Re: [PATCH mm-new 1/2] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

[Lance Yang]

On 2025/10/4 00:33, Dev Jain wrote:
> On 02/10/25 1:02 pm, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > As pointed out by Dev, the PTE checks for disjoint conditions in the
> > scanning loops can be optimized. is_swap_pte, (pte_none && is_zero_pfn),
> > and pte_uffd_wp are mutually exclusive.
> >
> > This patch refactors the loops in both __collapse_huge_page_isolate() and
> > hpage_collapse_scan_pmd() to use a continuous if-else-if-else-if chain
> > instead of separate if blocks.
> >
> > Also, this is a preparatory step to make it easier to merge the
> > almost-duplicated scanning logic in these two functions, as suggested
> > by David.
> >
> > Suggested-by: Dev Jain <dev.jain@xxxxxxx>
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >   mm/khugepaged.c | 12 ++++--------
> >   1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index f4f57ba69d72..808523f92c7b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -548,8 +548,7 @@ static int __collapse_huge_page_isolate(struct 
> > vm_area_struct *vma,
> >       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> >            _pte++, addr += PAGE_SIZE) {
> >           pte_t pteval = ptep_get(_pte);
> > -        if (pte_none(pteval) || (pte_present(pteval) &&
> > -                is_zero_pfn(pte_pfn(pteval)))) {
> > +        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>
> Should have mentioned in the description that pte_present() is not required
> here, so removing it.

Yep, got it.

> Reviewed-by: Dev Jain <dev.jain@xxxxxxx>

Cheers,
Lance

> >               ++none_or_zero;
> >               if (!userfaultfd_armed(vma) &&
> >                   (!cc->is_khugepaged ||
> > @@ -560,12 +559,10 @@ static int __collapse_huge_page_isolate(struct 
> > vm_area_struct *vma,
> >                   count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> >                   goto out;
> >               }
> > -        }
> > -        if (!pte_present(pteval)) {
> > +        } else if (!pte_present(pteval)) {
> >               result = SCAN_PTE_NON_PRESENT;
> >               goto out;
> > -        }
> > -        if (pte_uffd_wp(pteval)) {
> > +        } else if (pte_uffd_wp(pteval)) {
> >               result = SCAN_PTE_UFFD_WP;
> >               goto out;
> >           }
> > @@ -1316,8 +1313,7 @@ static int hpage_collapse_scan_pmd(struct 
> > mm_struct *mm,
> >                   count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> >                   goto out_unmap;
> >               }
> > -        }
> > -        if (pte_uffd_wp(pteval)) {
> > +        } else if (pte_uffd_wp(pteval)) {
> >               /*
> >                * Don't collapse the page if any of the small
> >                * PTEs are armed with uffd write protection.