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

[Longlong Xia]

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;
 	}
 
 	return freed == 0 ? SHRINK_STOP : freed;
-- 
2.43.0