Re: [PATCH v3 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/29/26 4:38 PM, David Hildenbrand (Arm) wrote:
> On 9/23/26 17:29, 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.
>>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
>> ---
>>   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 2aa7c9197158..b0cb02bf1a73 100644
>> --- a/tools/testing/selftests/mm/khugepaged.c
>> +++ b/tools/testing/selftests/mm/khugepaged.c
>> @@ -618,6 +618,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>>   		usleep(TICK);
>>   	}
>>   
>> +	if (is_anon(ops))
>> +		madvise(p, len, MADV_NOHUGEPAGE);
>> +
> 
> Any reason we just do that unconditionally?

Although it's a bit messy, as I mentioned before [1], unconditionally 
setting MADV_NOHUGEPAGE will break shmem testing. Maybe add some comments.

[1] 
https://lore.kernel.org/all/24690e82-3aab-4f2d-95a7-3bba332ca5bc@linux.alibaba.com/