linux-kernel - Re: [PATCH v1 3/9] selftests/mm: Skip soft-dirty tests on arm64

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Date:   Thu, 13 Jul 2023 15:03:29 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Mark Brown <broonie@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Florent Revest <revest@...omium.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1 3/9] selftests/mm: Skip soft-dirty tests on arm64

On 13/07/2023 14:56, David Hildenbrand wrote:
> On 13.07.23 15:54, Ryan Roberts wrote:
>> arm64 does not support the soft-dirty PTE bit. However there are tests
>> in `madv_populate` and `soft-dirty` which assume it is supported and
>> cause spurious failures to be reported when preferred behaviour would be
>> to mark the tests as skipped.
>>
>> Unfortunately, the only way to determine if the soft-dirty dirty bit is
>> supported is to write to a page, then see if the bit is set in
>> /proc/self/pagemap. But the tests that we want to conditionally execute
>> are testing precicesly this. So if we introduced this feature check, we
>> could accedentally turn a real failure (on a system that claims to
>> support soft-dirty) into a skip.
>>
>> So instead, do the check based on architecture; for arm64, we report
>> that soft-dirty is not supported. This is wrapped up into a utility
>> function `system_has_softdirty()`, which is used to skip the whole
>> `soft-dirty` suite, and mark the soft-dirty tests in the `madv_populate`
>> suite as skipped.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>   tools/testing/selftests/mm/madv_populate.c | 18 +++++++++++++-----
>>   tools/testing/selftests/mm/soft-dirty.c    |  3 +++
>>   tools/testing/selftests/mm/vm_util.c       | 17 +++++++++++++++++
>>   tools/testing/selftests/mm/vm_util.h       |  1 +
>>   4 files changed, 34 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/madv_populate.c
>> b/tools/testing/selftests/mm/madv_populate.c
>> index 60547245e479..5a8c176d7fec 100644
>> --- a/tools/testing/selftests/mm/madv_populate.c
>> +++ b/tools/testing/selftests/mm/madv_populate.c
>> @@ -232,6 +232,14 @@ static bool range_is_not_softdirty(char *start, ssize_t
>> size)
>>       return ret;
>>   }
>>
>> +#define ksft_test_result_if_softdirty(cond, ...)    \
>> +do {                            \
>> +    if (system_has_softdirty())            \
>> +        ksft_test_result(cond, __VA_ARGS__);    \
>> +    else                        \
>> +        ksft_test_result_skip(__VA_ARGS__);    \
>> +} while (0)
>> +
>>   static void test_softdirty(void)
>>   {
>>       char *addr;
>> @@ -246,19 +254,19 @@ static void test_softdirty(void)
>>
>>       /* Clear any softdirty bits. */
>>       clear_softdirty();
>> -    ksft_test_result(range_is_not_softdirty(addr, SIZE),
>> +    ksft_test_result_if_softdirty(range_is_not_softdirty(addr, SIZE),
>>                "range is not softdirty\n");
>>
>>       /* Populating READ should set softdirty. */
>>       ret = madvise(addr, SIZE, MADV_POPULATE_READ);
>> -    ksft_test_result(!ret, "MADV_POPULATE_READ\n");
>> -    ksft_test_result(range_is_not_softdirty(addr, SIZE),
>> +    ksft_test_result_if_softdirty(!ret, "MADV_POPULATE_READ\n");
>> +    ksft_test_result_if_softdirty(range_is_not_softdirty(addr, SIZE),
>>                "range is not softdirty\n");
>>
>>       /* Populating WRITE should set softdirty. */
>>       ret = madvise(addr, SIZE, MADV_POPULATE_WRITE);
>> -    ksft_test_result(!ret, "MADV_POPULATE_WRITE\n");
>> -    ksft_test_result(range_is_softdirty(addr, SIZE),
>> +    ksft_test_result_if_softdirty(!ret, "MADV_POPULATE_WRITE\n");
>> +    ksft_test_result_if_softdirty(range_is_softdirty(addr, SIZE),
>>                "range is softdirty\n");
> 
> We probably want to skip the whole test_*softdirty* test instead of adding this
> (IMHO suboptimal) ksft_test_result_if_softdirty.

Yeah I thought about doing it that way, but then the output just looks like
there were fewer tests and they all passed. But thinking about it now, I guess
the TAP header outputs the number of planned tests and the number of tests
executed are fewer, so a machine parser would still notice. I just don't like
that it outputs skipped:0.

But it a lightly held view. Happy to just do:

	if (system_has_softdirty())
		test_softdirty()

If you insist. ;-)

>