[PATCH v5] zram: break the strict dependency from lzo

[Rui Salvaterra <rsalvaterra@gmail.com>]

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.

Suggested-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Suggested-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
---
v5: incorporate Minchan's feedback. Allow the user to choose a default algorithm.
v4: incorporate Sergey's feedback and fix a small typo.
v3: fix the default selection when lzo isn't present. Rebase against 5.10-rc1.
v2: fix the dependency on CRYPTO.

I believe this is the final version, but it does deserve some comment. Given the
choice of having more preprocessor #if/#endif directives in C files or making
the kconfig a bit more complex, I went for the latter. However, since kconfig
choices can only be boolean, I had to devise a way to make a string selection
based on the boolean choice, hence the ZRAM_DEF_COMP symbol.
I also tried to make the ZRAM_AUTOSEL_ALGO definition a bit less painful to the
eye, while still allowing for the compression algorithms to be selected as
modules, as per Sergey's suggestion.

 drivers/block/zram/Kconfig    | 49 ++++++++++++++++++++++++++++++++++-
 drivers/block/zram/zcomp.c    |  2 ++
 drivers/block/zram/zram_drv.c |  2 +-
 3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index fe7a4b7d30cf..cde089c30ffb 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -2,7 +2,6 @@
 config ZRAM
 	tristate "Compressed RAM block device support"
 	depends on BLOCK && SYSFS && ZSMALLOC && CRYPTO
-	select CRYPTO_LZO
 	help
 	  Creates virtual block devices called /dev/zramX (X = 0, 1, ...).
 	  Pages written to these disks are compressed and stored in memory
@@ -14,6 +13,54 @@ config ZRAM
 
 	  See Documentation/admin-guide/blockdev/zram.rst for more information.
 
+choice
+	prompt "Default zram compression algorithm"
+	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_AUTOSEL_ALGO
+	def_bool y
+	depends on ZRAM && \
+		!(CRYPTO_LZ4=m   || CRYPTO_LZ4=y   || \
+		  CRYPTO_LZ4HC=m || CRYPTO_LZ4HC=y || \
+		  CRYPTO_842=m   || CRYPTO_842=y   || \
+		  CRYPTO_ZSTD=m  || CRYPTO_ZSTD=y)
+	select CRYPTO_LZO
+
 config ZRAM_WRITEBACK
        bool "Write back incompressible or idle page to backing device"
        depends on ZRAM
diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 33e3b76c4fa9..052aa3f65514 100644
--- a/drivers/block/zram/zcomp.c
+++ b/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
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 1b697208d661..9ddccb968c68 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/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;
-- 
2.29.2