Re: [PATCH] khugepaged: convert redundant check to WARN_ON

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2/19/26 1:48 PM, 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)
> 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, convert this to a VM_WARN_ON.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---

Make sense to me. Personally, I’d like to keep this VM_WARN_ON() to 
catch unexpected behavior.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> Based on mm-unstable (d9982f38eb6e). mm-selftests pass.
> 
>   mm/khugepaged.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index fa1e57fd2c469..f27cbb4d1f62c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2000,8 +2000,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>   		 * we locked the first folio, then a THP might be there already.
>   		 * This will be discovered on the first iteration.
>   		 */
> -		if (folio_order(folio) == HPAGE_PMD_ORDER &&
> -		    folio->index == start) {
> +		if (folio_order(folio) == HPAGE_PMD_ORDER) {
> +			VM_WARN_ON(folio->index != start);
> +
>   			/* Maybe PMD-mapped */
>   			result = SCAN_PTE_MAPPED_HUGEPAGE;
>   			goto out_unlock;
> @@ -2329,8 +2330,9 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>   			continue;
>   		}
>   
> -		if (folio_order(folio) == HPAGE_PMD_ORDER &&
> -		    folio->index == start) {
> +		if (folio_order(folio) == HPAGE_PMD_ORDER) {
> +			VM_WARN_ON(folio->index != start);
> +
>   			/* Maybe PMD-mapped */
>   			result = SCAN_PTE_MAPPED_HUGEPAGE;
>   			/*