Re: [PATCH v3] mm/damon/core: fix unconditionally skip last region

[SJ Park <sj@kernel.org>]

On Tue,  8 Sep 2026 06:47:38 -0700 SJ Park <sj@kernel.org> wrote:

> From: Liew Rui Yan <aethernet65535@gmail.com>
> 
> Once quota set, the charge_{target,addr}_from unconditionally skips and
> resets at the last region of the tracked target, so the last region can
> be skipped even when it has not been processed.
> 
> Example:
> 
>     1. Target has 2 regions: R1 (0-100 bytes) and R2 (100-200 bytes).
>     2. Quota is configured to process only 100 bytes per window.
>     3. Window 1: Processes R1 (0-100).  Quota is full.  charge_{target,
>        addr}_from is saved at (Target, 100).
>     4. Window 2: The loop reaches R2.  Because R2 is
>        damon_last_region(t), the old code unconditionally returns true,
>        skipping R2 entirely and resetting the charge_{target,addr}_from.
> 
>     Result: R2 is permanently skipped even though it has never been
>     processed.
> 
> However, it is important to note that this is a very minor issue.  This
> is because it is triggered only when the previous window saved/kept
> charge_{target,addr}_from, and in the next window, all regions except
> the last region were skipped by damos_skip_charged_region().
> 
> Fix this by only resets the charge_{target,addr}_from when last region
> is reached, only skips when it applied or cannot split.

Sashiko found a pre-existing issue that I'm separately working on.  It is no
blocker for this patch.


Thanks,
SJ

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