[RFC PATCH 0/6] Split huge pages to any lower order pages.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

Hi all,

With Matthew's THP in pagecache patches[1], we will be able to handle any size
pagecache THPs, but currently split_huge_page can only split a THP to order-0
pages. This can easily erase the benefit of having pagecache THPs, when
operations like truncate might want to keep pages larger than order-0. In
response, here is the patches to add support for splitting a THP to any lower
order pages. In addition, this patchset prepares for my PUD THP patchset[2],
since splitting a PUD THP to multiple PMD THPs can be handled by
split_huge_page_to_list_to_order function added by this patchset, which reduces
a lot of redundant code without just replicating split_huge_page for PUD THP.

The patchset is on top of Matthew's pagecache/next tree[3].

To ease the tests of split_huge_page functions, I added a new debugfs interface
at <debugfs>/split_huge_pages_in_range_pid, so developers can split THPs in a
given range from a process with the given pid by writing
"<pid>,<vaddr_start>,<vaddr_end>,<to_order>" to the interface. I also added a
new test program to test 1) split PMD THPs, 2) split pagecache THPs to any lower
order, and 3) truncating a pagecache THP to a page with a lower order.

Suggestions and comments are welcome. Thanks.


[1] https://lore.kernel.org/linux-mm/20201029193405.29125-1-willy@xxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20200928175428.4110504-1-zi.yan@xxxxxxxx/
[3] https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/next

Zi Yan (6):
  mm: huge_memory: add new debugfs interface to trigger split huge page
    on any page range.
  mm: memcg: make memcg huge page split support any order split.
  mm: page_owner: add support for splitting to any order in split
    page_owner.
  mm: thp: add support for split huge page to any lower order pages.
  mm: truncate: split thp to a non-zero order if possible.
  mm: huge_memory: enable debugfs to split huge pages to any order.

 include/linux/huge_mm.h                       |   8 +
 include/linux/memcontrol.h                    |   5 +-
 include/linux/page_owner.h                    |   7 +-
 mm/huge_memory.c                              | 177 ++++++++++--
 mm/internal.h                                 |   1 +
 mm/memcontrol.c                               |   4 +-
 mm/migrate.c                                  |   2 +-
 mm/page_alloc.c                               |   2 +-
 mm/page_owner.c                               |   6 +-
 mm/swap.c                                     |   1 -
 mm/truncate.c                                 |  22 +-
 tools/testing/selftests/vm/Makefile           |   1 +
 .../selftests/vm/split_huge_page_test.c       | 255 ++++++++++++++++++
 13 files changed, 453 insertions(+), 38 deletions(-)
 create mode 100644 tools/testing/selftests/vm/split_huge_page_test.c

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2.28.0