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

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Oct 01, 2025 at 11:22:51AM +0800, Lance Yang wrote:
>From: Lance Yang <lance.yang@linux.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@redhat.com>
>Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>Signed-off-by: Lance Yang <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 | 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)) {

It looks is_swap_pte() is mis-leading?

>+		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) {
>@@ -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)) {

I am not sure why we want to change this. There is no description in the
change log.

Would you mind giving some hint on this?

> 					result = SCAN_PTE_UFFD_WP;
> 					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
>-- 
>2.49.0

-- 
Wei Yang
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