Overhaul multi-page lookups for THP

This THP prep patchset changes several page cache iteration APIs to only
return head pages.

 - It's only possible to tag head pages in the page cache, so only
   return head pages, not all their subpages.
 - Factor a lot of common code out of the various batch lookup routines
 - Add mapping_seek_hole_data()
 - Unify find_get_entries() and pagevec_lookup_entries()
 - Make find_get_entries only return head pages, like find_get_entry().

These are only loosely connected, but they seem to make sense together
as a series.

v4:
 - Add FGP_ENTRY, remove find_get_entry and find_lock_entry
 - Rename xas_find_get_entry to find_get_entry
 - Add "Optimise get_shadow_from_swap_cache"
 - Move "iomap: Use mapping_seek_hole_data" to this patch series
 - Rebase against next-20201112

Matthew Wilcox (Oracle) (16):
  mm: Make pagecache tagged lookups return only head pages
  mm/shmem: Use pagevec_lookup in shmem_unlock_mapping
  mm/swap: Optimise get_shadow_from_swap_cache
  mm: Add FGP_ENTRY
  mm/filemap: Rename find_get_entry to mapping_get_entry
  mm/filemap: Add helper for finding pages
  mm/filemap: Add mapping_seek_hole_data
  iomap: Use mapping_seek_hole_data
  mm: Add and use find_lock_entries
  mm: Add an 'end' parameter to find_get_entries
  mm: Add an 'end' parameter to pagevec_lookup_entries
  mm: Remove nr_entries parameter from pagevec_lookup_entries
  mm: Pass pvec directly to find_get_entries
  mm: Remove pagevec_lookup_entries
  mm/truncate,shmem: Handle truncates that split THPs
  mm/filemap: Return only head pages from find_get_entries

 fs/iomap/seek.c         | 125 ++------------
 include/linux/pagemap.h |   6 +-
 include/linux/pagevec.h |   4 -
 mm/filemap.c            | 351 +++++++++++++++++++++++++---------------
 mm/internal.h           |   7 +-
 mm/shmem.c              | 218 ++++++-------------------
 mm/swap.c               |  38 +----
 mm/swap_state.c         |   7 +-
 mm/truncate.c           | 249 +++++++++++-----------------
 9 files changed, 390 insertions(+), 615 deletions(-)

-- 
2.28.0