memcg: use round-robin victim selection in refill_stock

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.

Link: https://lore.kernel.org/20260521223751.3794625-1-shakeel.butt@linux.dev
Fixes: f735eebe55f8f ("memcg: multi-memcg percpu charge cache")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: Harry Yoo <harry@kernel.org>
Closes: https://lore.kernel.org/4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org/
Acked-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 177732fef010bc7a13b2ef38777d591062f63d60..1a4fd2504bcdfc7469b04f1d6acc01db36815965 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2011,6 +2011,7 @@ 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) = {
@@ -2194,7 +2195,9 @@ 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);