+ mm-khugepaged-skip-collapsing-mthp-to-smaller-orders.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: mm/khugepaged: skip collapsing mTHP to smaller orders
has been added to the -mm mm-unstable branch.  Its filename is
     mm-khugepaged-skip-collapsing-mthp-to-smaller-orders.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-khugepaged-skip-collapsing-mthp-to-smaller-orders.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nico Pache <npache@redhat.com>
Subject: mm/khugepaged: skip collapsing mTHP to smaller orders
Date: Fri, 22 May 2026 09:00:02 -0600

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 check is also not done during the scan phase as the
current collapse order is unknown at that time.

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

Link: https://lore.kernel.org/20260522150009.121603-8-npache@redhat.com
Link: https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@arm.com/ [1]
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>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rafael Aquini <raquini@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Takashi Iwai (SUSE) <tiwai@suse.de>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/mm/khugepaged.c~mm-khugepaged-skip-collapsing-mthp-to-smaller-orders
+++ a/mm/khugepaged.c
@@ -689,6 +689,14 @@ static enum scan_result __collapse_huge_
 				goto out;
 			}
 		}
+		/*
+		 * TODO: In some cases of partially-mapped folios, we'd actually
+		 * want to collapse.
+		 */
+		if (!is_pmd_order(order) && folio_order(folio) >= order) {
+			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			goto out;
+		}
 
 		if (folio_test_large(folio)) {
 			struct folio *f;
_

Patches currently in -mm which might be from npache@redhat.com are

mm-khugepaged-generalize-hugepage_vma_revalidate-for-mthp-support.patch
mm-khugepaged-rework-max_ptes_-handling-with-helper-functions.patch
mm-khugepaged-generalize-__collapse_huge_page_-for-mthp-support.patch
mm-khugepaged-require-collapse_huge_page-to-enter-exit-with-the-lock-dropped.patch
mm-khugepaged-generalize-collapse_huge_page-for-mthp-collapse.patch
mm-khugepaged-skip-collapsing-mthp-to-smaller-orders.patch
mm-khugepaged-add-per-order-mthp-collapse-failure-statistics.patch
mm-khugepaged-improve-tracepoints-for-mthp-orders.patch
mm-khugepaged-introduce-collapse_allowable_orders-helper-function.patch
mm-khugepaged-introduce-mthp-collapse-support.patch
mm-khugepaged-avoid-unnecessary-mthp-collapse-attempts.patch
documentation-mm-update-the-admin-guide-for-mthp-collapse.patch