Thread (91 messages) 91 messages, 11 authors, 2025-11-26
STALE195d REVIEWED: 4 (2M)
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[PATCH v12 mm-new 08/15] khugepaged: skip collapsing mTHP to smaller orders

From: Nico Pache <npache@redhat.com>
Date: 2025-10-22 18:39:50
Also in: linux-doc, linux-mm, lkml
Subsystem: memory management, memory management - thp (transparent huge page), the rest · Maintainers: Andrew Morton, David Hildenbrand, Lorenzo Stoakes, Linus Torvalds

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)

Reviewed-by: Lorenzo Stoakes <redacted>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <redacted>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 8 ++++++++
 1 file changed, 8 insertions(+)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 75e7ebdccc36..d741af15e18c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -629,6 +629,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 				goto out;
 			}
 		}
+		/*
+		 * TODO: In some cases of partially-mapped folios, we'd actually
+		 * want to collapse.
+		 */
+		if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
 
 		if (folio_test_large(folio)) {
 			struct folio *f;
-- 
2.51.0
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