zram: break the strict dependency from lzo

From the beginning, the zram block device always enabled CRYPTO_LZO,
since lzo-rle is hardcoded as the fallback compression algorithm.  As a
consequence, on systems where another compression algorithm is chosen
(e.g.  CRYPTO_ZSTD), the lzo kernel module becomes unused, while still
having to be built/loaded.

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

Link: https://lkml.kernel.org/r/20201207121245.50529-1-rsalvaterra@gmail.com
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
Suggested-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Suggested-by: Minchan Kim <minchan@kernel.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/Kconfig

@@ -2,7 +2,7 @@
 config ZRAM
 	tristate "Compressed RAM block device support"
 	depends on BLOCK && SYSFS && ZSMALLOC && CRYPTO
-	select CRYPTO_LZO
+	depends on CRYPTO_LZO || CRYPTO_ZSTD || CRYPTO_LZ4 || CRYPTO_LZ4HC || CRYPTO_842
 	help
 	  Creates virtual block devices called /dev/zramX (X = 0, 1, ...).
 	  Pages written to these disks are compressed and stored in memory
@@ -14,6 +14,46 @@ config ZRAM
 
 	  See Documentation/admin-guide/blockdev/zram.rst for more information.
 
+choice
+	prompt "Default zram compressor"
+	default ZRAM_DEF_COMP_LZORLE
+	depends on ZRAM
+
+config ZRAM_DEF_COMP_LZORLE
+	bool "lzo-rle"
+	depends on CRYPTO_LZO
+
+config ZRAM_DEF_COMP_ZSTD
+	bool "zstd"
+	depends on CRYPTO_ZSTD
+
+config ZRAM_DEF_COMP_LZ4
+	bool "lz4"
+	depends on CRYPTO_LZ4
+
+config ZRAM_DEF_COMP_LZO
+	bool "lzo"
+	depends on CRYPTO_LZO
+
+config ZRAM_DEF_COMP_LZ4HC
+	bool "lz4hc"
+	depends on CRYPTO_LZ4HC
+
+config ZRAM_DEF_COMP_842
+	bool "842"
+	depends on CRYPTO_842
+
+endchoice
+
+config ZRAM_DEF_COMP
+	string
+	default "lzo-rle" if ZRAM_DEF_COMP_LZORLE
+	default "zstd" if ZRAM_DEF_COMP_ZSTD
+	default "lz4" if ZRAM_DEF_COMP_LZ4
+	default "lzo" if ZRAM_DEF_COMP_LZO
+	default "lz4hc" if ZRAM_DEF_COMP_LZ4HC
+	default "842" if ZRAM_DEF_COMP_842
+
 config ZRAM_WRITEBACK
        bool "Write back incompressible or idle page to backing device"
        depends on ZRAM

drivers/block/zram/zcomp.c

@@ -15,8 +15,10 @@
 #include "zcomp.h"
 
 static const char * const backends[] = {
+#if IS_ENABLED(CONFIG_CRYPTO_LZO)
 	"lzo",
 	"lzo-rle",
+#endif
 #if IS_ENABLED(CONFIG_CRYPTO_LZ4)
 	"lz4",
 #endif

drivers/block/zram/zram_drv.c

@@ -42,7 +42,7 @@ static DEFINE_IDR(zram_index_idr);
 static DEFINE_MUTEX(zram_index_mutex);
 
 static int zram_major;
-static const char *default_compressor = "lzo-rle";
+static const char *default_compressor = CONFIG_ZRAM_DEF_COMP;
 
 /* Module params (documentation at end) */
 static unsigned int num_devices = 1;