In this test case, the parent is allowed to alloc THP, but the child
forked by it can't alloc THP.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 .../selftests/bpf/prog_tests/thp_adjust.c     | 59 +++++++++++++++++++
 .../selftests/bpf/progs/test_thp_adjust.c     | 39 ++++++++++++
 2 files changed, 98 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/thp_adjust.c b/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
index a4a34ee28301..bf367c6e6f52 100644
--- a/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
+++ b/tools/testing/selftests/bpf/prog_tests/thp_adjust.c
@@ -1,5 +1,11 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#define _GNU_SOURCE
+#include <sched.h>
+#include <sys/wait.h>
+#include <sys/syscall.h>
+#include <linux/sched.h>
+
 #include <math.h>
 #include <sys/mman.h>
 #include <test_progs.h>
@@ -170,6 +176,57 @@ static void subtest_thp_policy(void)
 	bpf_link__destroy(ops_link);
 }
 
+/*
+ * In this test case, we clone the child process directly into the root cgroup.
+ * Consequently, the child process is not permitted to alloc THP.
+ */
+static void subtest_thp_fork(void)
+{
+	struct clone_args args = {
+		.flags = CLONE_INTO_CGROUP,
+		.exit_signal = SIGCHLD,
+	};
+	struct bpf_link *ops_link;
+	int status, err;
+	pid_t pid;
+
+	skel->bss->ppid = getpid();
+	if (!ASSERT_GT(skel->bss->ppid, 0, "getpid"))
+		return;
+	args.cgroup = get_root_cgroup();
+	if (!ASSERT_GE(args.cgroup, 0, "get_root_cgrp_fd"))
+		return;
+
+	ops_link = bpf_map__attach_struct_ops(skel->maps.thp_fork_ops);
+	if (!ASSERT_OK_PTR(ops_link, "attach struct_ops"))
+		return;
+
+	pid = syscall(__NR_clone3, &args, sizeof(args));
+	if (!ASSERT_GE(pid, 0, "clone3"))
+		goto detach_ops;
+
+	if (pid == 0) {
+		/* child */
+		if (!ASSERT_NEQ(thp_alloc(), -1, "THP alloc"))
+			exit(EXIT_FAILURE);
+		thp_free();
+		exit(EXIT_SUCCESS);
+	}
+
+	err = waitpid(pid, &status, 0);
+	if (!ASSERT_EQ(err, pid, "waitpid"))
+		goto detach_ops;
+	ASSERT_EQ(skel->bss->fork_fail, 0, "fork_fail");
+	ASSERT_GT(skel->bss->fork_succeed, 0, "fork_succeed");
+
+	if (!ASSERT_NEQ(thp_alloc(), -1, "THP alloc"))
+		goto detach_ops;
+	thp_free();
+	ASSERT_GT(skel->bss->parent_succeed, 0, "parent_succeed");
+detach_ops:
+	bpf_link__destroy(ops_link);
+}
+
 static int thp_adjust_setup(void)
 {
 	int err, cgrp_fd, cgrp_id, pmd_order;
@@ -249,6 +306,8 @@ void test_thp_adjust(void)
 
 	if (test__start_subtest("alloc_in_khugepaged"))
 		subtest_thp_policy();
+	if (test__start_subtest("khugepaged_fork"))
+		subtest_thp_fork();
 
 	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 635915f31786..034086ce2f3d 100644
--- a/tools/testing/selftests/bpf/progs/test_thp_adjust.c
+++ b/tools/testing/selftests/bpf/progs/test_thp_adjust.c
@@ -6,6 +6,7 @@
 
 char _license[] SEC("license") = "GPL";
 
+int ppid, fork_fail, fork_succeed, parent_succeed;
 int pf_alloc, pf_disallow, khugepaged_disallow;
 struct mm_struct *target_mm;
 int pmd_order, cgrp_id;
@@ -74,3 +75,41 @@ SEC(".struct_ops.link")
 struct bpf_thp_ops khugepaged_ops = {
 	.get_suggested_order = (void *)alloc_in_khugepaged,
 };
+
+
+SEC("struct_ops/get_suggested_order")
+int BPF_PROG(thp_fork_test, struct mm_struct *mm, struct vm_area_struct *vma__nullable,
+	     u64 vma_flags, enum tva_type tva_flags, int orders)
+{
+	struct task_struct *p = bpf_get_current_task_btf();
+	struct mem_cgroup *memcg;
+	int suggested_orders = 0;
+
+	/* Only works when CONFIG_MEMCG is enabled. */
+	memcg = bpf_mm_get_mem_cgroup(mm);
+	if (!memcg)
+		return 0;
+
+	/* The tasks under this specific cgroup are allowed to alloc THP */
+	if (memcg->css.cgroup->kn->id == cgrp_id)
+		suggested_orders = orders;
+
+	if (p->parent->pid == ppid) {
+		/* The child is forked into root cgrp, so it can't alloc THP */
+		if (suggested_orders)
+			fork_fail++;
+		else
+			fork_succeed++;
+	} else if (p->pid == ppid) {
+		/* The parent can alloc THP */
+		if (suggested_orders)
+			parent_succeed++;
+	}
+	bpf_put_mem_cgroup(memcg);
+	return suggested_orders;
+}
+
+SEC(".struct_ops.link")
+struct bpf_thp_ops thp_fork_ops = {
+	.get_suggested_order = (void *)thp_fork_test,
+};
-- 
2.47.3