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

[Dev Jain <dev.jain@arm.com>]

On 20/02/26 4:03 am, Andrew Morton wrote:
> 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?

But then shouldn't I remove it just now :) try_collapse_pte_mapped_thp is
anways going to bail out if such a bug occurs, so nothing goes wrong. In
short this check is actually useless, apart from the fact that this lets
us spot a bug (which I doubt won't be figured out by other code paths already).