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

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

On 2025/6/12 18:08, David Hildenbrand wrote:
> On 12.06.25 05:54, Baolin Wang wrote:
>> When running the khugepaged selftest for shmem (./khugepaged all:shmem),
> 
> Hmm, this combination is not run automatically through run_tests.sh, 
> right? IIUC, it only runs "./khugepaged" which tests anon only ...
> 
> Should we add it there? Then I would probably have noticed that myself 
> earlier :)

Yes, see patch 2.

>> 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
>> set with MADV_NOHUGEPAGE flag, so shmem cannot establish PMD mappings 
>> during
>> refault.
> 
> Right. It's always problematic when we have some contradicting 
> information in the VMA vs. pagecache.
> 
>>
>> 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;
>>   }
>> @@ -585,6 +583,7 @@ static void khugepaged_collapse(const char *msg, 
>> char *p, int nr_hpages,
>>       if (ops != &__anon_ops)
>>           ops->fault(p, 0, nr_hpages * hpage_pmd_size);
>> +    madvise(p, nr_hpages * hpage_pmd_size, MADV_NOHUGEPAGE);
>>       if (ops->check_huge(p, expect ? nr_hpages : 0))
>>           success("OK");
>>       else
> 
> It's a shame we have this weird interface: there is no way we can clear 
> VM_HUGEPAGE without setting VM_NOHUGEPAGE :(

Right.

> But, do we even care about setting MADV_NOHUGEPAGE at all? IIUC, we'll 
> almost immediately later call cleanup_area() where we munmap(), right?

I tested removing the MADV_NOHUGEPAGE setting, and the khugepaged test 
cases all passed.

However, a potential impact of removing MADV_NOHUGEPAGE is that, 
khugepaged might report 'timeout', but check_huge() would still report 
'success' (assuming khugepaged tries to scan the VMA and successfully 
collapses it after the timeout). Such test result could be confusing.

I know this kind of case is very rare and may not even be detectable in 
tests. At least, I couldn't reproduce it. But I prefer to keep the 
original test logic and fix the issue without making further changes.