linux-kernel - Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs

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Message-ID: <f6073afe-a604-485a-8cea-7aa08e46a0cf@arm.com>
Date: Thu, 18 Sep 2025 13:07:46 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 ioworker0@...il.com, kirill@...temov.name, hughd@...gle.com,
 mpenttil@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard
 PTEs


On 18/09/25 10:34 am, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> Guard PTE markers are installed via MADV_GUARD_INSTALL to create
> lightweight guard regions.
>
> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when
> encountering such a range.
>
> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in
> the special marker in __collapse_huge_page_swapin().
>
> hpage_collapse_scan_pmd()
>   `- collapse_huge_page()
>       `- __collapse_huge_page_swapin() -> fails!
>
> khugepaged's behavior is slightly different due to its max_ptes_swap limit
> (default 64). It won't fail as deep, but it will still needlessly scan up
> to 64 swap entries before bailing out.
>
> IMHO, we can and should detect this much earlier.
>
> This patch adds a check directly inside the PTE scan loop. If a guard
> marker is found, the scan is aborted immediately with SCAN_PTE_NON_PRESENT,
> avoiding wasted work.
>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   mm/khugepaged.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9ed1af2b5c38..70ebfc7c1f3e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>   					result = SCAN_PTE_UFFD_WP;
>   					goto out_unmap;
>   				}
> +				/*
> +				 * Guard PTE markers are installed by
> +				 * MADV_GUARD_INSTALL. Any collapse path must
> +				 * not touch them, so abort the scan immediately
> +				 * if one is found.
> +				 */
> +				if (is_guard_pte_marker(pteval)) {
> +					result = SCAN_PTE_NON_PRESENT;
> +					goto out_unmap;
> +				}
>   				continue;
>   			} else {
>   				result = SCAN_EXCEED_SWAP_PTE;
>
>

I would like to hear everyone else's thoughts on
https://lore.kernel.org/linux-mm/750a06dc-db3d-43c6-b234-95efb393a9df@arm.com/
wherein I suggest that we should not continue to try collapsing other regions
but immediately exit. The SCAN_PTE_NON_PRESENT case does not exit.