Re: [RFC PATCH 6.6.y 1/3] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park <sj@kernel.org>]

On Sat, 07 Nov 2026 22:33:38 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Medium] mm/damon: false tried stats on split failure in
>   damos_apply_scheme()
> --
> 
> commit ae5a2312a946a6108ba6414ca8a1f6fb984dd078
> 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
> damos_apply_scheme() when splitting a region to fit within the remaining
> 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 b5b6d00d7a3b5..9d4a78deaa06a 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -1004,7 +1004,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
> >  					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;
> >  		}
> >  		if (damos_filter_out(c, t, r, s))
> >  			return;
> 
> [Severity: Medium]
> Does jumping to update_stat when damon_split_region_at() fails inflate
> s->stat.nr_tried and s->stat.sz_tried?
> 
> At this point sz is non-zero, so damos_update_stat(s, sz, sz_applied)
> increments s->stat.nr_tried and adds the remaining quota size sz to
> s->stat.sz_tried even though scheme application was skipped before
> damos_filter_out() or c->ops.apply_scheme() ran.  Because quota->charged_sz
> is not updated either, every subsequent matching region in the same walk
> that fails to split will add sz to s->stat.sz_tried again.  Returning
> directly instead of jumping to update_stat would avoid accounting the
> skipped region as tried.

Good finding, but no blocker.  The user impact is trivial.  Also same issue
exists in the mainline.  I will think about whether to fix it in mainline and
backport to stable.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=1
> 


Thanks,
SJ