Patch series "mm/hugetlb: follow_hugetlb_page() improvements", v2.

While looking at ZONE_DEVICE struct page reuse particularly the last
patch[0], I found two possible improvements for follow_hugetlb_page()
which is solely used for get_user_pages()/pin_user_pages().

The first patch batches page refcount updates while the second tidies up
storing the subpages/vmas.  Both together bring the cost of slow variant
of gup() cost from ~87.6k usecs to ~5.8k usecs.

libhugetlbfs tests seem to pass as well gup_test benchmarks with hugetlbfs
vmas.

This patch (of 2):

follow_hugetlb_page() once it locks the pmd/pud, checks all its N subpages
in a huge page and grabs a reference for each one.  Similar to gup-fast,
have follow_hugetlb_page() grab the head page refcount only after counting
all its subpages that are part of the just faulted huge page.

Consequently we reduce the number of atomics necessary to pin said huge
page, which improves non-fast gup() considerably:

  - 16G with 1G huge page size
  gup_test -f /mnt/huge/file -m 16384 -r 10 -L -S -n 512 -w

PIN_LONGTERM_BENCHMARK: ~87.6k us -> ~12.8k us

Link: https://lkml.kernel.org/r/20210128182632.24562-1-joao.m.martins@oracle.com
Link: https://lkml.kernel.org/r/20210128182632.24562-2-joao.m.martins@oracle.comSigned-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>