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/[email protected]/
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
