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 23:38:14 +1300
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Ryan Roberts <ryan.roberts@....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 11:22 PM Lance Yang <ioworker0@...il.com> wrote:
>
> On Tue, Feb 27, 2024 at 5:14 PM David Hildenbrand <david@...hat.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@...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.
>
> 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?

I don't understand how this will be different for arm64 and others.
It seems pte_batch_hint with one value > 1 only helps move the
PTE pointer faster to finish the call.

Thanks
Barry