Re: [PATCH v6 2/2] mm/arm64: override clear_young_dirty_ptes() batch helper

[Lance Yang <ioworker0@gmail.com>]

On Mon, Apr 15, 2024 at 4:59 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 13/04/2024 01:22, Lance Yang wrote:
> > The per-pte get_and_clear/modify/set approach would result in
> > unfolding/refolding for contpte mappings on arm64. So we need
> > to override clear_young_dirty_ptes() for arm64 to avoid it.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Suggested-by: Barry Song <21cnbao@gmail.com>
> > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>
> No, afraid I haven't signed off yet!

Actually, you've done most of this change, and I just do the legwork :)
But I'll remove this s-o-b.

>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 37 ++++++++++++++++++++++++++++++++
> >  arch/arm64/mm/contpte.c          | 28 ++++++++++++++++++++++++
> >  2 files changed, 65 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 9fd8613b2db2..f951774dd2d6 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -1223,6 +1223,28 @@ static inline void __wrprotect_ptes(struct mm_struct *mm, unsigned long address,
> >               __ptep_set_wrprotect(mm, address, ptep);
> >  }
> >
> > +static inline void __clear_young_dirty_ptes(struct mm_struct *mm,
> > +                                         unsigned long addr, pte_t *ptep,
> > +                                         unsigned int nr, cydp_t flags)
> > +{
> > +     pte_t pte;
> > +
> > +     for (;;) {
> > +             pte = __ptep_get(ptep);
> > +
> > +             if (flags | CYDP_CLEAR_YOUNG)
>
> bug: should be bitwise AND (&).

Good spot! Thanks!

>
> > +                     pte = pte_mkold(pte);
> > +             if (flags | CYDP_CLEAR_DIRTY)
> > +                     pte = pte_mkclean(pte);
> > +
> > +             __set_pte(ptep, pte);
>
> The __ptep_get() and __set_pte() are not atomic. This is only safe when you are
> clearing BOTH access and dirty (as I explained in the previous version). If you
> are only clearing one of the flags, you will need a similar cmpxchg loop as for
> __ptep_test_and_clear_young(). Otherwise you can race with the HW and lose
> information.

Thanks again for your patience and explanation!
I still got it wrong :(

>
> > +             if (--nr == 0)
> > +                     break;
> > +             ptep++;
> > +             addr += PAGE_SIZE;
> > +     }
> > +}
> > +
> >  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >  #define __HAVE_ARCH_PMDP_SET_WRPROTECT
> >  static inline void pmdp_set_wrprotect(struct mm_struct *mm,
> > @@ -1379,6 +1401,9 @@ extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> >  extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> >                               unsigned long addr, pte_t *ptep,
> >                               pte_t entry, int dirty);
> > +extern void contpte_clear_young_dirty_ptes(struct mm_struct *mm,
> > +                             unsigned long addr, pte_t *ptep,
> > +                             unsigned int nr, cydp_t flags);
> >
> >  static __always_inline void contpte_try_fold(struct mm_struct *mm,
> >                               unsigned long addr, pte_t *ptep, pte_t pte)
> > @@ -1603,6 +1628,17 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
> >       return contpte_ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> >  }
> >
> > +#define clear_young_dirty_ptes clear_young_dirty_ptes
> > +static inline void clear_young_dirty_ptes(struct mm_struct *mm,
> > +                                       unsigned long addr, pte_t *ptep,
> > +                                       unsigned int nr, cydp_t flags)
> > +{
> > +     if (likely(nr == 1 && !pte_cont(__ptep_get(ptep))))
> > +             __clear_young_dirty_ptes(mm, addr, ptep, nr, flags);
> > +     else
> > +             contpte_clear_young_dirty_ptes(mm, addr, ptep, nr, flags);
> > +}
> > +
> >  #else /* CONFIG_ARM64_CONTPTE */
> >
> >  #define ptep_get                             __ptep_get
> > @@ -1622,6 +1658,7 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
> >  #define wrprotect_ptes                               __wrprotect_ptes
> >  #define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
> >  #define ptep_set_access_flags                        __ptep_set_access_flags
> > +#define clear_young_dirty_ptes                       __clear_young_dirty_ptes
> >
> >  #endif /* CONFIG_ARM64_CONTPTE */
> >
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index 1b64b4c3f8bf..bf3b089d9641 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -361,6 +361,34 @@ void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> >  }
> >  EXPORT_SYMBOL_GPL(contpte_wrprotect_ptes);
> >
> > +void contpte_clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr,
> > +                                 pte_t *ptep, unsigned int nr, cydp_t flags)
> > +{
> > +     /*
> > +      * We can safely clear access/dirty without needing to unfold from
> > +      * the architectures perspective, even when contpte is set. If the
> > +      * range starts or ends midway through a contpte block, we can just
> > +      * expand to include the full contpte block. While this is not
> > +      * exactly what the core-mm asked for, it tracks access/dirty per
> > +      * folio, not per page. And since we only create a contpte block
> > +      * when it is covered by a single folio, we can get away with
> > +      * clearing access/dirty for the whole block.
> > +      */
> > +     unsigned int start = addr;
> > +     unsigned int end = start + nr;
>
> There are addresses; they should be unsigned long. May have been my error
> originally when I sent you the example snippet.

Got it. I'll sort it.

Thanks again for your time!

Thanks,
Lance

>
> Thanks,
> Ryan
>
> > +
> > +     if (pte_cont(__ptep_get(ptep + nr - 1)))
> > +             end = ALIGN(end, CONT_PTE_SIZE);
> > +
> > +     if (pte_cont(__ptep_get(ptep))) {
> > +             start = ALIGN_DOWN(start, CONT_PTE_SIZE);
> > +             ptep = contpte_align_down(ptep);
> > +     }
> > +
> > +     __clear_young_dirty_ptes(mm, start, ptep, end - start, flags);
> > +}
> > +EXPORT_SYMBOL_GPL(contpte_clear_young_dirty_ptes);
> > +
> >  int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
> >                                       unsigned long addr, pte_t *ptep,
> >                                       pte_t entry, int dirty)
>