Re: [RFC PATCH 6.18.y 1/5] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park <sj@kernel.org>]

On Sat, 07 Nov 2026 20:18:33 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Low] mm/damon: damos_apply_scheme() inflates stats on region split
>   failure
> --
> 
> commit dd2b638af6fec217ed7123ab31fe119102f17f78
> 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 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 8fa788b0d709c..4c536a9b958e4 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -2000,7 +2000,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
> >  					c->min_sz_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: Low]
> 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 holds the non-zero aligned quota remainder, so
> damos_update_stat() increments s->stat.nr_tried and adds sz to
> s->stat.sz_tried even though damos_filter_out() and c->ops.apply_scheme()
> were skipped.  Because quota->charged_sz is not updated on this failure
> path, subsequent oversized matching regions in the walk will repeatedly add
> sz to s->stat.sz_tried while allocation keeps failing.

Good finding.  But too trivial to block this patch, in my opinion.

It happens under small memory allocation failure, which should be very rare or
even could be argued as too-small-to-fail.  The inflation would also be also
capped to quota size.  Hence it is trivial.

Also, same issue is in the mainline.  I will consider fixing it in mainline
first and add Cc:stable@ tag.  I might conclude not to fix this, since the
user impact is quite trivial.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261107201001.206120-1-sj@kernel.org?part=1
> 


Thanks,
SJ