Re: [PATCH v4 1/4] mm/huge_memory: scale thp_underused() to the folio size

["Zi Yan" <ziy@nvidia.com>]

On Tue Nov 3, 2026 at 4:20 PM EST, Joanne Koong wrote:
> thp_underused() decides whether a large folio on the deferred split list
> is underused and should be split so that its zero-filled subpages can be
> reclaimed.
>
> Today it only ever runs on PMD-sized folios, so it assumes that the
> folio has HPAGE_PMD_NR pages and compares the zero-filled page count
> directly against khugepaged_max_ptes_none. A later patch will queue
> anonymous mTHP folios onto the deferred split list as well, at which
> point that assumption will no longer hold.
>
> Make thp_underused() folio-size-aware, and scale
> khugepaged_max_ptes_none proportionally to the number of pages in the
> folio (eg every folio tolerates the same fraction of zero-filled pages
> the user configured for a PMD-sized folio).
>
> khugepaged deliberately doesn't scale intermediate values when
> collapsing to mTHP orders (to avoid the memory footprint creeping up
> through repeated collapses), but if/when collapse later gains support
> for intermediate values, a folio that khugepaged collapses won't then be
> split right back by the shrinker (and vice versa) since both collapse
> and split would use the same threshold for a given folio size.
>
> There is no functional change for PMD-sized folios, as for those,
> nr_pages is HPAGE_PMD_NR.
>
> Suggested-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  mm/huge_memory.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 14599d0fff01..3720c12bdb0d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4625,25 +4625,33 @@ 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.

It is better to document max_ptes_none is different from what khugepaged does,
especially khugepaged_max_ptes_none is mentioned above.

> +	 */
> +	max_ptes_none = 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;
>  		} else {
>  			/*
>  			 * Another path for early exit once the number
>  			 * of non-zero filled pages exceeds threshold.
>  			 */
> -			if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none)
> +			if (++num_filled_pages >= nr_pages - max_ptes_none)
>  				return false;
>  		}
>  	}

Otherwise, LGTM.

Acked-by: Zi Yan <ziy@nvidia.com>


-- 
Best Regards,
Yan, Zi