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Subject[PATCH 0/7] Split huge pages to any lower order pages and selftests.
Date
From: Zi Yan <ziy@nvidia.com>

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.

To help the tests of splitting huge pages, 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) splitting PMD THPs, 2) splitting PTE-mapped THPs,
3) splitting pagecache THPs to any lower order, 4) truncating a pagecache
THP to a page with a lower order, and 5) punching holes in a pagecache THP to
cause splitting THPs to lower order THPs.

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

* Patch 1 is cherry-picked from Matthew's recent xarray fix [4] just to make sure
Patch 3 to 7 can run without problem. I let Matthew decide how it should get
picked up.
* Patch 2 is self-contained and can be merged if it looks OK.

Comments and/or suggestions are welcome.

ChangeLog
===
From RFC:
1. Fixed debugfs to handle splitting PTE-mapped THPs properly and added stats
for split THPs.
2. Added a new test case for splitting PTE-mapped THPs. Each of the four PTEs
points to a different subpage from four THPs and used kpageflags to check
whether a PTE points to a THP or not (AnonHugePages from smap does not show
PTE-mapped THPs).
3. mem_cgroup_split_huge_fixup() takes order instead of nr.
4. split_page_owner takes old_order and new_order instead of nr and new_order.
5. Corrected __split_page_owner declaration and fixed its implementation when
splitting a THP to a new order.
6. Renamed left to remaining in truncate_inode_partial_page().
7. Use VM_BUG_ON instead of WARN_ONCE when splitting a THP to the unsupported
order-0 and splitting anonymous THPs to non-zero orders.
8. Added punching holes in a file as a new pagecache THP split test case, which
uncovered an xarray bug.


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


Matthew Wilcox (Oracle) (1):
XArray: Fix splitting to non-zero orders

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: 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 | 10 +-
lib/test_xarray.c | 26 +-
lib/xarray.c | 4 +-
mm/huge_memory.c | 219 ++++++--
mm/internal.h | 1 +
mm/memcontrol.c | 6 +-
mm/migrate.c | 2 +-
mm/page_alloc.c | 2 +-
mm/page_owner.c | 13 +-
mm/swap.c | 1 -
mm/truncate.c | 29 +-
tools/testing/selftests/vm/.gitignore | 1 +
tools/testing/selftests/vm/Makefile | 1 +
.../selftests/vm/split_huge_page_test.c | 479 ++++++++++++++++++
16 files changed, 742 insertions(+), 65 deletions(-)
create mode 100644 tools/testing/selftests/vm/split_huge_page_test.c

--
2.28.0

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