[RFC v2 9/9] khugepaged: skip collapsing mTHP to smaller orders

khugepaged may try to collapse a mTHP to a smaller mTHP, resulting in
some pages being unmapped. Skip these cases until we have a way to check
if its ok to collapse to a smaller mTHP size (like in the case of a
partially mapped folio).

This patch is inspired by Dev Jain's work on khugepaged mTHP support [1].

[1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@arm.com/ (local)

Signed-off-by: Nico Pache <redacted>
---
 mm/khugepaged.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e2ba18e57064..fc30698b8e6e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -622,6 +622,11 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		folio = page_folio(page);
 		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
 
+		if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
+
 		/* See khugepaged_scan_pmd(). */
 		if (folio_likely_mapped_shared(folio)) {
 			++shared;

-- 
2.48.1