Re: [PATCH v2 1/2] selftests: khugepaged: fix the shmem collapse failure

[David Hildenbrand <david@redhat.com>]

On 13.06.25 03:49, 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 ("mm: don't install PMD mappings when
> THPs are disabled by the hw/process/vma"), PMD mappings are prevented if the
> VMA is set with MADV_NOHUGEPAGE flag, so shmem cannot establish PMD mappings
> during refault.
> 
> One way to fix this issue is to move the MADV_NOHUGEPAGE setting after the
> shmem refault. After shmem refault and check huge, the test case will unmap
> the shmem immediately. So it seems unnecessary to set the MADV_NOHUGEPAGE.
> 
> Then we can simply drop the MADV_NOHUGEPAGE setting, and all khugepaged test
> cases passed.
> 
> Fixes: 2b0f922323cc ("mm: don't install PMD mappings when THPs are disabled by the hw/process/vma")
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb