mm: 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 __init functions have to remain in
memory after the init phase, just in case they are needed later,
but the total size is negligible.

See: #3432

Signed-off-by: Phil Elwell <[email protected]>

Author: pelwell

mm/zswap.c

@@ -212,6 +212,7 @@ static bool zswap_has_pool;
 static int zswap_writeback_entry(struct zpool *pool, unsigned long handle);
 static int zswap_pool_get(struct zswap_pool *pool);
 static void zswap_pool_put(struct zswap_pool *pool);
+static int init_zswap(void);
 
 static const struct zpool_ops zswap_zpool_ops = {
 	.evict = zswap_writeback_entry
@@ -243,13 +244,13 @@ static void zswap_update_total_size(void)
 **********************************/
 static struct kmem_cache *zswap_entry_cache;
 
-static int __init zswap_entry_cache_create(void)
+static int zswap_entry_cache_create(void)
 {
 	zswap_entry_cache = KMEM_CACHE(zswap_entry, 0);
 	return zswap_entry_cache == NULL;
 }
 
-static void __init zswap_entry_cache_destroy(void)
+static void zswap_entry_cache_destroy(void)
 {
 	kmem_cache_destroy(zswap_entry_cache);
 }
@@ -573,7 +574,7 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 	return NULL;
 }
 
-static __init struct zswap_pool *__zswap_pool_create_fallback(void)
+static struct zswap_pool *__zswap_pool_create_fallback(void)
 {
 	bool has_comp, has_zpool;
 
@@ -770,28 +771,30 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 static int zswap_compressor_param_set(const char *val,
 				      const struct kernel_param *kp)
 {
-	return __zswap_param_set(val, kp, zswap_zpool_type, NULL);
+	return __zswap_param_set(val, kp, NULL, zswap_compressor);
 }
 
 static int zswap_zpool_param_set(const char *val,
 				 const struct kernel_param *kp)
 {
-	return __zswap_param_set(val, kp, NULL, zswap_compressor);
+	return __zswap_param_set(val, kp, zswap_zpool_type, NULL);
 }
 
 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)
+		ret = init_zswap();
+
+	return ret;
 }
 
 /*********************************
@@ -1256,7 +1259,7 @@ static struct frontswap_ops zswap_frontswap_ops = {
 
 static struct dentry *zswap_debugfs_root;
 
-static int __init zswap_debugfs_init(void)
+static int zswap_debugfs_init(void)
 {
 	if (!debugfs_initialized())
 		return -ENODEV;
@@ -1294,7 +1297,7 @@ static void __exit zswap_debugfs_exit(void)
 	debugfs_remove_recursive(zswap_debugfs_root);
 }
 #else
-static int __init zswap_debugfs_init(void)
+static int zswap_debugfs_init(void)
 {
 	return 0;
 }
@@ -1305,13 +1308,16 @@ static void __exit zswap_debugfs_exit(void) { }
 /*********************************
 * module init and exit
 **********************************/
-static int __init init_zswap(void)
+static int init_zswap(void)
 {
 	struct zswap_pool *pool;
 	int ret;
 
 	zswap_init_started = true;
 
+	if (!zswap_enabled)
+	    return 0;
+
 	if (zswap_entry_cache_create()) {
 		pr_err("entry cache creation failed\n");
 		goto cache_fail;
@@ -1340,6 +1346,7 @@ static int __init init_zswap(void)
 	} else {
 		pr_err("pool creation failed\n");
 		zswap_enabled = false;
+		zswap_init_failed = true;
 	}
 
 	frontswap_register_ops(&zswap_frontswap_ops);