mode ();
> +
>    /* Enable THP if MALLOC_DEFAULT_THP_PAGESIZE is non-zero.  */
>    if (MALLOC_DEFAULT_THP_PAGESIZE > 0)
>      {
> -      mp_.thp_mode = thp_mode_madvise;
> +      /* If thp_mode is unknown, THP segment load is disabled by
> +	 GLIBC_TUNABLES=glibc.elf.thp=0.  In this case, set
> +	 mp_.thp_mode to madvise.  Otherwise, set it to thp_mode to
> +	 keep mp_.thp_mode in sync with GL(dl_thp_mode).  */
> +      if (thp_mode == thp_mode_unknown)
> +	mp_.thp_mode = thp_mode_madvise;
> +      else
> +	mp_.thp_mode = thp_mode;
>        mp_.thp_pagesize = MALLOC_DEFAULT_THP_PAGESIZE;
>      }
>  

When glibc.elf.thp=1, it now mirrors the real kernel mode instead of defaulting
to thp_mode_madvise. The madvise_thp will then stops issuing MADV_HUGEPAGE, which
I think benign/beneficial (kernel already covers always; never madvise was wasteful).

But it means glibc.malloc.hugetlb behavior on aarch64 now depends on glibc.elf.thp;
two previously independent tunables. The default is unchanged:

  elf.thp unset -> disabled -> unknown -> old forced-madvise

So it only matters when the tunable is set. Maybe add a note in the manual/NEWS
about this change.

> @@ -4680,9 +4708,27 @@ do_set_hugetlb (size_t value)
>        if (MALLOC_DEFAULT_THP_PAGESIZE > 0)
>  	return 0;
>  
> -      mp_.thp_mode = __get_thp_mode ();
> -      if (mp_.thp_mode == thp_mode_madvise || mp_.thp_mode == thp_mode_always)
> -	mp_.thp_pagesize = __get_thp_size ();
> +      if (thp_mode == thp_mode_unknown)
> +	{
> +	  /* Call __get_thp_mode and __get_thp_size when THP segment load
> +	     is disabled.  */
> +	  mp_.thp_mode = __get_thp_mode ();
> +	  if (mp_.thp_mode == thp_mode_madvise
> +	      || mp_.thp_mode == thp_mode_always)
> +	    mp_.thp_pagesize = __get_thp_size ();
> +	}
> +      else
> +	{
> +	  /* THP segment load is enabled.  GL(dl_elf_thp_pagesize) is
> +	     set to DL_MAP_DEFAULT_THP_PAGESIZE if it isn't zero .  In

Stray space after 'zero'.

> +	     this case, call __get_thp_size () instead of using
> +	     DL_MAP_DEFAULT_THP_PAGESIZE for malloc.  */
> +	  mp_.thp_mode = thp_mode;
> +	  if (DL_MAP_DEFAULT_THP_PAGESIZE != 0)
> +	    mp_.thp_pagesize = __get_thp_size ();
> +	  else
> +	    mp_.thp_pagesize = get_dl_elf_thp_pagesize ();
> +	}
>      }
>    else if (value >= 2)
>      __get_hugepage_config (value == 2 ? 0 : value, &mp_.hp_pagesize,

When elf.thp=1, malloc uses GL(dl_elf_thp_pagesize), which ld.so caps at 
MAX_THP_PAGESIZE; when elf.thp=0/unset, malloc uses the uncapped __get_thp_size(). 

I think we should not matter for current supported ABIs, but maybe we should
consolidate the logic.

> diff --git a/sysdeps/generic/hugepages.h b/sysdeps/generic/hugepages.h
> index 8e54661527..49251d94ab 100644
> --- a/sysdeps/generic/hugepages.h
> +++ b/sysdeps/generic/hugepages.h
> @@ -37,7 +37,8 @@ unsigned long int __get_thp_size (void) attribute_hidden;
>  
>  enum thp_mode_t
>  {
> -  thp_mode_not_supported = 0,
> +  thp_mode_unknown = 0,
> +  thp_mode_not_supported,
>    thp_mode_always,
>    thp_mode_madvise,
>    thp_mode_never
> -- 
> 2.54.0
>