Re: [PATCH v3 13/14] mm/gup: grab head page refcount once for group of subpages

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jul 14, 2021 at 12:36 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> Use try_grab_compound_head() for device-dax GUP when configured with a
> compound pagemap.
>
> Rather than incrementing the refcount for each page, do one atomic
> addition for all the pages to be pinned.
>
> Performance measured by gup_benchmark improves considerably
> get_user_pages_fast() and pin_user_pages_fast() with NVDIMMs:
>
>  $ gup_test -f /dev/dax1.0 -m 16384 -r 10 -S [-u,-a] -n 512 -w
> (get_user_pages_fast 2M pages) ~59 ms -> ~6.1 ms
> (pin_user_pages_fast 2M pages) ~87 ms -> ~6.2 ms
> [altmap]
> (get_user_pages_fast 2M pages) ~494 ms -> ~9 ms
> (pin_user_pages_fast 2M pages) ~494 ms -> ~10 ms
>
>  $ gup_test -f /dev/dax1.0 -m 129022 -r 10 -S [-u,-a] -n 512 -w
> (get_user_pages_fast 2M pages) ~492 ms -> ~49 ms
> (pin_user_pages_fast 2M pages) ~493 ms -> ~50 ms
> [altmap with -m 127004]
> (get_user_pages_fast 2M pages) ~3.91 sec -> ~70 ms
> (pin_user_pages_fast 2M pages) ~3.97 sec -> ~74 ms
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  mm/gup.c | 53 +++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 33 insertions(+), 20 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 42b8b1fa6521..9baaa1c0b7f3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2234,31 +2234,55 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  }
>  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
>
> +
> +static int record_subpages(struct page *page, unsigned long addr,
> +                          unsigned long end, struct page **pages)
> +{
> +       int nr;
> +
> +       for (nr = 0; addr != end; addr += PAGE_SIZE)
> +               pages[nr++] = page++;
> +
> +       return nr;
> +}
> +
>  #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
>  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>                              unsigned long end, unsigned int flags,
>                              struct page **pages, int *nr)
>  {
> -       int nr_start = *nr;
> +       int refs, nr_start = *nr;
>         struct dev_pagemap *pgmap = NULL;
>
>         do {
> -               struct page *page = pfn_to_page(pfn);
> +               struct page *pinned_head, *head, *page = pfn_to_page(pfn);
> +               unsigned long next;
>
>                 pgmap = get_dev_pagemap(pfn, pgmap);
>                 if (unlikely(!pgmap)) {
>                         undo_dev_pagemap(nr, nr_start, flags, pages);
>                         return 0;
>                 }
> -               SetPageReferenced(page);
> -               pages[*nr] = page;
> -               if (unlikely(!try_grab_page(page, flags))) {
> -                       undo_dev_pagemap(nr, nr_start, flags, pages);
> +
> +               head = compound_head(page);
> +               /* @end is assumed to be limited at most one compound page */
> +               next = PageCompound(head) ? end : addr + PAGE_SIZE;

Please no ternary operator for this check, but otherwise this patch
looks good to me.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>


> +               refs = record_subpages(page, addr, next, pages + *nr);
> +
> +               SetPageReferenced(head);
> +               pinned_head = try_grab_compound_head(head, refs, flags);
> +               if (!pinned_head) {
> +                       if (PageCompound(head)) {
> +                               ClearPageReferenced(head);
> +                               put_dev_pagemap(pgmap);
> +                       } else {
> +                               undo_dev_pagemap(nr, nr_start, flags, pages);
> +                       }
>                         return 0;
>                 }
> -               (*nr)++;
> -               pfn++;
> -       } while (addr += PAGE_SIZE, addr != end);
> +               *nr += refs;
> +               pfn += refs;
> +       } while (addr += (refs << PAGE_SHIFT), addr != end);
>
>         if (pgmap)
>                 put_dev_pagemap(pgmap);
> @@ -2318,17 +2342,6 @@ static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
>  }
>  #endif
>
> -static int record_subpages(struct page *page, unsigned long addr,
> -                          unsigned long end, struct page **pages)
> -{
> -       int nr;
> -
> -       for (nr = 0; addr != end; addr += PAGE_SIZE)
> -               pages[nr++] = page++;
> -
> -       return nr;
> -}
> -
>  #ifdef CONFIG_ARCH_HAS_HUGEPD
>  static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
>                                       unsigned long sz)
> --
> 2.17.1
>