zswap: Defer zswap initialisation

Enabling zswap support in the kernel configuration costs about 1.5MB
of RAM, even when zswap is not enabled at runtime. This cost can be
reduced significantly by deferring initialisation (including pool
creation) until the "enabled" parameter is set to true. There is a
small cost to this in that some initialisation code has to remain in
memory after the init phase, just in case they are needed later,
but the total size increase is negligible.

See: https://github.com/raspberrypi/linux/pull/3432

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

mm/zswap.c

@@ -594,8 +594,9 @@ error:
 	return NULL;
 }
 
-static __init struct zswap_pool *__zswap_pool_create_fallback(void)
+static bool zswap_try_pool_create(void)
 {
+	struct zswap_pool *pool;
 	bool has_comp, has_zpool;
 
 	has_comp = crypto_has_comp(zswap_compressor, 0, 0);
@@ -629,9 +630,21 @@ static __init struct zswap_pool *__zswap_pool_create_fallback(void)
 	}
 
 	if (!has_comp || !has_zpool)
-		return NULL;
+		return false;
 
-	return zswap_pool_create(zswap_zpool_type, zswap_compressor);
+	pool = zswap_pool_create(zswap_zpool_type, zswap_compressor);
+
+	if (pool) {
+		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
+			zpool_get_type(pool->zpool));
+		list_add(&pool->list, &zswap_pools);
+		zswap_has_pool = true;
+	} else {
+		pr_err("pool creation failed\n");
+		zswap_enabled = false;
+	}
+
+	return zswap_enabled;
 }
 
 static void zswap_pool_destroy(struct zswap_pool *pool)
@@ -804,16 +817,19 @@ static int zswap_zpool_param_set(const char *val,
 static int zswap_enabled_param_set(const char *val,
 				   const struct kernel_param *kp)
 {
+	int ret;
+
 	if (zswap_init_failed) {
 		pr_err("can't enable, initialization failed\n");
 		return -ENODEV;
 	}
-	if (!zswap_has_pool && zswap_init_started) {
-		pr_err("can't enable, no pool configured\n");
-		return -ENODEV;
-	}
 
-	return param_set_bool(val, kp);
+	ret = param_set_bool(val, kp);
+	if (!ret && zswap_enabled && zswap_init_started && !zswap_has_pool)
+		if (!zswap_try_pool_create())
+			ret = -ENODEV;
+
+	return ret;
 }
 
 /*********************************
@@ -1314,7 +1330,6 @@ static void __exit zswap_debugfs_exit(void) { }
 **********************************/
 static int __init init_zswap(void)
 {
-	struct zswap_pool *pool;
 	int ret;
 
 	zswap_init_started = true;
@@ -1338,29 +1353,19 @@ static int __init init_zswap(void)
 	if (ret)
 		goto hp_fail;
 
-	pool = __zswap_pool_create_fallback();
-	if (pool) {
-		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
-			zpool_get_type(pool->zpool));
-		list_add(&pool->list, &zswap_pools);
-		zswap_has_pool = true;
-	} else {
-		pr_err("pool creation failed\n");
-		zswap_enabled = false;
-	}
-
 	shrink_wq = create_workqueue("zswap-shrink");
 	if (!shrink_wq)
-		goto fallback_fail;
+		goto hp_fail;
 
 	frontswap_register_ops(&zswap_frontswap_ops);
 	if (zswap_debugfs_init())
 		pr_warn("debugfs initialization failed\n");
+
+	if (zswap_enabled)
+		zswap_try_pool_create();
+
 	return 0;
 
-fallback_fail:
-	if (pool)
-		zswap_pool_destroy(pool);
 hp_fail:
 	cpuhp_remove_state(CPUHP_MM_ZSWP_MEM_PREPARE);
 dstmem_fail: