mm/swap_slots.c: add warning if swap slots cache failed to initialize

Add a warning diagnostics to user if we failed to allocate swap slots
cache and use it.

[akpm@linux-foundation.org: use WARN_ONCE return value, fix grammar in message]
Link: http://lkml.kernel.org/r/20170328234827.GA10107@linux.intel.com
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_slots.c

@@ -241,8 +241,10 @@ int enable_swap_slots_cache(void)
 
 	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "swap_slots_cache",
 				alloc_swap_slot_cache, free_slot_cache);
-	if (ret < 0)
+	if (WARN_ONCE(ret < 0, "Cache allocation failed (%s), operating "
+			       "without swap slots cache.\n", __func__))
 		goto out_unlock;
+
 	swap_slot_cache_initialized = true;
 	__reenable_swap_slots_cache();
 out_unlock: