Re: [PATCH v2 1/2] mm/damon/core: split a fraction of regions when nr_regions exceeds max/2

[SeongJae Park <sj@kernel.org>]

On Fri, 26 Jun 2026 09:09:15 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Data race on the globally shared static variable `rotation` causes undefined behavior and inter-context interference.
> --
> 
> commit 2895b546218b4ccc570b82e50e62f788bac4157b
> Author: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> mm/damon/core: split a fraction of regions when nr_regions exceeds max/2
> 
> This commit modifies kdamond_split_regions() to split only a fraction of
> regions rather than returning early when nr_regions is above max_nr_regions / 2.
> It uses a rotating offset to ensure different regions are picked for splitting
> over time as the region count smoothly approaches max_nr_regions.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > +/* Split one in every @split_step regions into two, from a rotating offset */
> > +static void damon_split_some_regions(struct damon_ctx *ctx,
> > +				     unsigned long split_step)
> > +{
> > +	static unsigned long rotation;
> > +	struct damon_target *t;
> > +	struct damon_region *r, *next;
> > +	unsigned long offset = rotation++ % split_step;
> 
> [Severity: Medium]
> Will this cause a data race if multiple DAMON contexts are running concurrently?
> 
> Since rotation is declared static and locklessly incremented, independent
> kdamond worker threads could read and mutate it simultaneously.
> 
> This unsynchronized read-modify-write on shared state might trigger automated
> data race reports. Additionally, does this mean different contexts will step
> on each other's rotation offsets, breaking the intended round-robin behavior
> for each individual context?

I don't think this matters.  We don't really need strict round-robin but a
level of fairness.  Let me know if I'm wrong.


Thanks,
SJ

[...]