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

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Message-ID: <6ceb38ce-c16d-48f2-baca-fef79f8fc058@redhat.com>
Date: Thu, 12 Jun 2025 13:45:56 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org, shuah@...nel.org,
 ziy@...dia.com, linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] selftests: khugepaged: fix the shmem collapse failure

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@...ux.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?


-- 
Cheers,

David / dhildenb