Re: [PATCH] mm: export folio_pte_batch as a couple of modules might need it

[Lance Yang <ioworker0@gmail.com>]

On Tue, Feb 27, 2024 at 5:14 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 27.02.24 10:07, Ryan Roberts wrote:
> > On 27/02/2024 02:40, Barry Song wrote:
> >> From: Barry Song <v-songbaohua@oppo.com>
> >>
> >> madvise and some others might need folio_pte_batch to check if a range
> >> of PTEs are completely mapped to a large folio with contiguous physcial
> >> addresses. Let's export it for others to use.
> >>
> >> Cc: Lance Yang <ioworker0@gmail.com>
> >> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >> Cc: David Hildenbrand <david@redhat.com>
> >> Cc: Yin Fengwei <fengwei.yin@intel.com>
> >> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> >> ---
> >>   -v1:
> >>   at least two jobs madv_free and madv_pageout depend on it. To avoid
> >>   conflicts and dependencies, after discussing with Lance, we prefer
> >>   this one can land earlier.
> >
> > I think this will also ultimately be useful for mprotect too, though I haven't
> > looked at it properly yet.
> >
>
> Yes, I think we briefly discussed that.
>
> >>
> >>   mm/internal.h | 13 +++++++++++++
> >>   mm/memory.c   | 11 +----------
> >>   2 files changed, 14 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/mm/internal.h b/mm/internal.h
> >> index 13b59d384845..8e2bc304f671 100644
> >> --- a/mm/internal.h
> >> +++ b/mm/internal.h
> >> @@ -83,6 +83,19 @@ static inline void *folio_raw_mapping(struct folio *folio)
> >>      return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
> >>   }
> >>
> >> +/* Flags for folio_pte_batch(). */
> >> +typedef int __bitwise fpb_t;
> >> +
> >> +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
> >> +#define FPB_IGNORE_DIRTY            ((__force fpb_t)BIT(0))
> >> +
> >> +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
> >> +#define FPB_IGNORE_SOFT_DIRTY               ((__force fpb_t)BIT(1))
> >> +
> >> +extern int folio_pte_batch(struct folio *folio, unsigned long addr,
> >> +            pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
> >> +            bool *any_writable);
> >> +
> >>   void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
> >>                                              int nr_throttled);
> >>   static inline void acct_reclaim_writeback(struct folio *folio)
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 1c45b6a42a1b..319b3be05e75 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -953,15 +953,6 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
> >>      set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
> >>   }
> >>
> >> -/* Flags for folio_pte_batch(). */
> >> -typedef int __bitwise fpb_t;
> >> -
> >> -/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
> >> -#define FPB_IGNORE_DIRTY            ((__force fpb_t)BIT(0))
> >> -
> >> -/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
> >> -#define FPB_IGNORE_SOFT_DIRTY               ((__force fpb_t)BIT(1))
> >> -
> >>   static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> >>   {
> >>      if (flags & FPB_IGNORE_DIRTY)
> >> @@ -982,7 +973,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> >>    * If "any_writable" is set, it will indicate if any other PTE besides the
> >>    * first (given) PTE is writable.
> >>    */
> >
> > David was talking in Lance's patch thread, about improving the docs for this
> > function now that its exported. Might be worth syncing on that.
>
> Here is my take:
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   mm/memory.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d0b855a1837a8..098356b8805ae 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -971,16 +971,28 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>         return pte_wrprotect(pte_mkold(pte));
>   }
>
> -/*
> +/**
> + * folio_pte_batch - detect a PTE batch for a large folio
> + * @folio: The large folio to detect a PTE batch for.
> + * @addr: The user virtual address the first page is mapped at.
> + * @start_ptep: Page table pointer for the first entry.
> + * @pte: Page table entry for the first page.

Nit:

- * @pte: Page table entry for the first page.
+ * @pte: Page table entry for the first page that must be the first subpage of
+ *               the folio excluding arm64 for now.

IIUC, pte_batch_hint is always 1 excluding arm64 for now.
I'm not sure if this modification will be helpful?

Thanks,
Lance

> + * @max_nr: The maximum number of table entries to consider.
> + * @flags: Flags to modify the PTE batch semantics.
> + * @any_writable: Optional pointer to indicate whether any entry except the
> + *               first one is writable.
> + *
>    * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> - * pages of the same folio.
> + * pages of the same large folio.
>    *
>    * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
>    * the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
>    * soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
>    *
> - * If "any_writable" is set, it will indicate if any other PTE besides the
> - * first (given) PTE is writable.
> + * start_ptep must map any page of the folio. max_nr must be at least one and
> + * must be limited by the caller so scanning cannot exceed a single page table.
> + *
> + * Return: the number of table entries in the batch.
>    */
>   static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>                 pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
> @@ -996,6 +1008,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>                 *any_writable = false;
>
>         VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> +       VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
> +       VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
>
>         nr = pte_batch_hint(start_ptep, pte);
>         expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
> --
> 2.43.2
>
>
> >
> >> -static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >> +int folio_pte_batch(struct folio *folio, unsigned long addr,
> >
> > fork() is very performance sensitive. Is there a risk we are regressing
> > performance by making this out-of-line? Although its in the same compilation
> > unit so the compiler may well inline it anyway?
>
> Easy to verify by looking at the generated asm I guess?
>
> >
> > Either way, perhaps we are better off making it inline in the header? That would
> > avoid needing to rerun David's micro-benchmarks for fork() and munmap().
>
> That way, the compiler can most certainly better optimize it also outside of mm/memory.c
>
> --
> Cheers,
>
> David / dhildenb
>