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Wed, 17 Sep 2025 14:47:42 GMT Received: from smtpav06.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D8F415803F; Wed, 17 Sep 2025 14:47:41 +0000 (GMT) Received: from smtpav06.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8CC4958055; Wed, 17 Sep 2025 14:47:38 +0000 (GMT) Received: from [9.109.215.183] (unknown [9.109.215.183]) by smtpav06.wdc07v.mail.ibm.com (Postfix) with ESMTP; Wed, 17 Sep 2025 14:47:38 +0000 (GMT) Message-ID: <0cf5dea0-715d-4fcf-ad58-0272d4d07809@linux.ibm.com> Date: Wed, 17 Sep 2025 20:17:37 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] selftests/mm: Added fork test to verify global ksm_zero_pages counter behavior To: David Hildenbrand , Andrew Morton Cc: Ritesh Harjani , Xu Xin , Chengming Zhou , Wei Yang , Aboorva Devarajan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Giorgi Tchankvetadze References: <5910cb6bfc7c43b169b679a0108667a56d7ebdb8.1757946863.git.donettom@linux.ibm.com> Content-Language: en-US From: Donet Tom In-Reply-To: Content-Type: text/plain; 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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 >> --- >>   .../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.