Re: [PATCH] mm/damon/core: reset invalid quota->charge_target_from

[SJ Park <sj@kernel.org>]

On Mon, 07 Sep 2026 17:13:46 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] The newly added fix for invalid `charge_target_from` pointers is bypassed if a scheme's quota is dynamically disabled, leaving the scheme permanently broken.
> --
> 
> commit f88287bb2670346a5987d086e0731c7ba6d0b19e
> Author: SJ Park <sj@kernel.org>
> 
>     mm/damon/core: reset invalid quota->charge_target_from
>     
>     DAMOS can suddenly stop working if a target process that quota just
>     fully charged in it is terminated.  Fix by catching and processing the
>     corner case.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index ce8c6f99106ed..2c58be9edbde4 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -3318,6 +3334,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
> >  			damos_trace_esz(c, s, quota);
> >  	}
> >  
> > +	damos_reset_invalid_charge_target_from(quota, c);
> > +
> >  	if (!c->ops.get_scheme_score)
> >  		return;
> >  
> 
> [Severity: High]
> This is a pre-existing issue, but does this bypass the newly added reset if a
> scheme's quota is dynamically disabled?
> 
> If a user hits the corner case where charge_target_from becomes invalid, and
> then dynamically disables the scheme's quota via sysfs (setting ms=0 and
> sz=0), damos_adjust_quota() will return early before the reset is reached:
> 
> mm/damon/core.c:damos_adjust_quota() {
> 	if (!quota->ms && !quota->sz && list_empty(&quota->goals))
> 		return;
> 	...
> }
> 
> Because damos_skip_charged_region() does not check if the quota is enabled
> before evaluating charge_target_from, it will continue to skip all regions
> forever:
> 
> mm/damon/core.c:damos_skip_charged_region() {
> 	...
> 		if (t != quota->charge_target_from)
> 			return true;
> 	...
> }
> 
> Could the pointer reset be moved before the early return in
> damos_adjust_quota(), or could the skipping logic be updated to handle
> disabled quotas?

Good catch.  I think we should unset the charge_{target,addr}_from in
damos_commit_quota().  I will do so in the next version.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260907165529.98874-1-sj@kernel.org?part=1


Thanks,
SJ