mm/memcg: protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

The per-CPU counter are modified with the non-atomic modifier.  The
consistency is ensured by disabling interrupts for the update.  On non
PREEMPT_RT configuration this works because acquiring a spinlock_t typed
lock with the _irq() suffix disables interrupts.  On PREEMPT_RT
configurations the RMW operation can be interrupted.

Another problem is that mem_cgroup_swapout() expects to be invoked with
disabled interrupts because the caller has to acquire a spinlock_t which
is acquired with disabled interrupts.  Since spinlock_t never disables
interrupts on PREEMPT_RT the interrupts are never disabled at this
point.

The code is never called from in_irq() context on PREEMPT_RT therefore
disabling preemption during the update is sufficient on PREEMPT_RT.  The
sections which explicitly disable interrupts can remain on PREEMPT_RT
because the sections remain short and they don't involve sleeping locks
(memcg_check_events() is doing nothing on PREEMPT_RT).

Disable preemption during update of the per-CPU variables which do not
explicitly disable interrupts.

Link: https://lkml.kernel.org/r/20220226204144.1008339-4-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -629,6 +629,35 @@ static DEFINE_SPINLOCK(stats_flush_lock);
 static DEFINE_PER_CPU(unsigned int, stats_updates);
 static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
 
+/*
+ * Accessors to ensure that preemption is disabled on PREEMPT_RT because it can
+ * not rely on this as part of an acquired spinlock_t lock. These functions are
+ * never used in hardirq context on PREEMPT_RT and therefore disabling preemtion
+ * is sufficient.
+ */
+static void memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_disable();
+#else
+      VM_BUG_ON(!irqs_disabled());
+#endif
+}
+
+static void __memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_disable();
+#endif
+}
+
+static void memcg_stats_unlock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_enable();
+#endif
+}
+
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	unsigned int x;
@@ -705,6 +734,27 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	/*
+	 * The caller from rmap relay on disabled preemption becase they never
+	 * update their counter from in-interrupt context. For these two
+	 * counters we check that the update is never performed from an
+	 * interrupt context while other caller need to have disabled interrupt.
+	 */
+	__memcg_stats_lock();
+	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		switch (idx) {
+		case NR_ANON_MAPPED:
+		case NR_FILE_MAPPED:
+		case NR_ANON_THPS:
+		case NR_SHMEM_PMDMAPPED:
+		case NR_FILE_PMDMAPPED:
+			WARN_ON_ONCE(!in_task());
+			break;
+		default:
+			WARN_ON_ONCE(!irqs_disabled());
+		}
+	}
+
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +762,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	memcg_stats_unlock();
 }
 
 /**
@@ -794,8 +845,10 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	memcg_stats_lock();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	memcg_stats_unlock();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7154,8 +7207,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
 	 */
-	VM_BUG_ON(!irqs_disabled());
+	memcg_stats_lock();
 	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	memcg_stats_unlock();
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);