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

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Message-ID: <ba6d3e50-d8dc-4ee4-ae44-bc1b57e4a2dd@redhat.com>
Date: Tue, 26 Aug 2025 15:22:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Donet Tom <donettom@...ux.ibm.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

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" ?

> 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.

-- 
Cheers

David / dhildenb