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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 Feb 2026 11:18:27 +0530 Dev Jain <dev.jain@arm.com> 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.
> 
> ...
>
> --- 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);

It's a bad sad to remove unneeded code by retaining that code and
adding even more code.

Perhaps add a comment reminding us to remove this altogether at a later
date?