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

[SJ Park <sj@kernel.org>]

On Sun, 08 Nov 2026 20:23:37 +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 17459070b1916b2d77f607f0fb7f0f9158e6224f
> 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() if 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 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;
> 
> [Severity: Medium]
> Does jumping to update_stat here falsely inflate s->stat.nr_tried and
> s->stat.sz_tried when damon_split_region_at() fails?
> 
> At this point sz is the non-zero remaining quota size, and
> damos_update_stat(s, sz, sz_applied) unconditionally 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 split failure, every subsequent
> matching region in the walk that exceeds the remaining quota and fails to
> split will also add sz to s->stat.sz_tried again.  Returning early here, as
> damos_filter_out() does just below, would avoid counting skipped regions as
> tried.

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

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261108201456.116824-1-sj@kernel.org?part=1
> 


Thanks,
SJ