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

[SJ Park <sj@kernel.org>]

On Sat, 07 Nov 2026 22:56:53 +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 2d405ecddea503227b3b4138fe63cbeb95dc50b9
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: skip applying scheme if region split for quota fails
> 
> This commit changes damon_split_region_at() to return an error on memory
> allocation failure and skips applying the DAMOS scheme in
> damon_do_apply_schemes() when splitting a region to fit within the size
> 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;
> 
> [Severity: Low]
> Does jumping to update_stat here inflate s->stat.nr_tried and
> s->stat.sz_tried when damon_split_region_at() fails?
> 
> Unlike the !sz path right above, sz holds the non-zero remaining quota
> when damon_split_region_at() is called, and update_stat unconditionally
> accounts it:
> 
> mm/damon/core.c:damon_do_apply_schemes() {
> 	...
> update_stat:
> 		s->stat.nr_tried++;
> 		s->stat.sz_tried += sz;
> 	...
> }
> 
> Because c->ops.apply_scheme() is skipped and quota->charged_sz is not
> advanced, any subsequent matching regions in the same pass that also fail
> to split will repeatedly add sz to s->stat.sz_tried.

Good finding, but the user impact is trivial.  Same behavior exists in the
mainline, too.  Hence no blocker for this patch.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261107224800.340250-1-sj@kernel.org?part=1
> 


Thanks,
SJ