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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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@linux.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@oracle.com>
>>> Signed-off-by: Lance Yang <lance.yang@linux.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.