The test will set the global system THP setting to never, madvise
or always depending on the fixture variant and the 2M setting to
inherit before it starts (and reset to original at teardown).

This tests if the process can:
- successfully set and get the policy to disable THPs completely.
- never get a hugepage when the THPs are completely disabled
   with the prctl, including with MADV_HUGE and MADV_COLLAPSE.
- successfully reset the policy of the process.
- after reset, only get hugepages with:
   - MADV_COLLAPSE when policy is set to never.
   - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
   - always when policy is set to "always".
- repeat the above tests in a forked process to make sure
   the policy is carried across forks.

Signed-off-by: Usama Arif <redacted>
---
  tools/testing/selftests/mm/.gitignore         |   1 +
  tools/testing/selftests/mm/Makefile           |   1 +
  .../testing/selftests/mm/prctl_thp_disable.c  | 173 ++++++++++++++++++
  tools/testing/selftests/mm/thp_settings.c     |   9 +-
  tools/testing/selftests/mm/thp_settings.h     |   1 +
  5 files changed, 184 insertions(+), 1 deletion(-)
  create mode 100644 tools/testing/selftests/mm/prctl_thp_disable.c

diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
index e7b23a8a05fe..eb023ea857b3 100644
--- a/tools/testing/selftests/mm/.gitignore
+++ b/tools/testing/selftests/mm/.gitignore

@@ -58,3 +58,4 @@ pkey_sighandler_tests_32
  pkey_sighandler_tests_64
  guard-regions
  merge
+prctl_thp_disable

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index d13b3cef2a2b..2bb8d3ebc17c 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile

@@ -86,6 +86,7 @@ TEST_GEN_FILES += on-fault-limit
  TEST_GEN_FILES += pagemap_ioctl
  TEST_GEN_FILES += pfnmap
  TEST_GEN_FILES += process_madv
+TEST_GEN_FILES += prctl_thp_disable
  TEST_GEN_FILES += thuge-gen
  TEST_GEN_FILES += transhuge-stress
  TEST_GEN_FILES += uffd-stress

diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
new file mode 100644
index 000000000000..ef150180daf4
--- /dev/null
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c

@@ -0,0 +1,173 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic tests for PR_GET/SET_THP_DISABLE prctl calls
+ *
+ * Author(s): Usama Arif <usamaarif642@gmail.com>
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+
+#include "../kselftest_harness.h"
+#include "thp_settings.h"
+#include "vm_util.h"
+
+static int sz2ord(size_t size, size_t pagesize)
+{
+	return __builtin_ctzll(size / pagesize);
+}