Re: [PATCH RESEND 1/1] selftests/mm: skip soft-dirty tests when CONFIG_MEM_SOFT_DIRTY is disabled

[Lance Yang <lance.yang@linux.dev>]

On 2025/9/17 18:51, David Hildenbrand wrote:
> On 17.09.25 07:59, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> The madv_populate and soft-dirty kselftests currently fail on systems 
>> where
>> CONFIG_MEM_SOFT_DIRTY is disabled.
>>
>> Introduce a new helper softdirty_is_supported() into vm_util.c/h to 
>> ensure
>> tests are properly skipped when the feature is not enabled.
> 
> I'll note that tools/testing/selftests/mm/config contains
> 
>      CONFIG_MEM_SOFT_DIRTY=y
> 
> But yes, the current arm64 handling is nasty, because some other archs 
> (e.g., riscv) also don't support it yet.

Yep.

> 
> LGTM, some nits below:

Thanks for taking time to review!

> 
>>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   tools/testing/selftests/mm/madv_populate.c | 21 ++--------------
>>   tools/testing/selftests/mm/soft-dirty.c    |  5 +++-
>>   tools/testing/selftests/mm/vm_util.c       | 28 ++++++++++++++++++++++
>>   tools/testing/selftests/mm/vm_util.h       |  1 +
>>   4 files changed, 35 insertions(+), 20 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/madv_populate.c b/tools/ 
>> testing/selftests/mm/madv_populate.c
>> index b6fabd5c27ed..43dac7783004 100644
>> --- a/tools/testing/selftests/mm/madv_populate.c
>> +++ b/tools/testing/selftests/mm/madv_populate.c
>> @@ -264,23 +264,6 @@ static void test_softdirty(void)
>>       munmap(addr, SIZE);
>>   }
>> -static int system_has_softdirty(void)
>> -{
>> -    /*
>> -     * There is no way to check if the kernel supports soft-dirty, other
>> -     * than by writing to a page and seeing if the bit was set. But the
>> -     * tests are intended to check that the bit gets set when it 
>> should, so
>> -     * doing that check would turn a potentially legitimate fail into a
>> -     * skip. Fortunately, we know for sure that arm64 does not support
>> -     * soft-dirty. So for now, let's just use the arch as a corse guide.
>> -     */
>> -#if defined(__aarch64__)
>> -    return 0;
>> -#else
>> -    return 1;
>> -#endif
>> -}
>> -
>>   int main(int argc, char **argv)
>>   {
>>       int nr_tests = 16;
>> @@ -288,7 +271,7 @@ int main(int argc, char **argv)
>>       pagesize = getpagesize();
>> -    if (system_has_softdirty())
>> +    if (softdirty_is_supported())
>>           nr_tests += 5;
>>       ksft_print_header();
>> @@ -300,7 +283,7 @@ int main(int argc, char **argv)
>>       test_holes();
>>       test_populate_read();
>>       test_populate_write();
>> -    if (system_has_softdirty())
>> +    if (softdirty_is_supported())
>>           test_softdirty();
>>       err = ksft_get_fail_cnt();
>> diff --git a/tools/testing/selftests/mm/soft-dirty.c b/tools/testing/ 
>> selftests/mm/soft-dirty.c
>> index 8a3f2b4b2186..98e42d2ac32a 100644
>> --- a/tools/testing/selftests/mm/soft-dirty.c
>> +++ b/tools/testing/selftests/mm/soft-dirty.c
>> @@ -200,8 +200,11 @@ int main(int argc, char **argv)
>>       int pagesize;
>>       ksft_print_header();
>> -    ksft_set_plan(15);
>> +    if (!softdirty_is_supported())
>> +        ksft_exit_skip("soft-dirty is not support\n");
>> +
>> +    ksft_set_plan(15);
>>       pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
>>       if (pagemap_fd < 0)
>>           ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
>> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/ 
>> selftests/mm/vm_util.c
>> index 56e9bd541edd..3173335df775 100644
>> --- a/tools/testing/selftests/mm/vm_util.c
>> +++ b/tools/testing/selftests/mm/vm_util.c
>> @@ -449,6 +449,34 @@ bool check_vmflag_pfnmap(void *addr)
>>       return check_vmflag(addr, "pf");
>>   }
>> +bool softdirty_is_supported(void)
> 
> I'd just call it "softdirty_supported" similar to 
> "pagemap_scan_supported()".

Got it.

> 
>> +{
>> +    char *addr;
>> +    int ret = 0;
> 
> bool supported = false;
> 
>> +    size_t pagesize;
>> +
>> +    /* We know for sure that arm64 does not support soft-dirty. */
>> +#if defined(__aarch64__)
>> +    return ret;
>> +#endif
> 
> Just drop this arm special case now.

OK

> 
>> +    pagesize = getpagesize();
> 
> const size_t pagesize = getpagesize();
> 
>> +    /*
>> +     * __mmap_complete() always sets VM_SOFTDIRTY for new VMAs, so we
>> +     * just mmap a small region and check its VmFlags in /proc/self/ 
>> smaps
>> +     * for the "sd" flag.
>> +     */
> 
> /* New mappings are expected to be marked with VM_SOFTDIRTY (sd). */

Cool. Much better!

> 
>> +    addr = mmap(0, pagesize, PROT_READ | PROT_WRITE,
>> +            MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
>> +    if (!addr)
>> +        ksft_exit_fail_msg("mmap failed\n");
>> +
>> +    if (check_vmflag(addr, "sd"))
>> +        ret = 1;
> 
> supported = true;

I'll adjust as you suggested ;)

> 
>> +
>> +    munmap(addr, pagesize);
>> +    return ret;
>> +}
>> +
> 

Cheers,
Lance