Re: [PATCH 1/2] selftests: khugepaged: fix the shmem collapse failure

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

On 2025/6/12 23:46, Zi Yan wrote:
> On 11 Jun 2025, at 23:54, Baolin Wang wrote:
> 
>> When running the khugepaged selftest for shmem (./khugepaged all:shmem),
>> I encountered the following test failures:
>> "
>> Run test: collapse_full (khugepaged:shmem)
>> Collapse multiple fully populated PTE table.... Fail
>> ...
>> Run test: collapse_single_pte_entry (khugepaged:shmem)
>> Collapse PTE table with single PTE entry present.... Fail
>> ...
>> Run test: collapse_full_of_compound (khugepaged:shmem)
>> Allocate huge page... OK
>> Split huge page leaving single PTE page table full of compound pages... OK
>> Collapse PTE table full of compound pages.... Fail
>> "
>>
>> The reason for the failure is that, it will set MADV_NOHUGEPAGE to prevent
>> khugepaged from continuing to scan shmem VMA after khugepaged finishes
>> scanning in the wait_for_scan() function. Moreover, shmem requires a refault
>> to establish PMD mappings.
>>
>> However, after commit 2b0f922323cc, PMD mappings are prevented if the VMA is
> 
> Can you add the title of the commit? It is easier to understand the context.
> 
> 2b0f922323cc ("mm: don't install PMD mappings when THPs are disabled by the hw/process/vma")

Sure.

> 
>> set with MADV_NOHUGEPAGE flag, so shmem cannot establish PMD mappings during
>> refault.
>>
>> To fix this issue, we can set the MADV_NOHUGEPAGE flag after the shmem refault.
>> With this fix, the shmem test case passes.
>>
>> Fixes: 2b0f922323cc ("mm: don't install PMD mappings when THPs are disabled by the hw/process/vma")
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   tools/testing/selftests/mm/khugepaged.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
>> index 8a4d34cce36b..d462f62d8116 100644
>> --- a/tools/testing/selftests/mm/khugepaged.c
>> +++ b/tools/testing/selftests/mm/khugepaged.c
>> @@ -561,8 +561,6 @@ static bool wait_for_scan(const char *msg, char *p, int nr_hpages,
>>   		usleep(TICK);
>>   	}
>>
>> -	madvise(p, nr_hpages * hpage_pmd_size, MADV_NOHUGEPAGE);
>> -
>>   	return timeout == -1;
>>   }
>>
> I assume you are going to just remove this madvise based on your discussion
> with David. With that, feel free to add Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks.