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

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Message-ID: <80218cc2-de6f-49dc-bdee-4b2560c619b5@arm.com>
Date: Thu, 12 Jun 2025 10:40:38 +0530
From: Dev Jain <dev.jain@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 david@...hat.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....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 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@...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

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()?