Re: [PATCH 6.18.y v2] mm/damon/core: disallow time-quota setting zero esz

[SeongJae Park <sj@kernel.org>]

On Wed, 06 May 2026 20:40:45 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Incomplete fix: DAMOS scheme can still unexpectedly deactivate if using quota goals without a time quota.

No valid issue.  Read below for details, please.

> --
> 
> commit ce7d104c8087fff14fb782b89848659312c204fa
> Author: SeongJae Park <sj@kernel.org>
> Subject: mm/damon/core: disallow time-quota setting zero esz
> 
> This commit addresses an issue where DAMOS time quota can reduce the
> effective size quota below the minimum region size. This causes no actions
> to be applied, unexpectedly deactivating the scheme. The fix enforces a
> lower bound of min_sz_region on the effective size quota when a time quota
> is used.
> 
> Link: https://lore.kernel.org/20260407003153.79589-1-sj@kernel.org
> Link: https://lore.kernel.org/20260405192504.110014-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 87b6c9c2d6471..feec1b0c2babe 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -2150,6 +2151,7 @@ static void damos_set_effective_quota(struct damos_quota *quota)
> >  		else
> >  			throughput = PAGE_SIZE * 1024;
> >  		esz = min(throughput * quota->ms, esz);
> > +		esz = max(ctx->min_sz_region, esz);
> >  	}
> >  
> >  	if (quota->sz && quota->sz < esz)
> 
> Could this still leave a case where the scheme is unexpectedly deactivated
> if quota goals are used without a time quota?
> 
> If a scheme is configured with quota goals (!list_empty(&quota->goals)) but
> no time quota (quota->ms == 0), the bounds check added here inside the
> if (quota->ms) block is bypassed.
> 
> When the system metric over-achieves, the feedback loop in
> damon_feed_loop_next_input() can limit quota->esz_bp to its minimum of
> 10000, which evaluates to an esz of 1:
> 
> 	if (!list_empty(&quota->goals)) {
> 		unsigned long score = damos_quota_score(quota);
> 
> 		quota->esz_bp = damon_feed_loop_next_input(
> 				max(quota->esz_bp, 10000UL),
> 				score);
> 		esz = quota->esz_bp / 10000;
> 	}
> 
> During scheme application, damos_apply_scheme() calculates the size using
> ALIGN_DOWN(1 - 0, ctx->min_sz_region). Since min_sz_region is typically
> PAGE_SIZE, this evaluates to 0 and the action is skipped entirely.
> 
> Because the feedback loop increases esz_bp multiplicatively, it would take
> multiple charge windows for esz to grow from 1 back to min_sz_region after
> the score drops. During this period, the scheme applies 0 actions despite
> the score indicating they are needed.

Yes, and this is the intended behavior.

> 
> Should the max(ctx->min_sz_region, esz) bound be applied outside the
> if (quota->ms) block to ensure any auto-tuned esz is properly bounded?

Because it is an intended behavior, no.


Thanks,
SJ

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