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

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Message-ID: <CAMXpfWtM3kMLdkfKpf3CHF1XVk8M18MaZNXHUvwkX7REgQ3ANQ@mail.gmail.com>
Date: Fri, 20 Jun 2025 08:13:17 +0200
From: Mario Casquero <mcasquer@...hat.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, 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

This patch has been successfully tested. All the khugepaged shmem tests passed.

# ./khugepaged khugepaged:shmem
Save THP and khugepaged settings... OK
Allocate huge page on fault... OK
Split huge PMD on MADV_DONTNEED... OK

Run test: collapse_full (khugepaged:shmem)
Collapse multiple fully populated PTE table.... OK

Run test: collapse_single_pte_entry (khugepaged:shmem)
Collapse PTE table with single PTE entry present.... OK

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.... OK
Restore THP and khugepaged settings... OK

Tested-by: Mario Casquero <mcasquer@...hat.com>

On Thu, Jun 12, 2025 at 5:55 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> 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
> --
> 2.43.5
>
>