Re: [PATCH 6.1.y 1/2] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park <sj@kernel.org>]

On Sun, 08 Nov 2026 20:36:18 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Low] mm/damon: inflated tried stats in damon_do_apply_schemes() on
>   split failure
> --
> 
> commit 3b087258b4341cba87012b7cb96ed676fd164c12
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: skip applying scheme if region split for quota fails
> 
> This commit updates damon_split_region_at() to return an error code on
> allocation failure and skips applying the DAMOS scheme in
> damon_do_apply_schemes() when splitting a region to fit the quota fails.
> 
> Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
> Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
> Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index f0da5cb2b88a0..9206737d05ae7 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -810,7 +810,8 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
> >  						DAMON_MIN_REGION);
> >  				if (!sz)
> >  					goto update_stat;
> > -				damon_split_region_at(t, r, sz);
> > +				if (damon_split_region_at(t, r, sz))
> > +					goto update_stat;
> >  			}
> >  			ktime_get_coarse_ts64(&begin);
> >  			sz_applied = c->ops.apply_scheme(c, t, r, s);
> 
> [Severity: Low]
> Does jumping to update_stat on damon_split_region_at() failure inflate
> s->stat.nr_tried and s->stat.sz_tried?
> 
> When damon_split_region_at() fails, sz is the non-zero remaining quota
> size, c->ops.apply_scheme() is skipped, and quota->charged_sz is not
> advanced:
> 
> mm/damon/core.c:damon_do_apply_schemes() {
> 	...
> update_stat:
> 		s->stat.nr_tried++;
> 		s->stat.sz_tried += sz;
> 	...
> }
> 
> Because quota->charged_sz remains unchanged, subsequent matching regions
> can repeatedly add sz to s->stat.sz_tried if allocation continues to fail.

Trivial user impact issue.  The same behavior exists in the mainline.  No
blocker for this patch.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261108202759.117377-1-sj@kernel.org?part=1
> 


Thanks,
SJ