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

[Lance Yang <lance.yang@linux.dev>]

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.

Yeah. Good spot! Will do ;)

Thanks,
Lance

> 
>> +					result = SCAN_PTE_NON_PRESENT;
>> +					goto out_unmap;
>> +				}
>>   				continue;
>>   			} else {
>>   				result = SCAN_EXCEED_SWAP_PTE;
>> -- 
>> 2.49.0
> 
> 
> Best Regards,
> Yan, Zi