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

[David Hildenbrand <david@redhat.com>]

On 15.09.25 17:03, Donet Tom wrote:
> 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 */

That only works when "use_zero_pages" is enabled, no?

> +	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;

Cleanup missing as for patch #2.

> +	}
> +	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();
>

What stops KSM from merging pages in other processes by accident 
concurrently and giving false failures?

Likely you would want to stop ksm before the fork.

-- 
Cheers

David / dhildenb