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

[Hao Li]

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,
Hao