zram: make compression algorithm selection possible

Add and document `comp_algorithm' device attribute.  This attribute allows
to show supported compression and currently selected compression
algorithms:

	cat /sys/block/zram0/comp_algorithm
	[lzo] lz4

and change selected compression algorithm:
	echo lzo > /sys/block/zram0/comp_algorithm

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 27 insertions, 5 deletions

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 0da9ed6b82ea..70ec992514d0 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -65,6 +65,14 @@ Description:
                 number of backend's zcomp_strm compression streams (number of
                 concurrent compress operations).
 
+What:           /sys/block/zram<id>/comp_algorithm
+Date:           February 2014
+Contact:        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+                The comp_algorithm file is read-write and lets to show
+                available and selected compression algorithms, change
+                compression algorithm selection.
+
 What:           /sys/block/zram<id>/notify_free
 Date:           August 2010
 Contact:        Nitin Gupta <ngupta@vflare.org>
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index aadfe60391b7..2604ffed51db 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -42,7 +42,21 @@ will not take any effect, because single stream compression backend implemented
 as a special case and does not support dynamic max_comp_streams. Only multi
 stream backend supports dynamic max_comp_streams adjustment.
 
-3) Set Disksize
+3) Select compression algorithm
+        Using comp_algorithm device attribute one can see available and
+        currently selected (shown in square brackets) compression algortithms,
+        change selected compression algorithm (once the device is initialised
+        there is no way to change compression algorithm).
+
+        Examples:
+        #show supported compression algorithms
+        cat /sys/block/zram0/comp_algorithm
+        lzo [lz4]
+
+        #select lzo compression algorithm
+        echo lzo > /sys/block/zram0/comp_algorithm
+
+4) Set Disksize
         Set disk size by writing the value to sysfs node 'disksize'.
         The value can be either in bytes or you can use mem suffixes.
         Examples:
@@ -59,14 +73,14 @@ There is little point creating a zram of greater than twice the size of memory
 since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
 size of the disk when not in use so a huge zram is wasteful.
 
-4) Activate:
+5) Activate:
         mkswap /dev/zram0
         swapon /dev/zram0
 
         mkfs.ext4 /dev/zram1
         mount /dev/zram1 /tmp
 
-5) Stats:
+6) Stats:
         Per-device statistics are exported as various nodes under
         /sys/block/zram<id>/
                 disksize
@@ -81,11 +95,11 @@ size of the disk when not in use so a huge zram is wasteful.
                 compr_data_size
                 mem_used_total
 
-6) Deactivate:
+7) Deactivate:
         swapoff /dev/zram0
         umount /dev/zram1
 
-7) Reset:
+8) Reset:
         Write any positive value to 'reset' sysfs node
         echo 1 > /sys/block/zram0/reset
         echo 1 > /sys/block/zram1/reset