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

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e87c151130216f5c62f0e172e3eaebbc4a5d794a

commit e87c151130216f5c62f0e172e3eaebbc4a5d794a
Author: Wilco Dijkstra <[email protected]>
Date:   Wed Apr 1 12:15:21 2026 +0000

    malloc: Show hugetlb tunable default in --list-tunables
    
    Update the hugetlb tunable default in elf/dl-tunables.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.  Switch off THP if
    glibc.malloc.hugetlb=0 is used - this behaves as if DEFAULT_THP_PAGESIZE==0.
    Fix the --list-tunables testcase.
    
    Reviewed-by: DJ Delorie <[email protected]>

Diff:
---
 elf/dl-tunables.c             |  5 +++++
 elf/tst-rtld-list-tunables.sh | 12 ++----------
 malloc/arena.c                |  9 ---------
 malloc/malloc.c               | 18 +++++++++++++++++-
 4 files changed, 24 insertions(+), 20 deletions(-)

diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index bdb1de4ceb..1440d3fa6a 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -33,6 +33,7 @@
 #include <array_length.h>
 #include <dl-minimal-malloc.h>
 #include <dl-symbol-redir-ifunc.h>
+#include <malloc-hugepages.h>
 
 #define TUNABLES_INTERNAL 1
 #include "dl-tunables.h"
@@ -296,6 +297,10 @@ __tunables_init (char **envp)
   char *envval = NULL;
   char **prev_envp = envp;
 
+  /* Default to glibc.malloc.hugetlb=1 if DEFAULT_THP_PAGESIZE is non-zero.  */
+  if (DEFAULT_THP_PAGESIZE > 0)
+    TUNABLE_SET (glibc, malloc, hugetlb, 1);
+
   /* Ignore tunables for AT_SECURE programs.  */
   if (__libc_enable_secure)
     return;
diff --git a/elf/tst-rtld-list-tunables.sh b/elf/tst-rtld-list-tunables.sh
index 669898d8c0..11b9b4597a 100755
--- a/elf/tst-rtld-list-tunables.sh
+++ b/elf/tst-rtld-list-tunables.sh
@@ -26,16 +26,8 @@ run_program_env=$3
 LC_ALL=C
 export LC_ALL
 
-# Unset tunables and their aliases.
-GLIBC_TUNABLES=
-MALLOC_ARENA_MAX=
-MALLOC_ARENA_TEST=
-MALLOC_CHECK_=
-MALLOC_MMAP_MAX_=
-MALLOC_MMAP_THRESHOLD_=
-MALLOC_PERTURB_=
-MALLOC_TOP_PAD_=
-MALLOC_TRIM_THRESHOLD_=
+# Unset tunables.
+export GLIBC_TUNABLES=glibc.malloc.hugetlb=0
 
 ${test_wrapper_env} \
 ${run_program_env} \
diff --git a/malloc/arena.c b/malloc/arena.c
index 727d0517c4..ddde32c712 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 querying 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 6455a1b0e0..6a888b0eb7 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -5017,10 +5017,26 @@ 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;
+    {
+      /* Turn off THP support completely.  */
+      mp_.thp_mode = malloc_thp_mode_never;
+      mp_.thp_pagesize = 0;
+    }
   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)