linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <a1df234c-d003-4696-8f1f-609a360fdeda@arm.com>
Date: Wed, 1 Oct 2025 15:50:37 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: david@...hat.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 baohua@...nel.org, baolin.wang@...ux.alibaba.com, hughd@...gle.com,
 ioworker0@...il.com, kirill@...temov.name, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mpenttil@...hat.com, npache@...hat.com,
 ryan.roberts@....com, ziy@...dia.com, richard.weiyang@...il.com
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries


On 01/10/25 8:52 am, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> Currently, special non-swap entries (like migration, hwpoison, or PTE
> markers) are not caught early in hpage_collapse_scan_pmd(), leading to
> failures deep 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 any one 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@...hat.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.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 | 32 ++++++++++++++++++--------------
>   1 file changed, 18 insertions(+), 14 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7ab2d1a42df3..d0957648db19 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1284,7 +1284,23 @@ 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 you are trying to merge this with the _isolate() conditions, we can do
a micro-optimization here - is_swap_pte, (pte_none && is_zero_pfn), and pte_uffd_wp
are disjoint conditions, so we can use if-else-if-else-if to write them.

> +			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) {
> @@ -1293,7 +1309,7 @@ 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)) {
>   					result = SCAN_PTE_UFFD_WP;

Could have mentioned in the changelog "while at it, convert pte_swp_uffd_wp_any to
pte_swp_uffd_wp since we are in the swap pte branch".

>   					goto out_unmap;
>   				}
> @@ -1304,18 +1320,6 @@ 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

Otherwise LGTM

Reviewed-by: Dev Jain <dev.jain@....com>