Re: [PATCH v2 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Peter, Andrea,

On Fri, May 7, 2021 at 7:26 PM Peter Xu <peterx@redhat.com> wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> has_pinned 32bit can be packed in the MMF_HAS_PINNED bit as a noop
> cleanup.
>
> Any atomic_inc/dec to the mm cacheline shared by all threads in
> pin-fast would reintroduce a loss of SMP scalability to pin-fast, so
> there's no future potential usefulness to keep an atomic in the mm for
> this.
>
> set_bit(MMF_HAS_PINNED) will be theoretically a bit slower than
> WRITE_ONCE (atomic_set is equivalent to WRITE_ONCE), but the set_bit
> (just like atomic_set after this commit) has to be still issued only
> once per "mm", so the difference between the two will be lost in the
> noise.
>
> will-it-scale "mmap2" shows no change in performance with enterprise
> config as expected.
>
> will-it-scale "pin_fast" retains the > 4000% SMP scalability
> performance improvement against upstream as expected.
>
> This is a noop as far as overall performance and SMP scalability are
> concerned.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> [peterx: Fix build for task_mmu.c, introduce mm_set_has_pinned_flag, fix
>  comment here and there]
> Signed-off-by: Peter Xu <peterx@redhat.com>

Thanks for your patch, which is now in linux-next.

> diff --git a/mm/gup.c b/mm/gup.c
> index 9933bc5c2eff2..bb130723a6717 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1270,6 +1270,17 @@ int fixup_user_fault(struct mm_struct *mm,
>  }
>  EXPORT_SYMBOL_GPL(fixup_user_fault);
>
> +/*
> + * Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
> + * lifecycle.  Avoid setting the bit unless necessary, or it might cause write
> + * cache bouncing on large SMP machines for concurrent pinned gups.
> + */
> +static inline void mm_set_has_pinned_flag(unsigned long *mm_flags)
> +{
> +       if (!test_bit(MMF_HAS_PINNED, mm_flags))
> +               set_bit(MMF_HAS_PINNED, mm_flags);
> +}
> +
>  /*
>   * Please note that this function, unlike __get_user_pages will not
>   * return 0 for nr_pages > 0 without FOLL_NOWAIT
> @@ -1292,8 +1303,8 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
>                 BUG_ON(*locked != 1);
>         }
>
> -       if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned))
> -               atomic_set(&mm->has_pinned, 1);
> +       if (flags & FOLL_PIN)
> +               mm_set_has_pinned_flag(&mm->flags);
>
>         /*
>          * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
> @@ -2617,8 +2628,8 @@ static int internal_get_user_pages_fast(unsigned long start,
>                                        FOLL_FAST_ONLY)))
>                 return -EINVAL;
>
> -       if ((gup_flags & FOLL_PIN) && !atomic_read(&current->mm->has_pinned))
> -               atomic_set(&current->mm->has_pinned, 1);
> +       if (gup_flags & FOLL_PIN)
> +               mm_set_has_pinned_flag(&current->mm->flags);

noreply@ellerman.id.au reports:

    FAILED linux-next/m5272c3_defconfig/m68k-gcc8 Wed May 12, 19:30
    http://kisskb.ellerman.id.au/kisskb/buildresult/14543658/
    Commit:   Add linux-next specific files for 20210512
          ec85c95b0c90a17413901b018e8ade7b9eae7cad
    Compiler: m68k-linux-gcc (GCC) 8.1.0 / GNU ld (GNU Binutils) 2.30

    mm/gup.c:2698:3: error: implicit declaration of function
'mm_set_has_pinned_flag'; did you mean 'set_tsk_thread_flag'?
[-Werror=implicit-function-declaration]

It's definition is inside the #ifdef CONFIG_MMU section, but the last
user isn't.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds