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

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Message-ID: <AFC17CA1-DF5A-48F0-8E63-E139005F5880@nvidia.com>
Date: Thu, 12 Jun 2025 11:46:59 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, shuah@...nel.org, 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 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")

> 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@...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;
>  }
>
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@...dia.com>

Thanks.

Best Regards,
Yan, Zi