Re: [PATCH v7 1/3] mm/madvise: introduce clear_young_dirty_ptes() batch helper

[Lance Yang <ioworker0@gmail.com>]

On Tue, Apr 16, 2024 at 11:03 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 16.04.24 05:34, Lance Yang wrote:
> > This commit introduces clear_young_dirty_ptes() to replace mkold_ptes().
> > By doing so, we can use the same function for both use cases
> > (madvise_pageout and madvise_free), and it also provides the flexibility
> > to only clear the dirty flag in the future if needed.
> >
> > Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >   include/linux/mm_types.h |  9 +++++
> >   include/linux/pgtable.h  | 74 ++++++++++++++++++++++++----------------
> >   mm/madvise.c             |  3 +-
> >   3 files changed, 55 insertions(+), 31 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index c432add95913..28822cd65d2a 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1367,6 +1367,15 @@ enum fault_flag {
> >
> >   typedef unsigned int __bitwise zap_flags_t;
> >
> > +/* Flags for clear_young_dirty_ptes(). */
> > +typedef int __bitwise cydp_t;
> > +
> > +/* Clear the access bit */
> > +#define CYDP_CLEAR_YOUNG             ((__force cydp_t)BIT(0))
> > +
> > +/* Clear the dirty bit */
> > +#define CYDP_CLEAR_DIRTY             ((__force cydp_t)BIT(1))
> > +
> >   /*
> >    * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
> >    * other. Here is what they mean, and how to use them:
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index e2f45e22a6d1..18019f037bae 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -361,36 +361,6 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> >   }
> >   #endif
> >
> > -#ifndef mkold_ptes
> > -/**
> > - * mkold_ptes - Mark PTEs that map consecutive pages of the same folio as old.
> > - * @vma: VMA the pages are mapped into.
> > - * @addr: Address the first page is mapped at.
> > - * @ptep: Page table pointer for the first entry.
> > - * @nr: Number of entries to mark old.
> > - *
> > - * May be overridden by the architecture; otherwise, implemented as a simple
> > - * loop over ptep_test_and_clear_young().
> > - *
> > - * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> > - * some PTEs might be write-protected.
> > - *
> > - * Context: The caller holds the page table lock.  The PTEs map consecutive
> > - * pages that belong to the same folio.  The PTEs are all in the same PMD.
> > - */
> > -static inline void mkold_ptes(struct vm_area_struct *vma, unsigned long addr,
> > -             pte_t *ptep, unsigned int nr)
> > -{
> > -     for (;;) {
> > -             ptep_test_and_clear_young(vma, addr, ptep);
> > -             if (--nr == 0)
> > -                     break;
> > -             ptep++;
> > -             addr += PAGE_SIZE;
> > -     }
> > -}
> > -#endif
> > -
> >   #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
> >   #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
> >   static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
> > @@ -489,6 +459,50 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >   }
> >   #endif
> >
> > +#ifndef clear_young_dirty_ptes
> > +/**
> > + * clear_young_dirty_ptes - Mark PTEs that map consecutive pages of the
> > + *           same folio as old/clean.
> > + * @mm: Address space the pages are mapped into.
> > + * @addr: Address the first page is mapped at.
> > + * @ptep: Page table pointer for the first entry.
> > + * @nr: Number of entries to mark old/clean.
> > + * @flags: Flags to modify the PTE batch semantics.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented by
> > + * get_and_clear/modify/set for each pte in the range.
> > + *
> > + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> > + * some PTEs might be write-protected.
> > + *
> > + * Context: The caller holds the page table lock.  The PTEs map consecutive
> > + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> > + */
> > +static inline void clear_young_dirty_ptes(struct vm_area_struct *vma,
> > +                                       unsigned long addr, pte_t *ptep,
> > +                                       unsigned int nr, cydp_t flags)
> > +{
> > +     pte_t pte;
> > +
> > +     for (;;) {
> > +             if (flags == CYDP_CLEAR_YOUNG)
> > +                     ptep_test_and_clear_young(vma, addr, ptep);
> > +             else {
> > +                     pte = ptep_get_and_clear(vma->vm_mm, addr, ptep);
> > +                     if (flags & CYDP_CLEAR_YOUNG)
> > +                             pte = pte_mkold(pte);
> > +                     if (flags & CYDP_CLEAR_DIRTY)
> > +                             pte = pte_mkclean(pte);
> > +                     set_pte_at(vma->vm_mm, addr, ptep, pte);
> > +             }
> > +             if (--nr == 0)
> > +                     break;
> > +             ptep++;
> > +             addr += PAGE_SIZE;
> > +     }
> > +}
>

Hey David,

Thanks for taking time to review!

> The complier *might* generate a bit faster code if you check for
> CYDP_CLEAR_YOUNG outside of the loop, so you don't have to recheck on
> each loop iteration.

Nice! I think moving the CYDP_CLEAR_YOUNG check outside of the loop
could speed up the code. I'll make this change in the next version.

>
> For now, nothing to lose sleep about
>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks again for the review!

Best,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>