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

Link: https://lkml.kernel.org/r/20210212130843.13865-3-joao.m.martins@oracle.comSigned-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>