Re: [RFC PATCH v3 0/3] mm/damon: introduce DAMOS_SPLIT action

[Gutierrez Asier <gutierrez.asier@huawei-partners.com>]

Hi Lian,

On 7/20/2026 6:03 AM, Lian Wang wrote:
> DAMOS_SPLIT splits large folios in a target region down to a
> configured target order, using the existing split_folio_to_order().
> No new core-mm code or exported symbols are introduced.
> 
> Based on mm-unstable at 61cccb8363fc ("mm/swap, PM: hibernate:
> atomically replace hibernation pin").
> 
> Different addresses within a PMD-mapped folio resolve to the same
> PMD Accessed bit.  Accesses to a small part of the folio can
> therefore coarsen DAMON's observed hot set relative to the actual
> working set.
> 
> DAMOS already provides promotion actions (HUGEPAGE, COLLAPSE) but
> has no corresponding demotion action.  DAMOS_SPLIT fills this gap.
> It is a mechanism, not a policy -- it does not decide which folios
> to split.  Selection is left to DAMON's existing access patterns,
> filters, and future probe/PMU signals.
You should mention why page split is be needed. The fact that page
collapsing exist doesn't necessarily mean that split should exist.

I agree that it is a nice feature, but it should be backed in the
cover letter.
> target_order selects the split target: 0 for order-0 base pages,
> or a supported smaller mTHP order.  Both anonymous and file-backed
> folios are supported.  The locking follows split_huge_pages_in_pid()
> in mm/huge_memory.c.
> 
> Tests
> =====
> 
> damos_split.py (VM + Kunpeng 920):
>   anon THP -> order-0 split: PASS
> 
> sangfor_exp.py (Kunpeng 920, tmpfs, 4096 MiB):
> 
>   - Created a 4 GiB PMD-mapped tmpfs workload.
>   - Applied DAMOS_SPLIT with target_order=0.
>   - ShmemPmdMapped dropped from 4194304 KiB to 0 in every round.
>   - Repeated for five rounds without functional failures.
> 
> The functional selftest (damos_split.py) is included in this series.
> Additional experiment scripts and raw results are available on
> request.  Performance characterization using masim [1] and KMB [2]
> is in progress.
> 
> [1] https://github.com/sjp38/masim
> [2] https://gitee.com/OpenCloudOS/kernel-multi-bench
> 
> Open questions
> ==============
> 
> - Selection policy: this series keeps folio selection outside the
>   action and relies on DAMOS access patterns, filters, and quotas.
>   Is this the appropriate layering for future probe-based signals?
> 
> - Hysteresis: khugepaged may re-collapse a just-split folio.
>   Should cooldown live in DAMON policy or khugepaged?
> 
> - File-backed folios: adjust target_order upward to filesystem
>   minimum, or keep current "fail and skip"?
> 
> Beyond the action API itself, feedback on real workloads that need
> proactive large-folio demotion is particularly welcome.  Follow-up
> work will evaluate candidate selection signals, including DAMON
> probes and hardware-assisted sampling, as well as target-order
> selection and split/collapse hysteresis.  Those policies are
> intentionally kept outside this series.
> Changes since v2 [3]
> ====================
> 
> - Split-only series (collapse deferred).
> - Dropped SPE feedback (mechanism/policy separation).
> - DAMOS_MTHP_SPLIT -> DAMOS_SPLIT.
> - order field in existing union (no struct size increase).
> - Added functional selftest (damos_split.py).
> - checkpatch: 0 errors, 0 warnings.
> 
> [3] https://lore.kernel.org/20260701123000.00000-1-lianux.mm@gmail.com/
Could you add v1 as well?
> 
> Lian Wang (Processmission) (3):
>   mm/damon: introduce DAMOS_SPLIT action
>   mm/damon/vaddr: implement DAMOS_SPLIT handler
>   selftests/damon: add functional test for DAMOS_SPLIT
> 
>  .../ABI/testing/sysfs-kernel-mm-damon         |  7 ++
>  Documentation/mm/damon/design.rst             |  5 +
>  include/linux/damon.h                         | 15 ++-
>  mm/damon/core.c                               |  2 +
>  mm/damon/sysfs-schemes.c                      | 48 +++++++
>  mm/damon/vaddr.c                              | 90 +++++++++++++
>  tools/testing/selftests/damon/Makefile        |  1 +
>  tools/testing/selftests/damon/_damon_sysfs.py |  9 +-
>  tools/testing/selftests/damon/damos_split.py  | 99 +++++++++++++++
>  tools/testing/selftests/damon/sysfs.py        | 11 +-
>  10 files changed, 279 insertions(+), 8 deletions(-)
> 

-- 
Asier Gutierrez
Huawei