Re: [PATCH mm-new 3/3] mm/khugepaged: abort collapse scan on guard PTEs

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

On 2025/9/15 22:08, Lorenzo Stoakes wrote:
> On Sun, Sep 14, 2025 at 10:35:47PM +0800, 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 ;)
> 
> No smileys in commit messages please... :)

Got it. Apparently, I'm a bit too fond of them ... Will remove it in v2.

> 
>>
>> This patch adds a check directly inside the PTE scan loop. If a guard
>> marker is found, the scan is aborted immediately with a new SCAN_PTE_GUARD
>> status, avoiding wasted work.
>>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/khugepaged.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index e54f99bb0b57..910a6f2ec8a9 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -59,6 +59,7 @@ enum scan_result {
>>   	SCAN_STORE_FAILED,
>>   	SCAN_COPY_MC,
>>   	SCAN_PAGE_FILLED,
>> +	SCAN_PTE_GUARD,
> 
> I wonder if we really need to have it literally called out though, can we just
> use:
> 
> SCAN_PTE_NON_PRESENT
> 
> Instead?
> 
> As it is, indeed, non-present :)

Makes sense to me. A guard PTE is indeed a special non-present case.

So, let's reuse SCAN_PTE_NON_PRESENT for that case ;)

Cheers,
Lance

> 
>>   };
>>
>>   #define CREATE_TRACE_POINTS
>> @@ -1317,6 +1318,16 @@ 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)) {
>> +					result = SCAN_PTE_GUARD;
>> +					goto out_unmap;
>> +				}
>>   				continue;
>>   			} else {
>>   				result = SCAN_EXCEED_SWAP_PTE;
>> @@ -2860,6 +2871,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>>   		case SCAN_PAGE_COMPOUND:
>>   		case SCAN_PAGE_LRU:
>>   		case SCAN_DEL_PAGE_LRU:
>> +		case SCAN_PTE_GUARD:
>>   			last_fail = result;
>>   			break;
>>   		default:
>> --
>> 2.49.0
>>
> 
> Cheers, Lorenzo