Re: [PATCH mm-unstable v14 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Lorenzo Stoakes]

On Thu, Jan 22, 2026 at 12:28:32PM -0700, Nico Pache wrote:
> The current mechanism for determining mTHP collapse scales the
> khugepaged_max_ptes_none value based on the target order. This
> introduces an undesirable feedback loop, or "creep", when max_ptes_none
> is set to a value greater than HPAGE_PMD_NR / 2.
>
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
>
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes:
>
> - max_ptes_none=0: never introduce new none-pages for mTHP collapse.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>   available mTHP order.
>
> This removes the possiblilty of "creep", while not modifying any uAPI
> expectations. A warning will be emitted if any non-supported
> max_ptes_none value is configured with mTHP enabled.
>
> The limits can be ignored by passing full_scan=true, this is useful for
> madvise_collapse (which ignores limits), or in the case of
> collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
> collapse is available.

Thanks, great commit msg!

>
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>

This LGTM in terms of logic, some nits below, with those addressed feel
free to add:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Cheers, Lorenzo

> ---
>  mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0f68902edd9a..9b7e05827749 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -460,6 +460,44 @@ void __khugepaged_enter(struct mm_struct *mm)
>  		wake_up_interruptible(&khugepaged_wait);
>  }
>
> +/**
> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
> + * @order: The folio order being collapsed to
> + * @full_scan: Whether this is a full scan (ignore limits)
> + *
> + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
> + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
> + *
> + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured
> + * khugepaged_max_ptes_none value.
> + *
> + * For mTHP collapses, we currently only support khugepaged_max_pte_none values
> + * of 0 or (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
> + * collapse will be attempted
> + *
> + * Return: Maximum number of empty PTEs allowed for the collapse operation
> + */
> +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> +{
> +	/* ignore max_ptes_none limits */
> +	if (full_scan)
> +		return HPAGE_PMD_NR - 1;

I wonder if, given we are effectively doing:

	const unsigned int nr_pages = collapse_max_ptes_none(order, /*full_scan=*/true);

	...

	foo(nr_pages);

In places where we ignore limits, whether we would be better off putting
HPAGE_PMD_NR - 1 into a define and just using that in these cases, like:

#define COLLAPSE_MAX_PTES_LIM (HPAGE_PMD_NR - 1)

Then instead doing:

	foo(COLLAPSE_MAX_PTES_LIM);

?

Seems somewhat silly to pass in a boolean that makes it return a set value in
cases where you know that should be the case at the outset.

> +
> +	if (is_pmd_order(order))
> +		return khugepaged_max_ptes_none;
> +
> +	/* Zero/non-present collapse disabled. */
> +	if (!khugepaged_max_ptes_none)
> +		return 0;
> +
> +	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)

Having a define for HPAGE_PMD_NR - 1 would also be handy here...

> +		return (1 << order) - 1;
> +
> +	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> +		      HPAGE_PMD_NR - 1);

...and here.

Also a MICRO nit here - the function returns unsigned int and thus we
express PTEs in this unit, so maybe use %u rather than %d?

> +	return -EINVAL;
> +}

Logic of this function looks correct though!

> +
>  void khugepaged_enter_vma(struct vm_area_struct *vma,
>  			  vm_flags_t vm_flags)
>  {
> @@ -548,7 +586,10 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	int none_or_zero = 0, shared = 0, referenced = 0;
>  	enum scan_result result = SCAN_FAIL;
>  	const unsigned long nr_pages = 1UL << order;
> -	int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> +	int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);

Yeah, the !cc->is_khugepaged is a bit gross here, so as per the above, maybe do:

	int max_ptes_none;

	if (cc->is_khugepaged)
		max_ptes_none = collapse_max_ptes_none(order);
	else	/* MADV_COLLAPSE is not limited. */
		max_ptes_none = COLLAPSE_MAX_PTES_LIM;

> +
> +	if (max_ptes_none == -EINVAL)
> +		return result;
>
>  	for (_pte = pte; _pte < pte + nr_pages;
>  	     _pte++, addr += PAGE_SIZE) {
> --
> 2.52.0
>