linux-kernel - Re: [PATCH v2 3/3] selftests/mm: Added fork test to verify global ksm_zero

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Message-ID: <0cf5dea0-715d-4fcf-ad58-0272d4d07809@linux.ibm.com>
Date: Wed, 17 Sep 2025 20:17:37 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc: Ritesh Harjani <ritesh.list@...il.com>, Xu Xin <xu.xin16@....com.cn>,
        Chengming Zhou <chengming.zhou@...ux.dev>,
        Wei Yang <richard.weiyang@...il.com>,
        Aboorva Devarajan <aboorvad@...ux.ibm.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Giorgi Tchankvetadze <giorgitchankvetadze1997@...il.com>
Subject: Re: [PATCH v2 3/3] selftests/mm: Added fork test to verify global
 ksm_zero_pages counter behavior


On 9/17/25 6:49 PM, David Hildenbrand wrote:
> 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@...ux.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.


Yes, you are right. I will fix this and send a v3.