Re: [RFC PATCH v1.2 14/19] mm/damon/core: get merge threshold from probe hits when weights are set

[SJ Park <sj@kernel.org>]

On Thu, 09 Jul 2026 14:49:01 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Missing validation for `apply_probes` callback when probe weights are configured leads to complete DAMON region collapse.
> --
> 
> --- Patch [14]: [RFC PATCH v1.2 14/19] mm/damon/core: get merge threshold from probe hits when weights are set ---
> commit a4d0e22d5d4d741ec6086d8f49fefbab2c3b57bd
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: get merge threshold from probe hits when weights are set
> 
> When probe weights are set, DAMON merges regions based on their probe
> hits weighted sum.  But the merge threshold is calculated based on the
> access frequency.  Update it to retrieve the maximum probe hits weighted
> sum in the snapshot from apply_probes() ops callback, and generate the
> threshold based on it.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 8837b2129baa6..12c66edecd783 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -3776,9 +3777,18 @@ static int kdamond_fn(void *data)
> >  
> >  		if (!access_check_disabled && ctx->ops.check_accesses)
> >  			max_merge_score = ctx->ops.check_accesses(ctx);
> > -		if (ctx->ops.apply_probes)
> > -			ctx->ops.apply_probes(ctx, access_check_disabled,
> > -					false);
> > +		if (ctx->ops.apply_probes) {
> > +			if (time_after_eq(ctx->passed_sample_intervals,
> > +						next_aggregation_sis) &&
> > +					access_check_disabled)
> > +				get_max_wsum = true;
> > +			else
> > +				get_max_wsum = false;
> > +			max_wsum = ctx->ops.apply_probes(ctx,
> > +					access_check_disabled, get_max_wsum);
> > +			if (get_max_wsum)
> > +				max_merge_score = max_wsum;
> > +		}
> 
> [Severity: High]
> If a configuration has probe weights enabled (so access_check_disabled is
> true) but the active operations do not implement the apply_probes callback
> (like vaddr or fvaddr), does max_merge_score remain 0 here?
> 
> It appears that skipping both check_accesses() and apply_probes() leaves
> max_merge_score at 0. Later, when kdamond_merge_regions() is called with a
> threshold of 0, and all regions have 0 probe hits, damon_merge_regions_of()
> would see a difference of 0 between adjacent regions.
> 
> Could this lead to aggressive merging of all regions up to sz_limit,
> effectively collapsing the fine-grained region layout?
> 
> Should there be a validation check in damon_valid_probe_params() to reject
> configurations where probe weights are set but ctx->ops.apply_probes is
> missing?

No, this is an intentional design.  The user should correctly config DAMON.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260709140600.90950-1-sj@kernel.org?part=14


Thanks,
SJ