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

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

On 2025/6/12 19:45, David Hildenbrand wrote:
> On 12.06.25 13:37, Baolin Wang wrote:
>>
>>
>> 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.
> 
> Yes, was pleasantly surprised when I found that :)
> 
>>
>>>> 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.
> 
> If we run into the timeout, we return "true" from wait_for_scan(), and 
> in khugepaged_collapse() returns immediately.
> 
> So we wouldn't issue another check_huge() call in khugepaged_collapse().
> 
> Did I miss something?

Ah, right. Sorry for the wrong example. Now I'm fine to drop the 
MADV_NOHUGEPAGE settiing. Thanks.