Re: [PATCH 1/1] mm/slab_common: fix shrink budget underflow in kfree_rcu_shrink_scan

[Longlong Xia]

在 2026/8/26 10:56, Hao Li 写道:
> On Mon, Aug 24, 2026 at 05:18:38PM +0800, Longlong Xia wrote:
> > From: Longlong Xia <xialonglong@xxxxxxxxxx>
> >
> > The kfree_rcu shrinker decremented sc->nr_to_scan (unsigned long)
> > and then tested the result with <= 0. When a single CPU's object
> > count exceeds the remaining budget, the subtraction wraps to a large
> > positive value and the <= 0 comparison, which is equivalent to == 0
> > for an unsigned type, never fires again. The scan loop then iterates
> > through every possible CPU instead of honouring the reclaim budget.
> >
> > Reorder the logic to compare count against nr_to_scan before
> > subtracting, so the loop exits as soon as the budget is met.
> >
> > Assisted-by: Codex:gpt-5.6-sol
> > Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
> > ---
> >   mm/slab_common.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 657fd75776ea..95ddbab290d4 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -2172,11 +2172,11 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> >   		count += drain_page_cache(krcp);
> >   		kfree_rcu_monitor(&krcp->monitor_work.work);
> >   
> > -		sc->nr_to_scan -= count;
> >   		freed += count;
> >   
> > -		if (sc->nr_to_scan <= 0)
> > +		if (count >= sc->nr_to_scan)
> >   			break;
> > +		sc->nr_to_scan -= count;
> thanks, this is indeed a problem.
> I think we could make the code more better, for example:
>
> 1. remove `count` variable
> 2. add count to freed directly (also change type to unsigned)
> 3. check `if (freed >= sc->nr_to_scan)` then break.
>
> How does this sound to you?
Thanks for the reply.

Sounds good -- I'll adopt it for v2.

v2 incoming, with Suggested-by from you.


Thanks，

Longlong