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

[Joao Martins <joao.m.martins@oracle.com>]

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;

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