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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/20/26 04:03, 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.
> 
> 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

Hi,

you give no real motivation and evaluation why this is required or why this
gives the user any benefit.

HUGEPAGE + COLLAPSE is clear: give me THPs in a size not controlled by user
space, because the expectation is that this memory will be performance sensitive.

A SPLIT with an explicit order is not really want we want and it does not fit
the existing primitives.

-- 
Cheers,

David