linux-kernel - [PATCH v3 08/12] khugepaged: skip collapsing mTHP to smaller orders

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Message-ID: <20250414220557.35388-9-npache@redhat.com>
Date: Mon, 14 Apr 2025 16:05:53 -0600
From: Nico Pache <npache@...hat.com>
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Subject: [PATCH v3 08/12] 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/

Co-developed-by: Dev Jain <dev.jain@....com>
Signed-off-by: Dev Jain <dev.jain@....com>
Signed-off-by: Nico Pache <npache@...hat.com>
---
 mm/khugepaged.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index de09de5a3f28..114056276003 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -625,7 +625,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 		folio = page_folio(page);
 		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
 
-		/* See hpage_collapse_scan_pmd(). */
+		if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
+
+		/* See khugepaged_scan_pmd(). */
 		if (folio_maybe_mapped_shared(folio)) {
 			++shared;
 			if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
-- 
2.48.1