Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap entries

[Kiryl Shutsemau <kirill@shutemov.name>]

On Wed, Sep 24, 2025 at 06:02:07PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> The existing check in hpage_collapse_scan_pmd() is specific to uffd-wp
> markers. Other special markers (e.g., GUARD, POISONED) would not be caught
> early, leading to failures deeper in the swap-in logic.
> 
> hpage_collapse_scan_pmd()
>  `- collapse_huge_page()
>      `- __collapse_huge_page_swapin() -> fails!
> 
> As David suggested[1], this patch skips any such non-swap entries early.
> If a special marker is found, the scan is aborted immediately with the
> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
> work.
> 
> [1] https://lore.kernel.org/linux-mm/7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com
> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  mm/khugepaged.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7ab2d1a42df3..e9778e7734b5 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1285,16 +1285,19 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  	     _pte++, addr += PAGE_SIZE) {
>  		pte_t pteval = ptep_get(_pte);
>  		if (is_swap_pte(pteval)) {
> +			swp_entry_t swp = pte_to_swp_entry(pteval);
>  			++unmapped;
>  			if (!cc->is_khugepaged ||
>  			    unmapped <= khugepaged_max_ptes_swap) {
>  				/*
> -				 * Always be strict with uffd-wp
> -				 * enabled swap entries.  Please see
> -				 * comment below for pte_uffd_wp().
> +				 * Always be strict with PTE markers, which are
> +				 * special non-swap entries (e.g., for UFFD_WP,
> +				 * POISONED, GUARD). We cannot collapse over
> +				 * them, so just abort the scan here.
>  				 */
> -				if (pte_swp_uffd_wp_any(pteval)) {
> -					result = SCAN_PTE_UFFD_WP;
> +				if (is_pte_marker_entry(swp) &&
> +				    pte_marker_get(swp)) {

Hm. Can we have a marker that have zero pte_marker_get()?

> +					result = SCAN_PTE_NON_PRESENT;
>  					goto out_unmap;
>  				}
>  				continue;
> -- 
> 2.49.0
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov