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 9:23 PM, Dan Williams wrote:
> 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.
> 
In addition to abiove, I'll be remove @pinned_head variable and retain the
current style that was with try_grab_page() while retaining the unlikely
that was there before. I'm assuming I can retain the Reviewed-by tag, but let
me know of otherwise (diff below).

diff --git a/mm/gup.c b/mm/gup.c
index ca64bc0b339e..398bee74105a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2256,7 +2256,7 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
        int ret = 1;

        do {
-               struct page *pinned_head, *head, *page = pfn_to_page(pfn);
+               struct page *head, *page = pfn_to_page(pfn);
                unsigned long next = addr + PAGE_SIZE;

                pgmap = get_dev_pagemap(pfn, pgmap);
@@ -2273,8 +2273,7 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
                refs = record_subpages(page, addr, next, pages + *nr);

                SetPageReferenced(head);
-               pinned_head = try_grab_compound_head(head, refs, flags);
-               if (unlikely(!pinned_head)) {
+               if (unlikely(!try_grab_compound_head(head, refs, flags))) {
                        if (PageCompound(head))
                                ClearPageReferenced(head);
                        else