Re: (sashiko review) [RFC PATCH v5 01/11] mm/damon/core: handle <min_region_sz remaining quota as empty

[SeongJae Park <sj@kernel.org>]

TL; DR: sashiko found a good finding that better to be done as a separate
cleanup work.  This patch still seems good to go as-is.

Forwarding sashiko review in a reply format with my inline
comments below, for details of my view and doing discussions
via mails if needed.

> # review url: https://sashiko.dev/#/patchset/20260410142034.83798-2-sj@kernel.org
> # start of sashiko.dev inline review
> commit 2ab7d52b1eb10efa95d43a8eee7c50fd9669d56d
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/core: handle <min_region_sz remaining quota as empty
> 
> This commit updates the core DAMOS logic to consider a quota fully charged
> if the remaining quota is less than the minimum region size. This prevents
> actions from being applied too frequently to specific memory areas.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 12544c60531d3..8dbc365bb8e97 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -2127,8 +2140,7 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
> >  			continue;
> >  
> >  		/* Check the quota */
> > -		if (damos_quota_is_set(quota) &&
> > -				quota->charged_sz >= quota->esz)
> > +		if (damos_quota_is_full(quota, c->min_region_sz))
> >  			continue;
> >  
> >  		if (damos_skip_charged_region(t, r, s, c->min_region_sz))
> 
> With this new check skipping regions when the remaining quota is less than
> the minimum region size, does the truncation check in damos_apply_scheme()
> become unreachable?
> 
> Because damon_do_apply_schemes() will now only call damos_apply_scheme()
> when quota->esz - quota->charged_sz >= c->min_region_sz, the ALIGN_DOWN
> calculation:
> 
> damos_apply_scheme() {
>     ...
> 	if (c->ops.apply_scheme) {
> 		if (damos_quota_is_set(quota) &&
> 				quota->charged_sz + sz > quota->esz) {
> 			sz = ALIGN_DOWN(quota->esz - quota->charged_sz,
> 					c->min_region_sz);
> 			if (!sz)
> 				goto update_stat;
>     ...
> }
> 
> will always result in a value greater than or equal to c->min_region_sz.
> 
> Could the if (!sz) check and the corresponding goto update_stat be safely
> removed since sz can no longer evaluate to 0?

Good finding, but doing that would better to be a separate cleanup work.  I
will add that to my todo list, but keep this patch as is.

> 
> 
> # end of sashiko.dev inline review
> # review url: https://sashiko.dev/#/patchset/20260410142034.83798-2-sj@kernel.org

Thanks,
SJ

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