[PATCH v2 0/2] mm/hugetlb: follow_hugetlb_page() improvements

[Joao Martins <joao.m.martins@oracle.com>]

Hey,

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.

v2:
  * switch from refs++ to ++refs;
  * add Mike's Rb on patch 1;
  * switch from page++ to mem_map_offset() on the second patch;
  
[0] https://lore.kernel.org/linux-mm/20201208172901.17384-11-joao.m.martins@oracle.com/

Joao Martins (2):
  mm/hugetlb: grab head page refcount once for group of subpages
  mm/hugetlb: refactor subpage recording

 include/linux/mm.h |  3 +++
 mm/gup.c           |  5 ++--
 mm/hugetlb.c       | 66 +++++++++++++++++++++++++++-------------------
 3 files changed, 44 insertions(+), 30 deletions(-)

-- 
2.17.1