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

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

The existing check in hpage_collapse_scan_pmd() is specific to uffd-wp
markers. Other special markers (e.g., GUARD, POISONED) would not be caught
early, leading to failures deeper 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 a special marker 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>
---
 mm/khugepaged.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 7ab2d1a42df3..e9778e7734b5 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1285,16 +1285,19 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 	     _pte++, addr += PAGE_SIZE) {
 		pte_t pteval = ptep_get(_pte);
 		if (is_swap_pte(pteval)) {
+			swp_entry_t swp = pte_to_swp_entry(pteval);
 			++unmapped;
 			if (!cc->is_khugepaged ||
 			    unmapped <= khugepaged_max_ptes_swap) {
 				/*
-				 * Always be strict with uffd-wp
-				 * enabled swap entries.  Please see
-				 * comment below for pte_uffd_wp().
+				 * Always be strict with PTE markers, which are
+				 * special non-swap entries (e.g., for UFFD_WP,
+				 * POISONED, GUARD). We cannot collapse over
+				 * them, so just abort the scan here.
 				 */
-				if (pte_swp_uffd_wp_any(pteval)) {
-					result = SCAN_PTE_UFFD_WP;
+				if (is_pte_marker_entry(swp) &&
+				    pte_marker_get(swp)) {
+					result = SCAN_PTE_NON_PRESENT;
 					goto out_unmap;
 				}
 				continue;
-- 
2.49.0