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

David Hildenbrand
2025-10-01T09:15:45Z

On 01.10.25 10:54, 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?

Yeah, it nowadays includes non-swap entries as well.

Maybe we should consider renaming it at some point to something better.

--
Cheers

David / dhildenb