Re: [PATCH v3 1/3] mm/huge_memory: scale thp_underused() to the folio size

[Lance Yang <lance.yang@linux.dev>]

On Sat, Oct 24, 2026 at 11:01:50PM +0000, sashiko-bot@kernel.org wrote:
>Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>- [Medium] mm/huge_memory: wrong rounding in thp_underused() max_ptes_none scaling
>--
>
>commit 56eb0cfccba9e5e4276f2a8250815399987bc848
>Author: Joanne Koong <joannelkoong@gmail.com>
>
>mm/huge_memory: scale thp_underused() to the folio size
>
>This commit updates thp_underused() to scale khugepaged_max_ptes_none
>proportionally to the number of pages in the folio rather than assuming
>the folio has HPAGE_PMD_NR pages.
>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index bd44d58e86845..218259940f24d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4613,25 +4613,35 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>>  
>>  static bool thp_underused(struct folio *folio)
>>  {
>> -	int num_zero_pages = 0, num_filled_pages = 0;
>> -	int i;
>> +	const unsigned long nr_pages = folio_nr_pages(folio);
>> +	unsigned long num_zero_pages = 0, num_filled_pages = 0;
>> +	unsigned long max_ptes_none = khugepaged_max_ptes_none;
>> +	unsigned long i;
>>  
>> -	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
>> +	if (max_ptes_none == HPAGE_PMD_NR - 1)
>>  		return false;
>>  
>>  	if (folio_contain_hwpoisoned_page(folio))
>>  		return false;
>>  
>> -	for (i = 0; i < folio_nr_pages(folio); i++) {
>> +	/*
>> +	 * khugepaged_max_ptes_none is a number of pages out of HPAGE_PMD_NR.
>> +	 * Scale it to this folio's size (rounding up so that the folio is never
>> +	 * split when fewer than the configured fraction of its pages are
>> +	 * zero-filled).
>> +	 */
>> +	max_ptes_none = DIV_ROUND_UP(max_ptes_none * nr_pages, HPAGE_PMD_NR);
>> +
>> +	for (i = 0; i < nr_pages; i++) {
>>  		if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {
>> -			if (++num_zero_pages > khugepaged_max_ptes_none)
>> +			if (++num_zero_pages > max_ptes_none)
>>  				return true;
>
>[Severity: Medium]
>Should max_ptes_none in thp_underused() be scaled with truncating integer
>division rather than DIV_ROUND_UP()?

Agreed, we already use a strict > comparison, so truncating division
is enough here.

[...]

Cheers, Lance