[PATCH v1 9/9] zram: add config and doc file for writeback feature

[Minchan Kim <minchan@kernel.org>]

This patch adds document and kconfig for using of writeback feature.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 Documentation/ABI/testing/sysfs-block-zram |  8 ++++++++
 Documentation/blockdev/zram.txt            | 11 +++++++++++
 drivers/block/zram/Kconfig                 | 12 ++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 451b6d8..c1513c7 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -90,3 +90,11 @@ Contact:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
 		device's debugging info useful for kernel developers. Its
 		format is not documented intentionally and may change
 		anytime without any notice.
+
+What:		/sys/block/zram<id>/backing_dev
+Date:		June 2017
+Contact:	Minchan Kim <minchan@kernel.org>
+Description:
+		The backing_dev file is read-write and set up backing
+		device for zram to write incompressible pages.
+		For using, user should enable CONFIG_ZRAM_WRITEBACK.
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 4fced8a..257e657 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -168,6 +168,7 @@ max_comp_streams  RW    the number of possible concurrent compress operations
 comp_algorithm    RW    show and change the compression algorithm
 compact           WO    trigger memory compaction
 debug_stat        RO    this file is used for zram debugging purposes
+backing_dev	  RW	set up backend storage for zram to write out
 
 
 User space is advised to use the following files to read the device statistics.
@@ -231,5 +232,15 @@ The stat file represents device's mm statistics. It consists of a single
 	resets the disksize to zero. You must set the disksize again
 	before reusing the device.
 
+* Optional Feature
+
+= writeback
+
+With incompressible pages, there is no memory saving with zram.
+Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
+to backing storage rather than keeping it in memory.
+User should set up backing device via /sys/block/zramX/backing_dev
+before disksize setting.
+
 Nitin Gupta
 ngupta@vflare.org
diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index b8ecba6..7cd4a8e 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -13,3 +13,15 @@ config ZRAM
 	  disks and maybe many more.
 
 	  See zram.txt for more information.
+
+config ZRAM_WRITEBACK
+       bool "Write back incompressible page to backing device"
+       depends on ZRAM
+       default n
+       help
+	 With incompressible page, there is no memory saving to keep it
+	 in memory. Instead, write it out to backing device.
+	 For this feature, admin should set up backing device via
+	 /sys/block/zramX/backing_dev.
+
+	 See zram.txt for more infomration.
-- 
2.7.4