Re: [PATCH v2 1/2] selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results

[Wei Yang]

On Tue, Sep 02, 2025 at 06:25:35PM +0200, David Hildenbrand wrote:
>When checking for actual tail or head pages of a folio, we must make
>sure that the KPF_COMPOUND_HEAD/KPF_COMPOUND_TAIL flag is paired with
>KPF_THP.
>
>For example, if we have another large folio after our large folio in
>physical memory, our "pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL" would
                                                               ^
One nit here, we missed                                        )

>trigger even though it's actually a head page of the next folio.
>
>If is_backed_by_folio() returns a wrong result, split_pte_mapped_thp()
>can fail with "Some THPs are missing during mremap".
>
>Fix it by checking for head/tail pages of folios properly. Add
>folio_tail_flags/folio_head_flags to improve readability and use these
>masks also when just testing for any compound page.
>
>Fixes: 169b456b0162 ("selftests/mm: reimplement is_backed_by_thp() with more precise check")
>Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Otherwise, 

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>

-- 
Wei Yang
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