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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <7ca12da2-4ddc-4b4e-a8da-a1d9d2c44e03@linux.alibaba.com>
Date: Fri, 19 Sep 2025 11:18:51 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lance Yang <lance.yang@...ux.dev>, 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 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.