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

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Message-ID: <8f49bd75-5314-43e4-aecc-e0ef3616f7fd@arm.com>
Date: Thu, 18 Sep 2025 16:41:56 +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>
> ---

Reviewed-by: Dev Jain <dev.jain@....com>