Re: [PATCH v4 1/2] mm/madvise: introduce mkold_clean_ptes() batch helper

[Lance Yang]

Hey Ryan,

Thanks for taking time to review!

On Thu, Apr 4, 2024 at 12:38 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 02/04/2024 13:40, Lance Yang wrote:
> > Change the code that clears young and dirty bits from the PTEs to use
> > ptep_get_and_clear_full() and set_pte_at(), via the new mkold_clean_ptes()
> > batch helper function.
> >
> > Unfortunately, the per-pte get_and_clear/modify/set approach would result
> > in unfolding/refolding for contpte mappings on arm64. So we need to
> > override mkold_clean_ptes() for arm64 to avoid it.
> >
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Suggested-by: Barry Song <21cnbao@xxxxxxxxx>
> > Suggested-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 36 ++++++++++++++++++++++++++++++++
> >  arch/arm64/mm/contpte.c          | 10 +++++++++
> >  include/linux/pgtable.h          | 30 ++++++++++++++++++++++++++
> >  3 files changed, 76 insertions(+)
>
> When I suggested splitting out this patch, this wasn't quite what I meant.
> Instead I intended for the first patch to make the MADV_FREE change and as part
> of that introduce mkold_clean_ptes() with a generic implementation. Then a
> second patch would do the arm64-specific specialization of mkold_clean_ptes()
> for an arm64-specific performance boost. In general it is often useful to
> separate core changes from arch changes where practical since they have
> different target audiences.

Thanks for clearing that up! I‘ll make sure to implement it as discussed.

>
>
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 9fd8613b2db2..b032c107090c 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -1086,6 +1086,27 @@ static inline bool pud_user_accessible_page(pud_t pud)
> >  }
> >  #endif
> >
> > +static inline void __mkold_clean_pte(struct vm_area_struct *vma,
> > +                                 unsigned long addr, pte_t *ptep)
> > +{
> > +     pte_t old_pte, pte;
> > +
> > +     pte = __ptep_get(ptep);
> > +     do {
> > +             old_pte = pte;
> > +             pte = pte_mkclean(pte_mkold(pte));
> > +             pte_val(pte) = cmpxchg_relaxed(&pte_val(*ptep),
> > +                                            pte_val(old_pte), pte_val(pte));
> > +     } while (pte_val(pte) != pte_val(old_pte));
> > +}
>
> I'm not sure how value it is to split this out, given we are only exposing a
> batched version to the core. Perhaps just include the body in mkold_clean_ptes()?

I agree that including the body in mkold_clean_ptes() would be better.

>
> > +
> > +static inline void mkold_clean_ptes(struct vm_area_struct *vma,
> > +                                 unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> > +{
> > +     for (; nr-- > 0; ptep++, addr += PAGE_SIZE)
> > +             __mkold_clean_pte(vma, addr, ptep);
> > +}
> > +
> >  /*
> >   * Atomic pte/pmd modifications.
> >   */
> > @@ -1379,6 +1400,8 @@ 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_ptep_mkold_clean(struct vm_area_struct *vma,
> > +                             unsigned long addr, pte_t *ptep);
>
> Let's have this follow the same naming pattern; contpte_mkold_clean_ptes()

Got it. Let’s keep the naming consistent with contpte_mkold_clean_ptes().

>
> >
> >  static __always_inline void contpte_try_fold(struct mm_struct *mm,
> >                               unsigned long addr, pte_t *ptep, pte_t pte)
> > @@ -1603,6 +1626,18 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
> >       return contpte_ptep_set_access_flags(vma, addr, ptep, entry, dirty);
> >  }
> >
> > +#define mkold_clean_ptes mkold_clean_ptes
> > +static inline void mkold_clean_ptes(struct vm_area_struct *vma,
> > +                                 unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> > +{
> > +     pte_t orig_pte = __ptep_get(ptep);
> > +
> > +     if (likely(!pte_valid_cont(orig_pte)))
> > +             return __mkold_clean_ptes(vma, addr, ptep, nr, full);
> > +
> > +     return contpte_ptep_mkold_clean(vma, addr, ptep);> +}
>
> This function is totally broken as far as I can tell. You are assuming if the
> first pte is not cont, then the whole range is not cont; you can't assume that.
> Then if you decide it is cont, you ignore nr and only fixup a single contpte block.
>
> Take a look at wrprotect_ptes() or another one of the batched helpers and follow
> that pattern.

Sorry for the mistake. I'll take a closer look at wrprotect_ptes() and
make sure to
follow that pattern.

>
> > +
> >  #else /* CONFIG_ARM64_CONTPTE */
> >
> >  #define ptep_get                             __ptep_get
> > @@ -1622,6 +1657,7 @@ 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 mkold_clean_ptes                     __mkold_clean_ptes
> >
> >  #endif /* CONFIG_ARM64_CONTPTE */
> >
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index 1b64b4c3f8bf..560622cfb2a9 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -322,6 +322,16 @@ int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
> >  }
> >  EXPORT_SYMBOL_GPL(contpte_ptep_test_and_clear_young);
> >
> > +void contpte_ptep_mkold_clean(struct vm_area_struct *vma, unsigned long addr,
> > +                           pte_t *ptep)
> > +{
> > +     ptep = contpte_align_down(ptep);
> > +     addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
> > +
> > +     __mkold_clean_ptes(vma, addr, ptep, CONT_PTES, 0);
>
> As above, this is broken as is.
>
> > +}
> > +EXPORT_SYMBOL_GPL(contpte_ptep_mkold_clean);
> > +
> >  int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
> >                                       unsigned long addr, pte_t *ptep)
> >  {
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index fa8f92f6e2d7..fd30779fe487 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -391,6 +391,36 @@ static inline void mkold_ptes(struct vm_area_struct *vma, unsigned long addr,
> >  }
> >  #endif
> >
> > +#ifndef mkold_clean_ptes
> > +/**
> > + * mkold_clean_ptes - Mark PTEs that map consecutive pages of the same folio
> > + *           as old and clean.
> > + * @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 and clean.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented as a simple
> > + * loop over ptep_get_and_clear_full().
>
> How about "otherwise, implemented by get_and_clear/modify/set for each pte in
> the range."?

Nice, this is clearer and more accurate than before.

>
> > + *
> > + * 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_clean_ptes(struct vm_area_struct *vma,
> > +                                 unsigned long addr, pte_t *ptep, unsigned int nr, int full)
>
> You've included the "full" param in the function but not in the docs. I don't
> think the full param is valuable though; it doesn't make sense to call this
> function when tearing down a process. So drop the parameter and just call
> ptep_get_and_clear().

Got it. I'll drop the "full" parameter.

>
> > +{
> > +     pte_t pte;
> > +
> > +     for (; nr-- > 0; ptep++, addr += PAGE_SIZE) {
>
> nit: This is using a different pattern to all the other batch helpers
> (set_ptes(), wrprotect_ptes() clear_full_ptes()).

Thanks for pointing that out.

Thanks,
Lance

>
> > +             pte = ptep_get_and_clear_full(vma->vm_mm, addr, ptep, full);
> > +             set_pte_at(vma->vm_mm, addr, ptep, pte_mkclean(pte_mkold(pte)));
> > +     }
> > +}
> > +#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,
>