Merge 4041d09a on remote branch

		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.

What:		/sys/block/zram<id>/idle

Date:		November 2018

Contact:	Minchan Kim <minchan@kernel.org>

Description:

		idle file is write-only and mark zram slot as idle.

		If system has mounted debugfs, user can see which slots

		are idle via /sys/kernel/debug/zram/zram<id>/block_state

What:		/sys/block/zram<id>/writeback

Date:		November 2018

Contact:	Minchan Kim <minchan@kernel.org>

Description:

		The writeback file is write-only and trigger idle and/or

		huge page writeback to backing device.

What:		/sys/block/zram<id>/bd_stat

Date:		November 2018

Contact:	Minchan Kim <minchan@kernel.org>

Description:

		The bd_stat file is read-only and represents backing device's

		statistics (bd_count, bd_reads, bd_writes) in a format

		similar to block layer statistics file format.

What:		/sys/block/zram<id>/writeback_limit_enable

Date:		November 2018

Contact:	Minchan Kim <minchan@kernel.org>

Description:

		The writeback_limit_enable file is read-write and specifies

		eanbe of writeback_limit feature. "1" means eable the feature.

		No limit "0" is the initial state.

What:		/sys/block/zram<id>/writeback_limit

Date:		November 2018

Contact:	Minchan Kim <minchan@kernel.org>

Description:

		The writeback_limit file is read-write and specifies the maximum

		amount of writeback ZRAM can do. The limit could be changed

		in run time.

mem_used_max      	WO	reset the `mem_used_max' counter (see later)

mem_limit         	WO	specifies the maximum amount of memory ZRAM can use

				to store the compressed data

writeback_limit   	WO	specifies the maximum amount of write IO zram can

				write out to backing device as 4KB unit

writeback_limit_enable  RW	show and set writeback_limit feature

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

idle		  	WO	mark allocated slot as idle

User space is advised to use the following files to read the device statistics.

 pages_compacted  the number of pages freed during compaction

 huge_pages	  the number of incompressible pages

File /sys/block/zram<id>/bd_stat

The stat file represents device's backing device statistics. It consists of

a single line of text and contains the following stats separated by whitespace:

 bd_count	size of data written in backing device.

		Unit: 4K bytes

 bd_reads	the number of reads from backing device

		Unit: 4K bytes

 bd_writes	the number of writes to backing device

		Unit: 4K bytes

9) Deactivate:

	swapoff /dev/zram0

	umount /dev/zram1

= writeback

With incompressible pages, there is no memory saving with zram.

Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page

With CONFIG_ZRAM_WRITEBACK, zram can write idle/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.

To use the feature, admin should set up backing device via

	"echo /dev/sda5 > /sys/block/zramX/backing_dev"

before disksize setting. It supports only partition at this moment.

If admin want to use incompressible page writeback, they could do via

	"echo huge > /sys/block/zramX/write"

To use idle page writeback, first, user need to declare zram pages

as idle.

	"echo all > /sys/block/zramX/idle"

From now on, any pages on zram are idle pages. The idle mark

will be removed until someone request access of the block.

IOW, unless there is access request, those pages are still idle pages.

Admin can request writeback of those idle pages at right timing via

	"echo idle > /sys/block/zramX/writeback"

With the command, zram writeback idle pages from memory to the storage.

If there are lots of write IO with flash device, potentially, it has

flash wearout problem so that admin needs to design write limitation

to guarantee storage health for entire product life.

To overcome the concern, zram supports "writeback_limit" feature.

The "writeback_limit_enable"'s default value is 0 so that it doesn't limit

any writeback. IOW, if admin want to apply writeback budget, he should

enable writeback_limit_enable via

	$ echo 1 > /sys/block/zramX/writeback_limit_enable

Once writeback_limit_enable is set, zram doesn't allow any writeback

until admin set the budget via /sys/block/zramX/writeback_limit.

(If admin doesn't enable writeback_limit_enable, writeback_limit's value

assigned via /sys/block/zramX/writeback_limit is meaninless.)

If admin want to limit writeback as per-day 400M, he could do it

like below.

	$ MB_SHIFT=20

	$ 4K_SHIFT=12

	$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \

		/sys/block/zram0/writeback_limit.

	$ echo 1 > /sys/block/zram0/writeback_limit_enable

If admin want to allow further write again once the bugdet is exausted,

he could do it like below

	$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \

		/sys/block/zram0/writeback_limit

If admin want to see remaining writeback budget since he set,

	$ cat /sys/block/zramX/writeback_limit

If admin want to disable writeback limit, he could do

	$ echo 0 > /sys/block/zramX/writeback_limit_enable

The writeback_limit count will reset whenever you reset zram(e.g.,

system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of

writeback happened until you reset the zram to allocate extra writeback

budget in next setting is user's job.

If admin want to measure writeback count in a certain period, he could

know it via /sys/block/zram0/bd_stat's 3rd column.

= memory tracking

If you enable the feature, you could see block state via

/sys/kernel/debug/zram/zram0/block_state". The output is as follows,

	  300    75.033841 .wh

	  301    63.806904 s..

	  302    63.806919 ..h

	  300    75.033841 .wh.

	  301    63.806904 s...

	  302    63.806919 ..hi

First column is zram's block index.

Second column is access time since the system was booted

Third column is state of the block.

(s: same page

w: written page to backing store

h: huge page)

h: huge page

i: idle page)

First line of above example says 300th block is accessed at 75.033841sec

and the block's state is huge so it is written back to the backing

    Example of optional parameters section:

        1 ignore_corruption

device_wait

    Wait (indefinitely) for target devices to show up. Useful for devices

    that are detected asynchronously.

ignore_corruption

    Log corrupted blocks, but allow read operations to proceed normally.

compatible = "qcom,sdm845-mtp"

compatible = "qcom,sdm845-qrd"

compatible = "qcom,sdm845-qvr"

compatible = "qcom,sdm845-vm"

compatible = "qcom,sda845-cdp"

compatible = "qcom,sda845-mtp"

compatible = "qcom,sda845-qrd"

- compatible

  Usage: required

  Value type: <string>

  Definition: Should be "qcom,csid170" or "qcom,csid-lite170".

  Definition: Should be "qcom,csid170", "qcom,csid175", "qcom,csid175_200",

		"qcom,csid-lite170" or "qcom,csid-lite175".

- cell-index

  Usage: required