Re: (sashiko review) [PATCH] mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses()

[SeongJae Park <sj@kernel.org>]

On Wed, 25 Mar 2026 07:36:20 +0000 Josh Law <objecting@objecting.org> wrote:

> 
> 
> On 25 March 2026 01:16:52 GMT, SeongJae Park <sj@kernel.org> wrote:
> >Forwarding Sashiko review for doing discussions via mails.
> >
> ># review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org
> ># start of sashiko.dev inline review
> >commit ff5e227a18cd69435749b0aae0dc2c762c4f0a47
> >Author: Josh Law <objecting@objecting.org>
> >
> >mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses()
> >    
> >Hardware integer division is slow. The function damon_max_nr_accesses(),
> >which is called very frequently, performs an integer division.
> >However, the struct damon_attrs already caches this exact ratio in the
> >internal field aggr_samples. We can eliminate the hardware division in
> >the hot path by simply returning aggr_samples.
> >
> >> diff --git a/include/linux/damon.h b/include/linux/damon.h
> >> index d9a3babbafc1..04c8a052fcfb 100644
> >> --- a/include/linux/damon.h
> >> +++ b/include/linux/damon.h
> >> @@ -978,8 +978,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx)
> >>  static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs)
> >>  {
> >>  	/* {aggr,sample}_interval are unsigned long, hence could overflow */
> >> -	return min(attrs->aggr_interval / attrs->sample_interval,
> >> -			(unsigned long)UINT_MAX);
> >> +	return min_t(unsigned long, attrs->aggr_samples, UINT_MAX);
> >>  }
> >
> >If an administrator dynamically updates aggr_interval via sysfs and commits
> >the inputs, will attrs->aggr_samples be correctly recalculated?
> >
> >If aggr_samples becomes stale, kdamond_fn() will compute the real aggregation
> >loop count and allow r->nr_accesses to increment up to the new bounds (for
> >example, 80 loops). Meanwhile, damon_max_nr_accesses() will return the
> >stale aggr_samples cache (for example, 20).
> >
> >When evaluating schemes, damon_hot_score() normalizes r->nr_accesses against
> >this stale maximum, producing a freq_subscore that severely exceeds
> >DAMON_MAX_SUBSCORE. This causes the calculated hotness to exceed
> >DAMOS_MAX_SCORE.
> >
> >For DAMOS_PAGEOUT actions used by damon_reclaim, damon_cold_score() returns
> >DAMOS_MAX_SCORE - hotness, yielding a highly negative score.
> >
> >Could this negative score cause an out-of-bounds write on the
> >kernel heap when used as an array index?
> >
> >mm/damon/core.c:damos_walk() {
> >    ...
> >    c->regions_score_histogram[score] += damon_sz_region(r);
> >    ...
> >}
> >
> >> diff --git a/mm/damon/core.c b/mm/damon/core.c
> >> index db6c67e52d2b..afb058f7da5c 100644
> >> --- a/mm/damon/core.c
> >> +++ b/mm/damon/core.c
> >> @@ -587,6 +587,7 @@ struct damon_ctx *damon_new_ctx(void)
> >>  	ctx->attrs.sample_interval = 5 * 1000;
> >>  	ctx->attrs.aggr_interval = 100 * 1000;
> >>  	ctx->attrs.ops_update_interval = 60 * 1000 * 1000;
> >> +	ctx->attrs.aggr_samples = 20;
> >
> >Since this value is cached here, but not updated when intervals change, does
> >it remain permanently stale during dynamic updates?
> >
> >
> ># end of sashiko.dev inline review
> ># review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org
> >#
> ># hkml [1] generated a draft of this mail.  It can be regenerated
> ># using below command:
> >#
> >#     hkml patch sashiko_dev --for_forwarding \
> >#             20260324154005.83651-1-objecting@objecting.org
> >#
> ># [1] https://github.com/sjp38/hackermail
> >
> >Sent using hkml (https://github.com/sjp38/hackermail)
> 
> 
> 
> So it's nacked? Or is it just a review

Just a review from an AI :)

I treat sashiko review as just another review comments.  I'm forwarding these
in this way for a case that someone is interested in the comments and want to
share some findings from those in a way easy for mailing list based
communication.


Thanks,
SJ

[...]