Re: [PATCH v2] khugepaged: remove redundant index check for pmd-folios

[Wei Yang]

On Fri, Feb 27, 2026 at 08:05:01PM +0530, Dev Jain wrote:
>Claim: folio_order(folio) == HPAGE_PMD_ORDER => folio->index == start.
>
>Proof: Both loops in hpage_collapse_scan_file and collapse_file, which
>iterate on the xarray, have the invariant that
>start <= folio->index < start + HPAGE_PMD_NR ... (i)
>
>A folio is always naturally aligned in the pagecache, therefore
>folio_order == HPAGE_PMD_ORDER => IS_ALIGNED(folio->index, HPAGE_PMD_NR) == true ... (ii)

This is because __filemap_add_folio() align the index to folio_order(), right?

>
>thp_vma_allowable_order -> thp_vma_suitable_order requires that the virtual
>offsets in the VMA are aligned to the order,
>=> IS_ALIGNED(start, HPAGE_PMD_NR) == true ... (iii)
>
>Combining (i), (ii) and (iii), the claim is proven.
>
>Therefore, remove this check.
>While at it, simplify the comments.
>
>Signed-off-by: Dev Jain <dev.jain@xxxxxxx>

-- 
Wei Yang
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