+ mm-khugepaged-abort-collapse-scan-on-non-swap-entries.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: mm/khugepaged: abort collapse scan on non-swap entries
has been added to the -mm mm-new branch.  Its filename is
     mm-khugepaged-abort-collapse-scan-on-non-swap-entries.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-khugepaged-abort-collapse-scan-on-non-swap-entries.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
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fixup patches in mm-new.

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------------------------------------------------------
From: Lance Yang <lance.yang@linux.dev>
Subject: mm/khugepaged: abort collapse scan on non-swap entries
Date: Wed, 1 Oct 2025 11:22:51 +0800

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.


Link: https://lkml.kernel.org/r/20251001032251.85888-1-lance.yang@linux.dev
Link: https://lore.kernel.org/linux-mm/7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com [1]
Link: https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local [2]
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Mika Penttilä <mpenttil@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

--- a/mm/khugepaged.c~mm-khugepaged-abort-collapse-scan-on-non-swap-entries
+++ a/mm/khugepaged.c
@@ -1281,7 +1281,23 @@ static int hpage_collapse_scan_pmd(struc
 	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 +1306,7 @@ static int hpage_collapse_scan_pmd(struc
 				 * 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;
 					goto out_unmap;
 				}
@@ -1301,18 +1317,6 @@ static int hpage_collapse_scan_pmd(struc
 				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
_

Patches currently in -mm which might be from lance.yang@linux.dev are

hung_task-fix-warnings-caused-by-unaligned-lock-pointers.patch
mm-thp-fix-mte-tag-mismatch-when-replacing-zero-filled-subpages.patch
mm-rmap-fix-soft-dirty-and-uffd-wp-bit-loss-when-remapping-zero-filled-mthp-subpage-to-shared-zeropage.patch
mm-clean-up-is_guard_pte_marker.patch
mm-khugepaged-abort-collapse-scan-on-non-swap-entries.patch