[PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs

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

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

Guard PTE markers are installed via MADV_GUARD_INSTALL to create
lightweight guard regions.

Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when
encountering such a range.

MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in
the special marker in __collapse_huge_page_swapin().

hpage_collapse_scan_pmd()
 `- collapse_huge_page()
     `- __collapse_huge_page_swapin() -> fails!

khugepaged's behavior is slightly different due to its max_ptes_swap limit
(default 64). It won't fail as deep, but it will still needlessly scan up
to 64 swap entries before bailing out.

IMHO, we can and should detect this much earlier.

This patch adds a check directly inside the PTE scan loop. If a guard
marker is found, the scan is aborted immediately with SCAN_PTE_NON_PRESENT,
avoiding wasted work.

Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 mm/khugepaged.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9ed1af2b5c38..70ebfc7c1f3e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 					result = SCAN_PTE_UFFD_WP;
 					goto out_unmap;
 				}
+				/*
+				 * Guard PTE markers are installed by
+				 * MADV_GUARD_INSTALL. Any collapse path must
+				 * not touch them, so abort the scan immediately
+				 * if one is found.
+				 */
+				if (is_guard_pte_marker(pteval)) {
+					result = SCAN_PTE_NON_PRESENT;
+					goto out_unmap;
+				}
 				continue;
 			} else {
 				result = SCAN_EXCEED_SWAP_PTE;
-- 
2.49.0