zswap: break the strict dependency from lz4

From the beginning, the zswap block device has always required CRYPTO_LZ4 to be enabled, since lz4 is hardcoded as the default compression algorithm. As a consequence, when a different compression algorithm is hardcoded and the respective CRYPTO_<algo> is not enabled, zswap would not be used therefore causing an issue when swap is needed.

This patch removes the hardcoded lz4 dependency and allows the user to select the default compression algorithm for zswap at build time. The previous behaviour is kept, as the default compression algorithm is still lz4.

This patch was inspired by [1].

[1]: torvalds/linux@3d711a3

Signed-off-by: Cyber Knight <cyberknight755@gmail.com>

Author: cyberknight777

mm/Kconfig

@@ -571,7 +571,7 @@ config MEM_SOFT_DIRTY
 config ZSWAP
 	bool "Compressed cache for swap pages (EXPERIMENTAL)"
 	depends on FRONTSWAP && CRYPTO=y
-	select CRYPTO_LZ4
+	depends on CRYPTO_LZ4 || CRYPTO_ZSTD || CRYPTO_LZO
 	select ZPOOL
 	select BTREE
 	default n
@@ -589,6 +589,31 @@ config ZSWAP
 	  they have not be fully explored on the large set of potential
 	  configurations and workloads that exist.
 
+choice
+        prompt "Default zswap compressor"
+        default ZSWAP_DEF_COMP_LZ4
+        depends on ZSWAP
+
+config ZSWAP_DEF_COMP_LZ4
+        bool "lz4"
+        depends on CRYPTO_LZ4
+
+config ZSWAP_DEF_COMP_ZSTD
+        bool "zstd"
+        depends on CRYPTO_ZSTD
+
+config ZSWAP_DEF_COMP_LZO
+        bool "lzo"
+        depends on CRYPTO_LZO
+
+endchoice
+
+config ZSWAP_DEF_COMP
+       string
+        default "lzo" if ZSWAP_DEF_COMP_LZO
+        default "zstd" if ZSWAP_DEF_COMP_ZSTD
+        default "lz4" if ZSWAP_DEF_COMP_LZ4
+
 config ZPOOL
 	tristate "Common API for compressed memory storage"
 	default n

mm/zswap.c

@@ -85,8 +85,7 @@ static const struct kernel_param_ops zswap_enabled_param_ops = {
 module_param_cb(enabled, &zswap_enabled_param_ops, &zswap_enabled, 0644);
 
 /* Crypto compressor to use */
-#define ZSWAP_COMPRESSOR_DEFAULT "lz4"
-static char *zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
+static char *zswap_compressor = CONFIG_ZSWAP_DEF_COMP;
 static int zswap_compressor_param_set(const char *,
 				      const struct kernel_param *);
 static const struct kernel_param_ops zswap_compressor_param_ops = {
@@ -513,11 +512,11 @@ static __init struct zswap_pool *__zswap_pool_create_fallback(void)
 	bool has_comp, has_zpool;
 
 	has_comp = crypto_has_comp(zswap_compressor, 0, 0);
-	if (!has_comp && strcmp(zswap_compressor, ZSWAP_COMPRESSOR_DEFAULT)) {
+	if (!has_comp && strcmp(zswap_compressor, CONFIG_ZSWAP_DEF_COMP)) {
 		pr_err("compressor %s not available, using default %s\n",
-		       zswap_compressor, ZSWAP_COMPRESSOR_DEFAULT);
+		       zswap_compressor, CONFIG_ZSWAP_DEF_COMP);
 		param_free_charp(&zswap_compressor);
-		zswap_compressor = ZSWAP_COMPRESSOR_DEFAULT;
+		zswap_compressor = CONFIG_ZSWAP_DEF_COMP;
 		has_comp = crypto_has_comp(zswap_compressor, 0, 0);
 	}
 	if (!has_comp) {