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

[David Hildenbrand]

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@xxxxxxxxx>
> >
> > 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@xxxxxxxxxx
> > [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
> >
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> > v1 -> v2:
> > - Skip all non-present entries except swap entries (per David) thanks!
> > - https://lore.kernel.org/linux-mm/20250924100207.28332-1-lance.yang@xxxxxxxxx/
> >
> > 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