Re: [PATCH v10 07/13] khugepaged: skip collapsing mTHP to smaller orders
From: Lorenzo Stoakes <hidden>
Date: 2025-08-21 12:06:54
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On Tue, Aug 19, 2025 at 07:41:59AM -0600, Nico Pache wrote:
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) Acked-by: David Hildenbrand <redacted> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com> 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>
Other than comment below, LGTM so: Reviewed-by: Lorenzo Stoakes <redacted>
quoted hunk ↗ jump to hunk
--- mm/khugepaged.c | 9 +++++++++ 1 file changed, 9 insertions(+)diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 1ad7e00d3fd6..6a4cf7e4a7cc 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c@@ -611,6 +611,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, folio = page_folio(page); VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); + /* + * TODO: In some cases of partially-mapped folios, we'd actually + * want to collapse. + */
Not a fan of adding todo's in code, they have a habit of being left forever. I'd maybe put a more written out comment something similar to the commit message.
+ if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
+ result = SCAN_PTE_MAPPED_HUGEPAGE;
+ goto out;
+ }
+
/* See collapse_scan_pmd(). */
if (folio_maybe_mapped_shared(folio)) {
++shared;
--
2.50.1