selftests/mm: add fork inheritance test for ksm_merging_pages counter

Add a new selftest to verify whether the `ksm_merging_pages` counter in
`mm_struct` is not inherited by a child process after fork.  This helps
ensure correctness of KSM accounting across process creation.

Link: https://lkml.kernel.org/r/e7bb17d374133bd31a3e423aa9e46e1122e74971.1758648700.git.donettom@linux.ibm.com
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: donettom-1

tools/testing/selftests/mm/ksm_functional_tests.c

@@ -602,6 +602,46 @@ static void test_prot_none(void)
 	munmap(map, size);
 }
 
+static void test_fork_ksm_merging_page_count(void)
+{
+	const unsigned int size = 2 * MiB;
+	char *map;
+	pid_t child_pid;
+	int status;
+
+	ksft_print_msg("[RUN] %s\n", __func__);
+
+	map = mmap_and_merge_range(0xcf, size, PROT_READ | PROT_WRITE, KSM_MERGE_MADVISE);
+	if (map == MAP_FAILED)
+		return;
+
+	child_pid = fork();
+	if (!child_pid) {
+		init_global_file_handles();
+		exit(ksm_get_self_merging_pages());
+	} else if (child_pid < 0) {
+		ksft_test_result_fail("fork() failed\n");
+		goto unmap;
+	}
+
+	if (waitpid(child_pid, &status, 0) < 0) {
+		ksft_test_result_fail("waitpid() failed\n");
+		goto unmap;
+	}
+
+	status = WEXITSTATUS(status);
+	if (status) {
+		ksft_test_result_fail("ksm_merging_page in child: %d\n", status);
+		goto unmap;
+	}
+
+	ksft_test_result_pass("ksm_merging_pages is not inherited after fork\n");
+
+unmap:
+	ksm_stop();
+	munmap(map, size);
+}
+
 static void init_global_file_handles(void)
 {
 	mem_fd = open("/proc/self/mem", O_RDWR);
@@ -620,7 +660,7 @@ static void init_global_file_handles(void)
 
 int main(int argc, char **argv)
 {
-	unsigned int tests = 8;
+	unsigned int tests = 9;
 	int err;
 
 	if (argc > 1 && !strcmp(argv[1], FORK_EXEC_CHILD_PRG_NAME)) {
@@ -652,6 +692,7 @@ int main(int argc, char **argv)
 	test_prctl_fork();
 	test_prctl_fork_exec();
 	test_prctl_unmerge();
+	test_fork_ksm_merging_page_count();
 
 	err = ksft_get_fail_cnt();
 	if (err)