Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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

On 2025/10/1 16:54, Wei Yang wrote:
> On Wed, Oct 01, 2025 at 11:22:51AM +0800, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> Currently, special non-swap entries (like migration, hwpoison, or PTE
>> markers) are not caught early in hpage_collapse_scan_pmd(), leading to
>> failures deep in the swap-in logic.
>>
>> hpage_collapse_scan_pmd()
>> `- collapse_huge_page()
>>      `- __collapse_huge_page_swapin() -> fails!
>>
>> As David suggested[1], this patch skips any such non-swap entries
>> early. If any one is found, the scan is aborted immediately with the
>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>> work.
>>
>> [1] https://lore.kernel.org/linux-mm/7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com
>> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
>>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> v1 -> v2:
>> - Skip all non-present entries except swap entries (per David) thanks!
>> - https://lore.kernel.org/linux-mm/20250924100207.28332-1-lance.yang@linux.dev/
>>
>> mm/khugepaged.c | 32 ++++++++++++++++++--------------
>> 1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 7ab2d1a42df3..d0957648db19 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1284,7 +1284,23 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>> 	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>> 	     _pte++, addr += PAGE_SIZE) {
>> 		pte_t pteval = ptep_get(_pte);
>> -		if (is_swap_pte(pteval)) {
> 
> It looks is_swap_pte() is mis-leading?

Hmm.. not to me, IMO. is_swap_pte() just means:

!pte_none(pte) && !pte_present(pte)

> 
>> +		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> +			++none_or_zero;
>> +			if (!userfaultfd_armed(vma) &&
>> +			    (!cc->is_khugepaged ||
>> +			     none_or_zero <= khugepaged_max_ptes_none)) {
>> +				continue;
>> +			} else {
>> +				result = SCAN_EXCEED_NONE_PTE;
>> +				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>> +				goto out_unmap;
>> +			}
>> +		} else if (!pte_present(pteval)) {
>> +			if (non_swap_entry(pte_to_swp_entry(pteval))) {
>> +				result = SCAN_PTE_NON_PRESENT;
>> +				goto out_unmap;
>> +			}
>> +
>> 			++unmapped;
>> 			if (!cc->is_khugepaged ||
>> 			    unmapped <= khugepaged_max_ptes_swap) {
>> @@ -1293,7 +1309,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>> 				 * enabled swap entries.  Please see
>> 				 * comment below for pte_uffd_wp().
>> 				 */
>> -				if (pte_swp_uffd_wp_any(pteval)) {
>> +				if (pte_swp_uffd_wp(pteval)) {
> 
> I am not sure why we want to change this. There is no description in the
> change log.
> 
> Would you mind giving some hint on this?

The reason is that pte_swp_uffd_wp_any(pte) is broader than what
we need :)

static inline bool pte_swp_uffd_wp_any(pte_t pte)
{
#ifdef CONFIG_PTE_MARKER_UFFD_WP
	if (!is_swap_pte(pte))
		return false;

	if (pte_swp_uffd_wp(pte))
		return true;

	if (pte_marker_uffd_wp(pte))
		return true;
#endif
	return false;
}

In the context within hpage_collapse_scan_pmd(), we are already inside
an is_swap_pte() block, and we have just handled all non-swap entries
(which would include pte_marker_uffd_wp()).

So we only need to check if the swap entry itself is write-protected
for userfaultfd ;)

Hope that explains it. I skipped it in the changelog as it's a tiny
cleanup ...

Thanks,
Lance

> 
>> 					result = SCAN_PTE_UFFD_WP;
>> 					goto out_unmap;
>> 				}
>> @@ -1304,18 +1320,6 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>> 				goto out_unmap;
>> 			}
>> 		}
>> -		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>> -			++none_or_zero;
>> -			if (!userfaultfd_armed(vma) &&
>> -			    (!cc->is_khugepaged ||
>> -			     none_or_zero <= khugepaged_max_ptes_none)) {
>> -				continue;
>> -			} else {
>> -				result = SCAN_EXCEED_NONE_PTE;
>> -				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>> -				goto out_unmap;
>> -			}
>> -		}
>> 		if (pte_uffd_wp(pteval)) {
>> 			/*
>> 			 * Don't collapse the page if any of the small
>> -- 
>> 2.49.0
>