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 <redacted>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

Looks fine

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

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