[PATCH 2.43] malloc: Show hugetlb tunable default in --list-tunables

[A. Sverdlin]

From: Wilco Dijkstra <wilco.dijkstra@arm.com>

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 <dj@redhat.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
(cherry picked from commit e87c151130216f5c62f0e172e3eaebbc4a5d794a)
---

The commit e87c15113021
("malloc: Show hugetlb tunable default in --list-tunables") fixes the
tunables hugetlb regression second time in a row.

The regression (considerable RAM footprint increase on embedded Aarch64
devices publicly reported at least once [1]) was initially caused by commit
321e1fc73f53 ("malloc: Enable 2MB THP by default on Aarch64").

Commit 1c588a2187a4 ("malloc: Improve thp_init") made mp_.thp_pagesize
configurable again, so that the regression could be mitigated via
glibc.malloc.hugetlb, but commit 2e8a940df14e
("malloc: Avoid accessing /sys/kernel/mm files") has broken the tunable
again (glibc.malloc.hugetlb didn't affect mp_.thp_pagesize any longer).

Fortunately e87c15113021
("malloc: Show hugetlb tunable default in --list-tunables") makes it again
possible to mitigate RAM footprint increase and its commit message body
even mentions the fix, but its importance in 2.43 stable branch is not
obvious from the commit subject alone.

[1] https://github.com/bottlerocket-os/bottlerocket-core-kit/pull/905

 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 d83b4db068..5bfcd7f972 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -276,15 +276,6 @@ __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 771e7d40b7..847168d647 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -5114,10 +5114,26 @@ 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)
-- 
2.55.0