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 28, 2021 at 1:08 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>
>
>
> On 7/28/21 8:55 PM, Dan Williams wrote:
> > 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.
> >
> OK. I take that you prefer this instead:
>
> unsigned long next = addr + PAGE_SIZE;
>
> [...]
>
> /* @end is assumed to be limited at most one compound page */
> if (PageCompound(head))
>         next = end;

Yup.