[PATCH] memcg: use round-robin victim selection in refill_stock

[Shakeel Butt <shakeel.butt@linux.dev>]

Harry Yoo reported that get_random_u32_below() is not safe to call in
the nmi context and memcg charge draining can happen in nmi context.

More specifically get_random_u32_below() is neither reentrant- nor
NMI-safe: it acquires a per-cpu local_lock via local_lock_irqsave() on
the batched_entropy_u32 state. An NMI that lands on a CPU mid-update of
the ChaCha batch state and recurses into the random subsystem would
corrupt that state. The memcg_stock local_trylock prevents re-entry
on the percpu stock itself, but cannot protect an unrelated
subsystem's per-cpu lock.

Replace the random pick with a per-cpu round-robin counter stored in
memcg_stock_pcp and serialized by the same local_trylock that already
guards cached[] and nr_pages[]. No atomics, no random calls, no extra
locks needed.

Fixes: f735eebe55f8f ("memcg: multi-memcg percpu charge cache")
Reported-by: Harry Yoo <harry@kernel.org>
Closes: https://lore.kernel.org/4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org/
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/memcontrol.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0eb50e639f0a..6392a2704441 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2031,6 +2031,7 @@ struct memcg_stock_pcp {
 
 	struct work_struct work;
 	unsigned long flags;
+	uint8_t drain_idx;
 };
 
 static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
@@ -2214,7 +2215,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	if (!success) {
 		i = empty_slot;
 		if (i == -1) {
-			i = get_random_u32_below(NR_MEMCG_STOCK);
+			i = stock->drain_idx++;
+			if (stock->drain_idx == NR_MEMCG_STOCK)
+				stock->drain_idx = 0;
 			drain_stock(stock, i);
 		}
 		css_get(&memcg->css);
-- 
2.53.0-Meta