linux-kernel - Re: [PATCH v9 08/14] khugepaged: skip collapsing mTHP to smaller orders

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Message-ID: <290d669f-e358-471a-95b7-c46f974742d0@redhat.com>
Date: Wed, 16 Jul 2025 16:32:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, ryan.roberts@....com,
 dev.jain@....com, corbet@....net, rostedt@...dmis.org, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
 baohua@...nel.org, willy@...radead.org, peterx@...hat.com,
 wangkefeng.wang@...wei.com, usamaarif642@...il.com, sunnanyong@...wei.com,
 vishal.moola@...il.com, thomas.hellstrom@...ux.intel.com,
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Subject: Re: [PATCH v9 08/14] khugepaged: skip collapsing mTHP to smaller
 orders

On 14.07.25 02:32, 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/
> 
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.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 5d7c5be9097e..a701d9f0f158 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -612,7 +612,12 @@ 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;
> +		}

Probably worth adding a TODO in the code like

/*
  * TODO: In some cases of partially-mapped folios, we'd actually
  * want to collapse.
  */

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb