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

[David Hildenbrand <david@redhat.com>]

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 :)

> 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 :(

But, do we even care about setting MADV_NOHUGEPAGE at all? IIUC, we'll 
almost immediately later call cleanup_area() where we munmap(), right?

-- 
Cheers,

David / dhildenb