Re: [PATCH mm-unstable v1 3/5] mm/khugepaged: define COLLAPSE_MAX_PTES_LIMIT as HPAGE_PMD_NR - 1

[Vernon Yang]

On Wed, Feb 11, 2026 at 07:18:33PM -0700, Nico Pache wrote:
> The value (HPAGE_PMD_NR - 1) is used often in the khugepaged code to
> signify the limit of the max_ptes_* values. Add a define for this to
> increase code readability and reuse.
>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c362b3b2e08a..3dcce6018f20 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -85,6 +85,7 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
>   *
>   * Note that these are only respected if collapse was initiated by khugepaged.
>   */
> +#define COLLAPSE_MAX_PTES_LIMIT (HPAGE_PMD_NR - 1)
>  unsigned int khugepaged_max_ptes_none __read_mostly;
>  static unsigned int khugepaged_max_ptes_swap __read_mostly;
>  static unsigned int khugepaged_max_ptes_shared __read_mostly;
> @@ -252,7 +253,7 @@ static ssize_t max_ptes_none_store(struct kobject *kobj,
>  	unsigned long max_ptes_none;
>
>  	err = kstrtoul(buf, 10, &max_ptes_none);
> -	if (err || max_ptes_none > HPAGE_PMD_NR - 1)
> +	if (err || max_ptes_none > COLLAPSE_MAX_PTES_LIMIT)
>  		return -EINVAL;
>
>  	khugepaged_max_ptes_none = max_ptes_none;
> @@ -277,7 +278,7 @@ static ssize_t max_ptes_swap_store(struct kobject *kobj,
>  	unsigned long max_ptes_swap;
>
>  	err  = kstrtoul(buf, 10, &max_ptes_swap);
> -	if (err || max_ptes_swap > HPAGE_PMD_NR - 1)
> +	if (err || max_ptes_swap > COLLAPSE_MAX_PTES_LIMIT)
>  		return -EINVAL;
>
>  	khugepaged_max_ptes_swap = max_ptes_swap;
> @@ -303,7 +304,7 @@ static ssize_t max_ptes_shared_store(struct kobject *kobj,
>  	unsigned long max_ptes_shared;
>
>  	err  = kstrtoul(buf, 10, &max_ptes_shared);
> -	if (err || max_ptes_shared > HPAGE_PMD_NR - 1)
> +	if (err || max_ptes_shared > COLLAPSE_MAX_PTES_LIMIT)
>  		return -EINVAL;
>
>  	khugepaged_max_ptes_shared = max_ptes_shared;
> @@ -384,7 +385,7 @@ int __init khugepaged_init(void)
>  		return -ENOMEM;
>
>  	khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
> -	khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
> +	khugepaged_max_ptes_none = COLLAPSE_MAX_PTES_LIMIT;
>  	khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;
>  	khugepaged_max_ptes_shared = HPAGE_PMD_NR / 2;
>
> @@ -1869,7 +1870,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	bool is_shmem = shmem_file(file);
>
>  	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> -	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> +	VM_BUG_ON(start & (COLLAPSE_MAX_PTES_LIMIT));
>
>  	result = alloc_charge_folio(&new_folio, mm, cc);
>  	if (result != SCAN_SUCCEED)
> @@ -2209,7 +2210,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	 * unwritten page.
>  	 */
>  	folio_mark_uptodate(new_folio);
> -	folio_ref_add(new_folio, HPAGE_PMD_NR - 1);
> +	folio_ref_add(new_folio, COLLAPSE_MAX_PTES_LIMIT);

Hi Nico,

Thank you for cleanup.

The readability is better in other places, but here it confuses me.
Why is it LIMIT? I had to look at the implementation code.
Would COLLAPSE_MAX_PTES be better?

>  	if (is_shmem)
>  		folio_mark_dirty(new_folio);
> @@ -2303,7 +2304,7 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>  	memset(cc->node_load, 0, sizeof(cc->node_load));
>  	nodes_clear(cc->alloc_nmask);
>  	rcu_read_lock();
> -	xas_for_each(&xas, folio, start + HPAGE_PMD_NR - 1) {
> +	xas_for_each(&xas, folio, start + COLLAPSE_MAX_PTES_LIMIT) {
>  		if (xas_retry(&xas, folio))
>  			continue;
>
> --
> 2.53.0
>
>

--
Cheers,
Vernon