Re: [PATCH v3 2/4] mm/gup: decrement head page once for group of subpages

[Jason Gunthorpe]

On Fri, Feb 05, 2021 at 08:41:25PM +0000, Joao Martins wrote:
> Rather than decrementing the head page refcount one by one, we
> walk the page array and checking which belong to the same
> compound_head. Later on we decrement the calculated amount
> of references in a single write to the head page. To that
> end switch to for_each_compound_head() does most of the work.
> 
> set_page_dirty() needs no adjustment as it's a nop for
> non-dirty head pages and it doesn't operate on tail pages.
> 
> This considerably improves unpinning of pages with THP and
> hugetlbfs:
> 
> - THP
> gup_test -t -m 16384 -r 10 [-L|-a] -S -n 512 -w
> PIN_LONGTERM_BENCHMARK (put values): ~87.6k us -> ~23.2k us
> 
> - 16G with 1G huge page size
> gup_test -f /mnt/huge/file -m 16384 -r 10 [-L|-a] -S -n 512 -w
> PIN_LONGTERM_BENCHMARK: (put values): ~87.6k us -> ~27.5k us
> 
> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  mm/gup.c | 29 +++++++++++------------------
>  1 file changed, 11 insertions(+), 18 deletions(-)

Looks fine

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

I was wondering why this only touches the FOLL_PIN path, it would make
sense to also use this same logic for release_pages()

        for (i = 0; i < nr; i++) {
                struct page *page = pages[i];
                page = compound_head(page);
                if (is_huge_zero_page(page))
                        continue; 

Jason