Introduce mt_dup() to improve the performance of fork()

A few weeks ago, Liam and I discussed "Fork & Dup tree + Delete DONT_COPY" in
[1]. Thanks Liam for a lot of useful information there.

I didn't use the scheme of linking the leaf nodes, nor did I use the scheme
of building the tree in BFS order. I ended up using the scheme of building the
tree in DFS order. I implemented this algorithm and it seems to be very
efficient.

Use bench_forking() in lib/test_maple_tree.c to test in user space, and get the
performance numbers of duplicating maple tree as follows:

before: 13.52s
after: 2.60s

Their meaning is the time consumed by duplicating 80,000 maple trees with 134
entries. These numbers do not include the time consumed by mt_validate() and
mtree_destroy(). It can be seen that the time consumed has been reduced by
80.77%.

The performance improvement of fork() can be summarized as follows:
With 23 VMAs, performance improves by about 3%, with 223 VMAs, performance
improves by about 15%, and with 4023 VMAs, performance improves by about 30%.
See patch[11/11] for details.

In addition, I would like to assist Liam in maintaining the maple tree, which
requires Liam's consent. In the future I will make some contributions to the
development of maple tree.

The layout of these patches:
001 - 003: Introduce some internal functions to facilitate the
           implementation of mt_dup().
004 - 005: Introduce __mt_dup() and mt_dup(), and their tests.
      006: Introduce mas_replace_entry() to efficiently replace an entry.
007 - 009: Follow-up work on introducing these things.
      010: Add myself as co-maintainer for maple tree.
      011: Use __mt_dup() to duplicate maple tree in dup_mmap().

[1] https://lore.kernel.org/lkml/463899aa-6cbd-f08e-0aca-077b...

Peng Zhang (11):
  maple_tree: Introduce ma_nonleaf_data_end{_nocheck}()
  maple_tree: Validate MAPLE_ENODE and ma_nonleaf_data_end()
  maple_tree: Add some helper functions
  maple_tree: Introduce interfaces __mt_dup() and mt_dup()
  maple_tree: Add test for mt_dup()
  maple_tree: Introduce mas_replace_entry() to directly replace an entry
  maple_tree: Update the documentation of maple tree
  maple_tree: Skip other tests when BENCH is enabled
  maple_tree: Update check_forking() and bench_forking()
  MAINTAINERS: Add co-maintainer for maple tree
  fork: Use __mt_dup() to duplicate maple tree in dup_mmap()

 Documentation/core-api/maple_tree.rst |  10 +
 MAINTAINERS                           |   1 +
 include/linux/maple_tree.h            |   4 +
 kernel/fork.c                         |  35 ++-
 lib/maple_tree.c                      | 389 ++++++++++++++++++++++++--
 lib/test_maple_tree.c                 |  67 ++---
 mm/mmap.c                             |  14 +-
 tools/testing/radix-tree/maple.c      | 204 ++++++++++++++
 8 files changed, 658 insertions(+), 66 deletions(-)

-- 
2.20.1