Re: [PATCH] selftests/mm: khugepaged: skip swap tests when swap is unavailable

[Dev Jain <dev.jain@arm.com>]

On 15/07/26 7:39 pm, David Hildenbrand (Arm) wrote:
> On 7/15/26 15:34, Jaeyeon Lee wrote:
>> The collapse_swapin_single_pte and collapse_max_ptes_swap tests use
>> MADV_PAGEOUT to swap out pages before verifying khugepaged collapse
>> behavior. On systems without swap configured, MADV_PAGEOUT succeeds
>> but the pages are not actually swapped out, so check_swap() returns
>> false and the tests report a failure.
>>
>> These tests cannot run meaningfully without swap, so a missing swap
>> area is an environment limitation rather than a test failure. Detect
>> the absence of swap via /proc/swaps and skip the affected tests
>> instead of failing them.
>>
>> Signed-off-by: Jaeyeon Lee <jaeyeon.lee.dev@gmail.com>
>> ---
>>  tools/testing/selftests/mm/khugepaged.c | 25 +++++++++++++++++++++++++
>>  1 file changed, 25 insertions(+)
>>
>> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
>> index 10e8dedcb087..f9b4ebe733ce 100644
>> --- a/tools/testing/selftests/mm/khugepaged.c
>> +++ b/tools/testing/selftests/mm/khugepaged.c
>> @@ -100,6 +100,22 @@ static void skip(const char *msg)
>>  	exit_status = KSFT_SKIP;
>>  }
>>  
>> +static bool swap_available(void)
>> +{
>> +	char buf[256];
>> +	bool ret;
>> +	FILE *fp = fopen("/proc/swaps", "r");
>> +
>> +	if (!fp)
>> +		return false;
>> +
>> +	fgets(buf, sizeof(buf), fp);
>> +	ret = !!fgets(buf, sizeof(buf), fp);
>> +
>> +	fclose(fp);
>> +	return ret;
>> +}
>> +
> 
> That doesn't mean that swapout will succeed.
> 
> Take a look at cow.c where we use pagemap_is_swapped() after MADV_PAGEOUT.

That suffers from the reverse problem - in case of a real swap bug, cow.c
will skip the test and anyone running the test won't bother.

I think this patch idea is reasonable, we need a way to say "swap definitely
won't succeed because there is no swap device, so skip" vs "swap failed,
need to dig in!" .

>