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

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

On 2025/6/12 13:10, Dev Jain wrote:
> 
> On 12/06/25 9:24 am, 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
>> 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;
>>   }
>> @@ -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
> 
> The idea looks sane to me, but do we need to add the madvise call to
> madvise_retracted_page_tables() too, since that also calls wait_for_scan()?

Yes, I also realized this after sending the patches:) Thanks.

To keep the original logic:

diff --git a/tools/testing/selftests/mm/khugepaged.c 
b/tools/testing/selftests/mm/khugepaged.c
index d462f62d8116..3452763e2fe3 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1074,6 +1074,7 @@ static void madvise_retracted_page_tables(struct 
collapse_context *c,
                 return;
         }
         success("OK");
+       madvise(p, nr_hpages * hpage_pmd_size, MADV_NOHUGEPAGE);
         c->collapse("Install huge PMD from page cache", p, nr_hpages, ops,
                     true);
         validate_memory(p, 0, size);