Re: [PATCH 2/2] selftests/mm: add fork inheritance test for ksm_merging_pages counter

[Donet Tom]

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@xxxxxxxxxxxxx>
> > ---
> >   .../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