linux-kernel - Re: [PATCH v2 2/3] selftests/mm: Added fork inheritance test for ksm_merging

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Message-ID: <659ba2d4-fee3-410d-b611-04a3586b5bb7@linux.ibm.com>
Date: Wed, 17 Sep 2025 20:15:57 +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 2/3] selftests/mm: Added fork inheritance test for
 ksm_merging_pages counter


On 9/17/25 6:45 PM, David Hildenbrand wrote:
> On 15.09.25 17:03, Donet Tom wrote:
>> Added 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.
>>
>> Signed-off-by: Donet Tom <donettom@...ux.ibm.com>
>> ---
>>   .../selftests/mm/ksm_functional_tests.c       | 42 ++++++++++++++++++-
>>   1 file changed, 41 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c 
>> b/tools/testing/selftests/mm/ksm_functional_tests.c
>> index 712f43c87736..645cefba2126 100644
>> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
>> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
>> @@ -602,6 +602,45 @@ 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");
>> +        return;
>
> Probably
>     goto out;
>
> in all these fail cases
>
>> +    }
>> +
>> +    if (waitpid(child_pid, &status, 0) < 0) {
>> +        ksft_test_result_fail("waitpid() failed\n");
>> +        return;
>> +    }
>> +
>> +    status = WEXITSTATUS(status);
>> +    if (status) {
>> +        ksft_test_result_fail("ksm_merging_page in child: %d\n", 
>> status);
>> +        return;
>> +    }
>> +
>> +    ksft_test_result_pass("ksm_merging_pages is not inherited after 
>> fork\n");
>> +
>
> And have
>
>     out:
>
> here


Sure, I will send a v3 with this change.


>
> With that:
>
> Acked-by: David Hildenbrand <david@...hat.com>
>