Re: [PATCH v2] mm/damon/core: fix unconditionally skip last region

[SJ Park]

On Sun, 30 Aug 2026 14:31:50 +0800 Liew Rui Yan <aethernet65535@xxxxxxxxx> wrote:

> 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.

Looks good to me!

> 
> Fixes: 50585192bc2e ("mm/damon/schemes: skip already charged targets and regions")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.16.x
> Signed-off-by: Liew Rui Yan <aethernet65535@xxxxxxxxx>

Reviewed-by: SJ Park <sj@xxxxxxxxxx>


Thanks,
SJ

[...]