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

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Date: Tue, 27 Feb 2024 17:57:00 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Barry Song <v-songbaohua@...o.com>, Yin Fengwei <fengwei.yin@...el.com>
Subject: Re: [PATCH] mm: export folio_pte_batch as a couple of modules might
 need it

On Tue, Feb 27, 2024 at 5:53 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 27.02.24 10:51, Lance Yang wrote:
> > On Tue, Feb 27, 2024 at 5:14 PM David Hildenbrand <david@...hatcom> wrote:
> >>
> >> On 27.02.24 10:07, Ryan Roberts wrote:
> >>> On 27/02/2024 02:40, Barry Song wrote:
> >>>> From: Barry Song <v-songbaohua@...o.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@...il.com>
> >>>> Cc: Ryan Roberts <ryan.roberts@....com>
> >>>> Cc: David Hildenbrand <david@...hat.com>
> >>>> Cc: Yin Fengwei <fengwei.yin@...el.com>
> >>>> Signed-off-by: Barry Song <v-songbaohua@...o.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@...hat.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.
> >> + * @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);
> >
> > Nit:
> > IIUC, the pte that maps to the first page.
> >   -       VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) !=
> > folio, folio);
> >   +       VM_WARN_ON_FOLIO(pte_pfn(pte) != folio_pfn(folio), folio);
>
> That would only work if the PTE would map the very first subpage of the
> folio, not any subpage?

You're right. I got it.

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