This test case exercises the BPF THP update mechanism by modifying an
existing policy. The behavior confirms that:
- EBUSY error occurs when attempting to install a new BPF program while
  another is active
- Updates to currently running programs are successfully processed

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 .../selftests/bpf/prog_tests/thp_adjust.c     | 23 +++++++++++++++++++
 .../selftests/bpf/progs/test_thp_adjust.c     | 14 +++++++++++
 2 files changed, 37 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/thp_adjust.c b/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
index b14f57040654..72b2ec31025a 100644
--- a/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
+++ b/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
@@ -208,6 +208,27 @@ static void subtest_thp_eligible(void)
 	bpf_link__destroy(ops_link);
 }
 
+static void subtest_thp_policy_update(void)
+{
+	struct bpf_link *old_link, *new_link;
+	int err;
+
+	old_link = bpf_map__attach_struct_ops(skel->maps.swap_ops);
+	if (!ASSERT_OK_PTR(old_link, "attach_old_link"))
+		return;
+
+	new_link = bpf_map__attach_struct_ops(skel->maps.thp_eligible_ops);
+	if (!ASSERT_NULL(new_link, "attach_new_link"))
+		goto destory_old;
+	ASSERT_EQ(errno, EBUSY, "attach_new_link");
+
+	err = bpf_link__update_map(old_link, skel->maps.thp_eligible_ops);
+	ASSERT_EQ(err, 0, "update_old_link");
+
+destory_old:
+	bpf_link__destroy(old_link);
+}
+
 static int thp_adjust_setup(void)
 {
 	int err = -1, pmd_order;
@@ -253,6 +274,8 @@ void test_thp_adjust(void)
 
 	if (test__start_subtest("thp_eligible"))
 		subtest_thp_eligible();
+	if (test__start_subtest("policy_update"))
+		subtest_thp_policy_update();
 
 	thp_adjust_destroy();
 }
diff --git a/tools/testing/selftests/bpf/progs/test_thp_adjust.c b/tools/testing/selftests/bpf/progs/test_thp_adjust.c
index ed8c510693a0..8f3bc4768edc 100644
--- a/tools/testing/selftests/bpf/progs/test_thp_adjust.c
+++ b/tools/testing/selftests/bpf/progs/test_thp_adjust.c
@@ -39,3 +39,17 @@ SEC(".struct_ops.link")
 struct bpf_thp_ops thp_eligible_ops = {
 	.thp_get_order = (void *)thp_eligible,
 };
+
+SEC("struct_ops/thp_get_order")
+int BPF_PROG(alloc_not_in_swap, struct vm_area_struct *vma, enum tva_type tva_type,
+	     unsigned long orders)
+{
+	if (tva_type == TVA_SWAP_PAGEFAULT)
+		return 0;
+	return -1;
+}
+
+SEC(".struct_ops.link")
+struct bpf_thp_ops swap_ops = {
+	.thp_get_order = (void *)alloc_not_in_swap,
+};
-- 
2.47.3