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

[SJ Park]

On Sun, 30 Aug 2026 11:14:52 -0700 SJ Park <sj@xxxxxxxxxx> wrote:

> 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>

This patch is applied 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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