[PATCH v3 0/2] mm/damon/core: detect internal variation above max_nr_regions/2

[SJ Park <sj@kernel.org>]

kdamond_split_regions() bails out early when nr_regions is already
above max_nr_regions / 2.  A large region that picks up new internal
variation after that point never gets split, so we lose visibility
into its hot/cold structure.

We hit this with damon-paddr on hugepage workloads and damon-vaddr
on processes that mmap a large anonymous range.

Example with max_nr_regions == 1500.  A target ends up with 799
small hot/cold regions plus one big region (an earlier merge
collapsed a uniformly-accessed range into a single piece):

H:hot
C:cold

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................|

    nr_regions = 800  >  max_nr_regions / 2 = 750

Now a cold subarea shows up inside r800:

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............|

The small regions can't merge with each other (their access counts
differ), so budget never frees up.  r800 can't be split because
nr_regions > max_nr_regions / 2 returns early.  The cold subarea
stays invisible.

Patch 1 keeps refining on this path: when nr_regions is above
max_nr_regions / 2 but still under the maximum, it splits a fraction
of the regions instead of returning.  The fraction shrinks as the
remaining budget shrinks, so the count approaches max_nr_regions
smoothly.  A useless split is undone by the next merge cycle.

Patch 2 adds a KUnit test for the case where nr_regions is already
above max_nr_regions / 2.

Thanks to SJ for the suggestion to drive the split fraction
from the remaining budget rather than an age-based filter.

Changes from v2
- v2: https://lore.kernel.org/20260626085851.70754-1-jiayuan.chen@linux.dev
- Collect R-b: from SJ.
- Rebase to latest mm-new.
Changes from v1
- v1: https://lore.kernel.org/damon/20260521045236.115749-1-jiayuan.chen@linux.dev/
- Some feedback from SJ.

Jiayuan Chen (2):
  mm/damon/core: split a fraction of regions when nr_regions exceeds
    max/2
  mm/damon/tests/core-kunit: test split above max_nr_regions/2

 mm/damon/core.c             | 49 +++++++++++++++++++++++++---
 mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+), 5 deletions(-)


base-commit: 86100fb7e27ebb5da22fc8a2810eebcf8cc897e8
-- 
2.47.3