Re: [PATCH v2 0/2] mm/damon/core: detect internal variation above max_nr_regions/2

[SeongJae Park <sj@kernel.org>]

On Fri, 26 Jun 2026 16:58:36 +0800 Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> Sorry for the late.

No worry!

> 
> 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.

I assume you confirmed this change is making somee progress in a test or on
your real use case?  It would be great if you can confirm that and/or even
share your measurements.

> 
> Patch 2 adds a KUnit test for the case where nr_regions is already
> above max_nr_regions / 2.
> 
> Thanks to SeongJae for the suggestion to drive the split fraction
> from the remaining budget rather than an age-based filter.

I'm glad to help this grateful improvement!

> 
> 
> v1 -> v2: Some feedback from SJ.
> v1: https://lore.kernel.org/damon/20260521045236.115749-1-jiayuan.chen@linux.dev/
> 
> 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

I have trivial comments to the second patch, but all looks good enough.

I will apply this series to damon/next [1] tree.  If this patch is not added to
mm.git in short term (~1 week?), I will ask mm.git maintainer (Andrew Morton)
to pick this.  So, no action from your side is needed for now.  If it seems I
also forgot doing that or you cannot wait for my action, please feel free to
directly ask that to Andrew.

[1] https://origin.kernel.org/doc/html/latest/mm/damon/maintainer-profile.html#scm-trees


Thanks,
SJ

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