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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/15/26 16:47, Dev Jain wrote:
> 
> 
> 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.

Yes, because there could be other reasons why MADV_PAGEOUT would not be
successful (speculative page references, out of swap space).

> 
> 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!" .

I would not pack a "let's test if swapping in the system even works" into a
khugeapged test.

I think we can reasonably assume here "swap subsystem is not completely broken".

And if we want to check for that, have a separate test that stresses swapping,
if available.

-- 
Cheers,

David