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

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Message-ID: <af66aaaa-9652-497a-9ff3-9ce8d9417acb@linux.dev>
Date: Fri, 19 Sep 2025 11:34:07 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, Zi Yan <ziy@...dia.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....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 2025/9/19 11:18, Baolin Wang wrote:
> 
> 
> On 2025/9/19 10:44, Lance Yang wrote:
>>
>>
>> On 2025/9/19 03:12, Zi Yan wrote:
>>> On 18 Sep 2025, at 1:04, 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)) {
>>>
>>> pteval is already is_swap_pte(), would is_guard_swp_entry() be a better
>>> choice here? Save one is_swap_pte() call.
> 
> Agree. Then seems we don't need to move the is_guard_pte_marker() into a 
> header file.

Got it. I'll only remove the redundant check in #01 patch.