mm/khugepaged: abort collapse scan on non-swap entries

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
Link: https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
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>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index abe54f0043c730e7eb8459e108205d75f018fbf5..f4f57ba69d72e5df28133a42bbd93f47f1e2b87e 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1281,7 +1281,23 @@ 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 +1306,7 @@ 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;
                                        goto out_unmap;
                                }
@@ -1301,18 +1317,6 @@ 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