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

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Message-ID: <0c724a5c-3317-4bc6-b9ba-98b073934b66@linux.ibm.com>
Date: Wed, 27 Aug 2025 23:33:11 +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
Subject: Re: [PATCH 2/2] selftests/mm: add fork inheritance test for
 ksm_merging_pages counter

Hi David

On 8/26/25 6:52 PM, David Hildenbrand wrote:
> On 26.08.25 14:49, Donet Tom wrote:
>> Add a new selftest to verify whether the `ksm_merging_pages` counter
>
> "to verify ... is *not* inherited" ?
>
I will change it in next version.

>
>
>> in `mm_struct` is 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..d971394c9567 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(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) {
>> +        int mpages;
>> +
>> +        init_global_file_handles();
>> +        mpages = ksm_get_self_merging_pages();
>> +        if (mpages > 0)
>> +            ksft_test_result_fail("ksm_merging_page in child: %d\n", 
>> mpages);
>> +
>> +        exit(0);
>> +    } 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;
>> +    }
>> +
>> +    ksft_test_result_pass("ksm_merging_pages is not inherited after 
>> fork\n");
>
> Won't this trigger a fail in the child and a pass in the parent process?
>
> You should likely instead let the child return the result (or the 
> number) and check that here.


Thanks for pointing this out. I will make the change in the next version