[PATCH v2 3/3] selftests/mm: Added fork test to verify global ksm_zero_pages counter behavior

[Donet Tom <donettom@linux.ibm.com>]

Added a selftest to verify the behavior of the global KSM zero-page
counter during fork. When a process forks, the per-process zero-page
counter is inherited by the child, and the global counter should be
updated with this inherited value. This test ensures that the global
counter is correctly updated after fork.

Signed-off-by: Donet Tom <donettom@linux.ibm.com>
---
 .../selftests/mm/ksm_functional_tests.c       | 74 ++++++++++++++++++-
 1 file changed, 73 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 645cefba2126..f23597ac8066 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -602,6 +602,77 @@ static void test_prot_none(void)
 	munmap(map, size);
 }
 
+long ksm_get_global_ksm_zero_pages(void)
+{
+	int global_ksm_zero_pages_fd;
+	char buf[10];
+	ssize_t ret;
+
+	global_ksm_zero_pages_fd = open("/sys/kernel/mm/ksm/ksm_zero_pages",
+								O_RDONLY);
+	if (global_ksm_zero_pages_fd < 0)
+		return -errno;
+
+	ret = pread(global_ksm_zero_pages_fd, buf, sizeof(buf) - 1, 0);
+	close(global_ksm_zero_pages_fd);
+	if (ret <= 0)
+		return -errno;
+	buf[ret] = 0;
+
+	return strtol(buf, NULL, 10);
+}
+
+static void test_fork_global_ksm_zero_pages_count(void)
+{
+	const unsigned int size = 2 * MiB;
+	char *map;
+	pid_t child_pid;
+	int status;
+	long g_zpages_before = 0, g_zpages_after = 0;
+
+	ksft_print_msg("[RUN] %s\n", __func__);
+
+	/* Unmerge all pages before test */
+	if (ksm_stop() < 0) {
+		ksft_test_result_fail("KSM unmerging failed\n");
+		return;
+	}
+	/* Get the global zero page count before test */
+	g_zpages_before = ksm_get_global_ksm_zero_pages();
+	/* Let KSM deduplicate zero pages. */
+	map = mmap_and_merge_range(0x00, size, PROT_READ | PROT_WRITE, KSM_MERGE_MADVISE);
+	if (map == MAP_FAILED)
+		return;
+
+	child_pid = fork();
+	if (!child_pid) {
+		exit(ksm_stop());
+	} else if (child_pid < 0) {
+		ksft_test_result_fail("fork() failed\n");
+		return;
+	}
+	if (waitpid(child_pid, &status, 0) < 0) {
+		ksft_test_result_fail("waitpid() failed\n");
+		return;
+	}
+	status = WEXITSTATUS(status);
+	if (status < 0) {
+		ksft_test_result_fail("KSM unmerging failed in child\n");
+		return;
+	}
+
+	/* Verify global zero-page count remains unchanged */
+	g_zpages_after = ksm_get_global_ksm_zero_pages();
+	if (g_zpages_before != g_zpages_after) {
+		ksft_test_result_fail("Incorrect global ksm zero page count after fork\n");
+		return;
+	}
+
+	ksft_test_result_pass("Global ksm zero page count is correct after fork\n");
+	ksm_stop();
+	munmap(map, size);
+}
+
 static void test_fork_ksm_merging_page_count(void)
 {
 	const unsigned int size = 2 * MiB;
@@ -659,7 +730,7 @@ static void init_global_file_handles(void)
 
 int main(int argc, char **argv)
 {
-	unsigned int tests = 9;
+	unsigned int tests = 10;
 	int err;
 
 	if (argc > 1 && !strcmp(argv[1], FORK_EXEC_CHILD_PRG_NAME)) {
@@ -692,6 +763,7 @@ int main(int argc, char **argv)
 	test_prctl_fork_exec();
 	test_prctl_unmerge();
 	test_fork_ksm_merging_page_count();
+	test_fork_global_ksm_zero_pages_count();
 
 	err = ksft_get_fail_cnt();
 	if (err)
-- 
2.51.0