linux-kernel - Re: [PATCH v3] zram: break the strict dependency from lzo

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Date:   Wed, 28 Oct 2020 19:19:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Rui Salvaterra <rsalvaterra@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        minchan@...nel.org, ngupta@...are.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v3] zram: break the strict dependency from lzo

Hi,

On (20/10/27 08:39), Rui Salvaterra wrote:
> Personally, I always use zram with zstd, and the only lzo dependency I
> have is zram. Disabling lzo saves me about 3 kiB in the final
> (xz-compressed) vmlinuz image. It's not much, for sure, but when your
> total storage is 4 MiB (and your RAM is 32 MiB), every bit counts. :)

Can the following work then?

Completely untested.

===8<===

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index fe7a4b7d30cf..f93eed40e155 100644
--- a/drivers/block/zram/Kconfig
+++ b/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_LZ4 || CRYPTO_LZ4HC || CRYPTO_842 || CRYPTO_ZSTD)
 	help
 	  Creates virtual block devices called /dev/zramX (X = 0, 1, ...).
 	  Pages written to these disks are compressed and stored in memory
diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 33e3b76c4fa9..98c7c46c9c3a 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
@@ -202,6 +204,21 @@ void zcomp_destroy(struct zcomp *comp)
 	kfree(comp);
 }
 
+const char *default_compressor(void)
+{
+	/*
+	 * Pick the first available one (there should be at least one).
+	 *
+	 * In theory, we can drop all the ifdefs from backends[] and
+	 * just iterate backends array doing crypto_has_comp(comp, 0, 0)
+	 * for each entry and return the first one which is recognized by
+	 * crypto. But crypto_has_comp() modprobes compression drivers,
+	 * so we may endup with extra loaded drivers, when the 'default'
+	 * compressor is not what zram is configured to use.
+	 */
+	return backends[0];
+}
+
 /*
  * search available compressors for requested algorithm.
  * allocate new zcomp and initialize it. return compressing
diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
index 40f6420f4b2e..f104be9eae9c 100644
--- a/drivers/block/zram/zcomp.h
+++ b/drivers/block/zram/zcomp.h
@@ -27,6 +27,7 @@ int zcomp_cpu_dead(unsigned int cpu, struct hlist_node *node);
 ssize_t zcomp_available_show(const char *comp, char *buf);
 bool zcomp_available_algorithm(const char *comp);
 
+const char *default_compressor(void);
 struct zcomp *zcomp_create(const char *comp);
 void zcomp_destroy(struct zcomp *comp);
 
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 1b697208d661..f02ee050c7bf 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -42,7 +42,6 @@ static DEFINE_IDR(zram_index_idr);
 static DEFINE_MUTEX(zram_index_mutex);
 
 static int zram_major;
-static const char *default_compressor = "lzo-rle";
 
 /* Module params (documentation at end) */
 static unsigned int num_devices = 1;
@@ -1960,7 +1959,9 @@ static int zram_add(void)
 	blk_queue_flag_set(QUEUE_FLAG_STABLE_WRITES, zram->disk->queue);
 	device_add_disk(NULL, zram->disk, zram_disk_attr_groups);
 
-	strlcpy(zram->compressor, default_compressor, sizeof(zram->compressor));
+	strlcpy(zram->compressor,
+		default_compressor(),
+		sizeof(zram->compressor));
 
 	zram_debugfs_register(zram);
 	pr_info("Added device: %s\n", zram->disk->disk_name);