Re: [PATCH mm-new 3/3] mm/khugepaged: abort collapse scan on guard PTEs

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Sun, Sep 14, 2025 at 10:35:47PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
>
> Guard PTE markers are installed via MADV_GUARD_INSTALL to create
> lightweight guard regions.
>
> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when
> encountering such a range.
>
> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in
> the special marker in __collapse_huge_page_swapin().
>
> hpage_collapse_scan_pmd()
>  `- collapse_huge_page()
>      `- __collapse_huge_page_swapin() -> fails!
>
> khugepaged's behavior is slightly different due to its max_ptes_swap limit
> (default 64). It won't fail as deep, but it will still needlessly scan up
> to 64 swap entries before bailing out.
>
> IMHO, we can and should detect this much earlier ;)

No smileys in commit messages please... :)

>
> This patch adds a check directly inside the PTE scan loop. If a guard
> marker is found, the scan is aborted immediately with a new SCAN_PTE_GUARD
> status, avoiding wasted work.
>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  mm/khugepaged.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e54f99bb0b57..910a6f2ec8a9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -59,6 +59,7 @@ enum scan_result {
>  	SCAN_STORE_FAILED,
>  	SCAN_COPY_MC,
>  	SCAN_PAGE_FILLED,
> +	SCAN_PTE_GUARD,

I wonder if we really need to have it literally called out though, can we just
use:

SCAN_PTE_NON_PRESENT

Instead?

As it is, indeed, non-present :)

>  };
>
>  #define CREATE_TRACE_POINTS
> @@ -1317,6 +1318,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  					result = SCAN_PTE_UFFD_WP;
>  					goto out_unmap;
>  				}
> +				/*
> +				 * Guard PTE markers are installed by
> +				 * MADV_GUARD_INSTALL. Any collapse path must
> +				 * not touch them, so abort the scan immediately
> +				 * if one is found.
> +				 */
> +				if (is_guard_pte_marker(pteval)) {
> +					result = SCAN_PTE_GUARD;
> +					goto out_unmap;
> +				}
>  				continue;
>  			} else {
>  				result = SCAN_EXCEED_SWAP_PTE;
> @@ -2860,6 +2871,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>  		case SCAN_PAGE_COMPOUND:
>  		case SCAN_PAGE_LRU:
>  		case SCAN_DEL_PAGE_LRU:
> +		case SCAN_PTE_GUARD:
>  			last_fail = result;
>  			break;
>  		default:
> --
> 2.49.0
>

Cheers, Lorenzo