[PATCH] zram: update documentation

[Sergey Senozhatsky]

From: Sergey SENOZHATSKY <sergey.senozhatsky@xxxxxxxxx>

Several documentation changes.

First, we never mentioned any errors and never required any
error checks in documentation.

Second, mention that there is another way to configure and
manage zram devices -- zramctl, provided by util-linux.

Third, add a bit of clarification on why `mem_used_max' attr
is RW and correct some typos.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 Documentation/blockdev/zram.txt | 44 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 62435bb..6b80cf5 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -14,8 +14,43 @@ Statistics for individual zram devices are exported through sysfs nodes at
 
 * Usage
 
+There are several ways to configure and manage zram device(-s):
+a) using zram and zram_control sysfs attributes
+b) using zramctl utility, provided by util-linux (util-linux@xxxxxxxxxxxxxxx).
+
+In this document we will describe only 'manual' zram configuration steps,
+IOW, zram and zram_control sysfs attributes.
+
+In order to get a better idea about zramctl please consult util-linux
+documentation, zramctl man-page or `zramctl --help'. Please be informed
+that zram maintainers do not develop/maintain util-linux or zramctl, should
+you have a questions please contact util-linux@xxxxxxxxxxxxxxx
+
 Following shows a typical sequence of steps for using zram.
 
+WARNING
+=======
+For the sake of simplicity we skip error checking parts in most of the
+examples below. However, it is your sole responsibility to handle errors.
+
+zram sysfs attributes always return negative values in case of errors.
+The list of possible return codes:
+-EBUSY	-- an attempt to modify an attribute that cannot be changed once
+the device has been initialised. Please reset device first;
+-ENOMEM	-- zram was not able to allocate enough memory to fulfil your
+needs;
+-EINVAL	-- invalid input has been provided.
+
+If you use 'echo', the returned value that is changed by 'echo' utility,
+and, in general case, something like:
+
+	echo 3 > /sys/block/zram0/max_comp_streams
+	if [ $? -ne 0 ];
+		handle_error
+	fi
+
+should suffice.
+
 1) Load Module:
 	modprobe zram num_devices=4
 	This creates 4 devices: /dev/zram{0,1,2,3}
@@ -47,7 +82,7 @@ max_comp_streams adjustment.
 
 3) Select compression algorithm
 	Using comp_algorithm device attribute one can see available and
-	currently selected (shown in square brackets) compression algortithms,
+	currently selected (shown in square brackets) compression algorithms,
 	change selected compression algorithm (once the device is initialised
 	there is no way to change compression algorithm).
 
@@ -119,7 +154,7 @@ execute
 8) Stats:
 Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 
-A brief description of exported device attritbutes. For more details please
+A brief description of exported device attributes. For more details please
 read Documentation/ABI/testing/sysfs-block-zram.
 
 Name            access            description
@@ -140,8 +175,9 @@ zero_pages        RO    the number of zero filled pages written to this disk
 orig_data_size    RO    uncompressed size of data stored in this disk
 compr_data_size   RO    compressed size of data stored in this disk
 mem_used_total    RO    the amount of memory allocated for this disk
-mem_used_max      RW    the maximum amount memory zram have consumed to
-                        store compressed data
+mem_used_max      RW    the maximum amount of memory zram have consumed to
+                        store the data (to reset this counter to the actual
+                        current value, write 1 to this attribute)
 mem_limit         RW    the maximum amount of memory ZRAM can use to store
                         the compressed data
 pages_compacted   RO    the number of pages freed during compaction
-- 
2.5.3

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