Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries

[Zi Yan <ziy@nvidia.com>]

On 7 Oct 2025, at 23:26, Lance Yang wrote:

> From: Lance Yang <lance.yang@linux.dev>
>
> Currently, special non-swap entries (like PTE markers) are not caught
> early in hpage_collapse_scan_pmd(), leading to failures deep in the
> swap-in logic.
>
> A function that is called __collapse_huge_page_swapin() and documented
> to "Bring missing pages in from swap" will handle other types as well.
>
> As analyzed by David[1], we could have ended up with the following
> entry types right before do_swap_page():
>
>   (1) Migration entries. We would have waited.
>       -> Maybe worth it to wait, maybe not. We suspect we don't stumble
>          into that frequently such that we don't care. We could always
>          unlock this separately later.
>
>   (2) Device-exclusive entries. We would have converted to non-exclusive.
>       -> See make_device_exclusive(), we cannot tolerate PMD entries and
>          have to split them through FOLL_SPLIT_PMD. As popped up during
>          a recent discussion, collapsing here is actually
>          counter-productive, because the next conversion will PTE-map
>          it again.
>       -> Ok to not collapse.
>
>   (3) Device-private entries. We would have migrated to RAM.
>       -> Device-private still does not support THPs, so collapsing right
>          now just means that the next device access would split the
>          folio again.
>       -> Ok to not collapse.
>
>   (4) HWPoison entries
>       -> Cannot collapse
>
>   (5) Markers
>       -> Cannot collapse
>
> First, this patch adds an early check for these non-swap entries. If
> any one is found, the scan is aborted immediately with the
> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
> work. While at it, convert pte_swp_uffd_wp_any() to pte_swp_uffd_wp()
> since we are in the swap pte branch.
>
> Second, as Wei pointed out[3], we may have a chance to get a non-swap
> entry, since we will drop and re-acquire the mmap lock before
> __collapse_huge_page_swapin(). To handle this, we also add a
> non_swap_entry() check there.
>
> Note that we can unlock later what we really need, and not account it
> towards max_swap_ptes.
>
> [1] https://lore.kernel.org/linux-mm/09eaca7b-9988-41c7-8d6e-4802055b3f1e@redhat.com
> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
> [3] https://lore.kernel.org/linux-mm/20251005010511.ysek2nqojebqngf3@master
>
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> v2 -> v3:
>  - Collect Acked-by from David - thanks!
>  - Collect Reviewed-by from Wei and Dev - thanks!
>  - Add a non_swap_entry() check in __collapse_huge_page_swapin() (per Wei
>    and David) - thanks!
>  - Rework the changelog to incorporate David's detailed analysis of
>    non-swap entry types - thanks!!!
>  - https://lore.kernel.org/linux-mm/20251001032251.85888-1-lance.yang@linux.dev/
>
> v1 -> v2:
>  - Skip all non-present entries except swap entries (per David) thanks!
>  - https://lore.kernel.org/linux-mm/20250924100207.28332-1-lance.yang@linux.dev/
>
>  mm/khugepaged.c | 37 +++++++++++++++++++++++--------------
>  1 file changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index abe54f0043c7..bec3e268dc76 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1020,6 +1020,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>  		if (!is_swap_pte(vmf.orig_pte))
>  			continue;
>
> +		if (non_swap_entry(pte_to_swp_entry(vmf.orig_pte))) {
> +			result = SCAN_PTE_NON_PRESENT;
> +			goto out;
> +		}
> +
>  		vmf.pte = pte;
>  		vmf.ptl = ptl;
>  		ret = do_swap_page(&vmf);
> @@ -1281,7 +1286,23 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>  	     _pte++, addr += PAGE_SIZE) {
>  		pte_t pteval = ptep_get(_pte);
> -		if (is_swap_pte(pteval)) {
> +		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> +			++none_or_zero;
> +			if (!userfaultfd_armed(vma) &&
> +			    (!cc->is_khugepaged ||
> +			     none_or_zero <= khugepaged_max_ptes_none)) {
> +				continue;
> +			} else {
> +				result = SCAN_EXCEED_NONE_PTE;
> +				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> +				goto out_unmap;
> +			}
> +		} else if (!pte_present(pteval)) {
> +			if (non_swap_entry(pte_to_swp_entry(pteval))) {
> +				result = SCAN_PTE_NON_PRESENT;
> +				goto out_unmap;
> +			}
> +
>  			++unmapped;
>  			if (!cc->is_khugepaged ||
>  			    unmapped <= khugepaged_max_ptes_swap) {
> @@ -1290,7 +1311,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  				 * enabled swap entries.  Please see
>  				 * comment below for pte_uffd_wp().
>  				 */
> -				if (pte_swp_uffd_wp_any(pteval)) {
> +				if (pte_swp_uffd_wp(pteval)) {

pte_swp_uffd_wp_any() returns true for both pte_swp_uffd_wp() and
pte_marker_uffd_wp(). Why is it OK to just check pte_swp_uffd_wp() here?

>  					result = SCAN_PTE_UFFD_WP;
>  					goto out_unmap;
>  				}
> @@ -1301,18 +1322,6 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  				goto out_unmap;
>  			}
>  		}
> -		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> -			++none_or_zero;
> -			if (!userfaultfd_armed(vma) &&
> -			    (!cc->is_khugepaged ||
> -			     none_or_zero <= khugepaged_max_ptes_none)) {
> -				continue;
> -			} else {
> -				result = SCAN_EXCEED_NONE_PTE;
> -				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> -				goto out_unmap;
> -			}
> -		}
>  		if (pte_uffd_wp(pteval)) {
>  			/*
>  			 * Don't collapse the page if any of the small
> -- 
> 2.49.0


--
Best Regards,
Yan, Zi