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

[Lance Yang]

Thanks, Ryan, Barry, David!

Best,
Lance

On Tue, Feb 27, 2024 at 6:53 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 27/02/2024 10:30, David Hildenbrand wrote:
> > On 27.02.24 11:21, Lance Yang wrote:
> >> On Tue, Feb 27, 2024 at 5:14 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>
> >>> On 27.02.24 10:07, Ryan Roberts wrote:
> >>>> On 27/02/2024 02:40, Barry Song wrote:
> >>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>
> >>>>> 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@xxxxxxxxx>
> >>>>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> >>>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >>>>> Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> >>>>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>> ---
> >>>>>    -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@xxxxxxxxxx>
> >>> ---
> >>>    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?
> >
> > IIRC, Ryan made sure that this also works when passing another subpage, after
> > when cont-pte is set. Otherwise this would already be broken for fork/zap.
> >
> > So I don't think this comment would actually be correct.
>
> Indeed, the spec for the function is exactly the same for arm64 as for other
> arches. It's just that arm64 can accelerate the implementation by skipping
> forward to the next contpte boundary when the current pte is part of a contpte
> block.
>
> There is no requirement for pte (or addr or start_ptep) to point to the first
> subpage of a folio - they can point to any subpage.
>
> pte, addr and start_ptep must all refer to the same entry, but I think that's
> clear from the existing text.
>
>