Index: sys/vm/uma_core.c
===================================================================
--- sys/vm/uma_core.c
+++ sys/vm/uma_core.c
@@ -3632,6 +3632,10 @@
 	return (cache_alloc_item(zone, cache, bucket, NULL, flags));
 }
 
+#if defined(INVARIANTS) && (defined(DDB) || defined(STACK))
+#include <sys/stack.h>
+#endif
+
 /* See uma.h */
 void *
 uma_zalloc_arg(uma_zone_t zone, void *udata, int flags)
@@ -3639,6 +3643,31 @@
 	uma_cache_bucket_t bucket;
 	uma_cache_t cache;
 
+	_Static_assert(M_NOWAIT != 0 && M_WAITOK != 0,
+	    "M_NOWAIT and M_WAITOK must be non-zero for this assertion:");
+#if 0
+	/*
+	 * Give people time to find problems with the #else option below,
+	 * then enable this instead.  (Remove <sys/stack.h> above, too.)
+	 */
+	KASSERT((flags & (M_NOWAIT|M_WAITOK)) == M_NOWAIT ||
+	    (flags & (M_NOWAIT|M_WAITOK)) == M_WAITOK,
+	    ("uma_zalloc_arg: must pass exactly one of M_NOWAIT or M_WAITOK"));
+#elif defined(INVARIANTS) && (defined(DDB) || defined(STACK))
+	if (__predict_false((flags & (M_NOWAIT|M_WAITOK)) != M_NOWAIT &&
+	    (flags & (M_NOWAIT|M_WAITOK)) != M_WAITOK)) {
+		static int stack_count;
+		struct stack st;
+
+		if (stack_count < 10) {
+			++stack_count;
+			printf("uma_zalloc_arg called with bad WAIT flags:\n");
+			stack_save(&st);
+			stack_print(&st);
+		}
+	}
+#endif
+
 	/* Enable entropy collection for RANDOM_ENABLE_UMA kernel option */
 	random_harvest_fast_uma(&zone, sizeof(zone), RANDOM_UMA);