Re: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function

Baolin Wang
2025-12-02T07:53:22Z

On 2025/12/2 01:46, 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.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8dab49c53128..f425238d5d4f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -463,6 +463,44 @@ 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;
> +
> + if (!is_mthp_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)
> + return (1 << order) - 1;
> +
> + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> + HPAGE_PMD_NR - 1);
> + return -EINVAL;
> +}

Thanks. That aligns with what we talked about previously. So
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>