Re: [PATCH v2 2/2] kselftest: mm: fix intermittent failure khugepaged test

[David Hildenbrand (Arm)]

On 9/23/26 12:54, Yeoreum Yun wrote:
>>
>>
>> On 9/21/26 6:35 PM, Yeoreum Yun wrote:
>>> There are intermittent failures in collapse_max_ptes_swap() and
>>> collapse_max_ptes_shared() when using the khugepaged_context:
>>>
>>>    // while running ./khugepaged -s 2
>>>
>>>    # Run test: collapse_max_ptes_shared (khugepaged:anon)
>>>    # Allocate huge page... OK
>>>    # Share huge page over fork()... OK
>>>    # Trigger CoW on page 1023 of 2048... OK
>>>    # Maybe collapse with max_ptes_shared exceeded.... OK
>>>    # Trigger CoW on page 1024 of 2048... Fail
>>>    Bail out! Unexpected huge page
>>>    # Planned tests != run tests (26 != 23)
>>>    # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0
>>>
>>>    # Run test: collapse_max_ptes_swap (khugepaged:anon)
>>>    # Swapout 257 of 2048 pages... OK
>>>    # Maybe collapse with max_ptes_swap exceeded.... OK
>>>    # Swapout 256 of 2048 pages... OK
>>>    Bail out! Unexpected huge page
>>>    # Planned tests != run tests (26 != 17)
>>>    # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0
>>>
>>> This happens because khugepaged may collapse the pages before wait_for_scan()
>>> is called, causing a sanity check that expects uncollapsed pages to fail.
>>>
>>> For example, in collapse_max_ptes_swap(), after faulting the pages back in
>>> and paging out up to max_ptes_swap pages, khugepaged may collapse them again
>>> before c->collapse() is called.
>>>
>>> To prevent this, mark the VMA with MADV_NOHUGEPAGE after it has been
>>> collapsed by wait_for_scan() for anon. This prevents khugepaged from
>>> collapsing it again before c->collapse() is called.
>>>
>>> This failure was observed on NVIDIA Spark with 16KB page.
>>>
>>> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
>>> ---
>>>   tools/testing/selftests/mm/khugepaged.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
>>> index c32244b565658..1aad4bb427ece 100644
>>> --- a/tools/testing/selftests/mm/khugepaged.c
>>> +++ b/tools/testing/selftests/mm/khugepaged.c
>>> @@ -578,6 +578,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>>>   		usleep(TICK);
>>>   	}
>>> +	if (!strncmp(ops->name, "anon", 4))
>>
>> We usually use the 'if (ops == &__anon_ops)' check in this file to identify
>> anonymous test cases.
>>
>> With that, LGTM.
>> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> 
> I think this is merged at mm/mm-unstable already So, I'll send
> another patch to change it with is_anon().

Unless it is in mm-stable we can reasonably squash stuff or resend. This
shouldn't be an independent patch.

-- 
Cheers,

David