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).
The fixture setup will also test if PR_SET_THP_DISABLE prctl call can
be made with PR_THP_DISABLE_EXCEPT_ADVISED and skip if it fails.

This tests if the process can:
- successfully get the policy to disable THPs expect for madvise.
- get hugepages only on MADV_HUGE and MADV_COLLAPSE if the global policy
  is madvise/always and only with MADV_COLLAPSE if the global policy is
  never.
- 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.

Test results:
./prctl_thp_disable
TAP version 13
1..12
ok 1 prctl_thp_disable_completely.never.nofork
ok 2 prctl_thp_disable_completely.never.fork
ok 3 prctl_thp_disable_completely.madvise.nofork
ok 4 prctl_thp_disable_completely.madvise.fork
ok 5 prctl_thp_disable_completely.always.nofork
ok 6 prctl_thp_disable_completely.always.fork
ok 7 prctl_thp_disable_except_madvise.never.nofork
ok 8 prctl_thp_disable_except_madvise.never.fork
ok 9 prctl_thp_disable_except_madvise.madvise.nofork
ok 10 prctl_thp_disable_except_madvise.madvise.fork
ok 11 prctl_thp_disable_except_madvise.always.nofork
ok 12 prctl_thp_disable_except_madvise.always.fork

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 .../testing/selftests/mm/prctl_thp_disable.c  | 107 ++++++++++++++++++
 1 file changed, 107 insertions(+)

diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
index 8845e9f414560..9bfed4598a1a6 100644
--- a/tools/testing/selftests/mm/prctl_thp_disable.c
+++ b/tools/testing/selftests/mm/prctl_thp_disable.c
@@ -16,6 +16,10 @@
 #include "thp_settings.h"
 #include "vm_util.h"
 
+#ifndef PR_THP_DISABLE_EXCEPT_ADVISED
+#define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
+#endif
+
 enum thp_collapse_type {
 	THP_COLLAPSE_NONE,
 	THP_COLLAPSE_MADV_HUGEPAGE,	/* MADV_HUGEPAGE before access */
@@ -165,4 +169,107 @@ TEST_F(prctl_thp_disable_completely, fork)
 	ASSERT_EQ(ret, 0);
 }
 
+static void prctl_thp_disable_except_madvise_test(struct __test_metadata *const _metadata,
+						  size_t pmdsize,
+						  enum thp_enabled thp_policy)
+{
+	ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL), 3);
+
+	/* tests after prctl overrides global policy */
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize), 0);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
+		  thp_policy == THP_NEVER ? 0 : 1);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 1);
+
+	/* Reset to global policy */
+	ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL), 0);
+
+	/* tests after prctl is cleared, and only global policy is effective */
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
+		  thp_policy == THP_ALWAYS ? 1 : 0);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
+		  thp_policy == THP_NEVER ? 0 : 1);
+
+	ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 1);
+}
+
+FIXTURE(prctl_thp_disable_except_madvise)
+{
+	struct thp_settings settings;
+	size_t pmdsize;
+};
+
+FIXTURE_VARIANT(prctl_thp_disable_except_madvise)
+{
+	enum thp_enabled thp_policy;
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, never)
+{
+	.thp_policy = THP_NEVER,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, madvise)
+{
+	.thp_policy = THP_MADVISE,
+};
+
+FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, always)
+{
+	.thp_policy = THP_ALWAYS,
+};
+
+FIXTURE_SETUP(prctl_thp_disable_except_madvise)
+{
+	if (!thp_available())
+		SKIP(return, "Transparent Hugepages not available\n");
+
+	self->pmdsize = read_pmd_pagesize();
+	if (!self->pmdsize)
+		SKIP(return, "Unable to read PMD size\n");
+
+	if (prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, NULL, NULL))
+		SKIP(return, "Unable to set PR_THP_DISABLE_EXCEPT_ADVISED\n");
+
+	thp_save_settings();
+	thp_read_settings(&self->settings);
+	self->settings.thp_enabled = variant->thp_policy;
+	self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
+	thp_write_settings(&self->settings);
+}
+
+FIXTURE_TEARDOWN(prctl_thp_disable_except_madvise)
+{
+	thp_restore_settings();
+}
+
+TEST_F(prctl_thp_disable_except_madvise, nofork)
+{
+	prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize, variant->thp_policy);
+}
+
+TEST_F(prctl_thp_disable_except_madvise, fork)
+{
+	int ret = 0;
+	pid_t pid;
+
+	/* Make sure prctl changes are carried across fork */
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (!pid)
+		prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize,
+						      variant->thp_policy);
+
+	wait(&ret);
+	if (WIFEXITED(ret))
+		ret = WEXITSTATUS(ret);
+	else
+		ret = -EINVAL;
+	ASSERT_EQ(ret, 0);
+}
+
 TEST_HARNESS_MAIN
-- 
2.47.3