Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+========================================
+zram: Compressed RAM-based block devices
+========================================
+
+Introduction
+============
+
+The zram module creates RAM-based block devices named /dev/zram<id>
+(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
+in memory itself. These disks allow very fast I/O and compression provides
+good amounts of memory savings. Some of the use cases include /tmp storage,
+use as swap disks, various caches under /var and maybe many more. :)
+
+Statistics for individual zram devices are exported through sysfs nodes at
+/sys/block/zram<id>/
+
+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@vger.kernel.org).
+
+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 any questions please contact util-linux@vger.kernel.org
+
+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 is set by the 'echo' utility,
+and, in general case, something like::
+
+	echo 3 > /sys/block/zram0/max_comp_streams
+	if [ $? -ne 0 ]; then
+		handle_error
+	fi
+
+should suffice.
+
+1) Load Module
+==============
+
+::
+
+	modprobe zram num_devices=4
+
+This creates 4 devices: /dev/zram{0,1,2,3}
+
+num_devices parameter is optional and tells zram how many devices should be
+pre-created. Default: 1.
+
+2) Set max number of compression streams
+========================================
+
+Regardless of the value passed to this attribute, ZRAM will always
+allocate multiple compression streams - one per online CPU - thus
+allowing several concurrent compression operations. The number of
+allocated compression streams goes down when some of the CPUs
+become offline. There is no single-compression-stream mode anymore,
+unless you are running a UP system or have only 1 CPU online.
+
+To find out how many streams are currently available::
+
+	cat /sys/block/zram0/max_comp_streams
+
+3) Select compression algorithm
+===============================
+
+Using comp_algorithm device attribute one can see available and
+currently selected (shown in square brackets) compression algorithms,
+or change the 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
+
+For the time being, the `comp_algorithm` content does not necessarily
+show every compression algorithm supported by the kernel. We keep this
+list primarily to simplify device configuration and one can configure
+a new device with a compression algorithm that is not listed in
+`comp_algorithm`. The thing is that, internally, ZRAM uses Crypto API
+and, if some of the algorithms were built as modules, it's impossible
+to list all of them using, for instance, /proc/crypto or any other
+method. This, however, has an advantage of permitting the usage of
+custom crypto compression modules (implementing S/W or H/W compression).
+
+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::
+
+	# Initialize /dev/zram0 with 50MB disksize
+	echo $((50*1024*1024)) > /sys/block/zram0/disksize
+
+	# Using mem suffixes
+	echo 256K > /sys/block/zram0/disksize
+	echo 512M > /sys/block/zram0/disksize
+	echo 1G > /sys/block/zram0/disksize
+
+Note:
+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.
+
+5) Set memory limit: Optional
+=============================
+
+Set memory limit by writing the value to sysfs node 'mem_limit'.
+The value can be either in bytes or you can use mem suffixes.
+In addition, you could change the value in runtime.
+Examples::
+
+	# limit /dev/zram0 with 50MB memory
+	echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
+
+	# Using mem suffixes
+	echo 256K > /sys/block/zram0/mem_limit
+	echo 512M > /sys/block/zram0/mem_limit
+	echo 1G > /sys/block/zram0/mem_limit
+
+	# To disable memory limit
+	echo 0 > /sys/block/zram0/mem_limit
+
+6) Activate
+===========
+
+::
+
+	mkswap /dev/zram0
+	swapon /dev/zram0
+
+	mkfs.ext4 /dev/zram1
+	mount /dev/zram1 /tmp
+
+7) Add/remove zram devices
+==========================
+
+zram provides a control interface, which enables dynamic (on-demand) device
+addition and removal.
+
+In order to add a new /dev/zramX device, perform a read operation on the hot_add
+attribute. This will return either the new device's device id (meaning that you
+can use /dev/zram<id>) or an error code.
+
+Example::
+
+	cat /sys/class/zram-control/hot_add
+	1
+
+To remove the existing /dev/zramX device (where X is a device id)
+execute::
+
+	echo X > /sys/class/zram-control/hot_remove
+
+8) Stats
+========
+
+Per-device statistics are exported as various nodes under /sys/block/zram<id>/
+
+A brief description of exported device attributes follows. For more details
+please read Documentation/ABI/testing/sysfs-block-zram.
+
+======================  ======  ===============================================
+Name            	access            description
+======================  ======  ===============================================
+disksize          	RW	show and set the device's disk size
+initstate         	RO	shows the initialization state of the device
+reset             	WO	trigger device reset
+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.
+
+File /sys/block/zram<id>/stat
+
+Represents block layer statistics. Read Documentation/block/stat.rst for
+details.
+
+File /sys/block/zram<id>/io_stat
+
+The stat file represents device's I/O statistics not accounted by block
+layer and, thus, not available in zram<id>/stat file. It consists of a
+single line of text and contains the following stats separated by
+whitespace:
+
+ =============    =============================================================
+ failed_reads     The number of failed reads
+ failed_writes    The number of failed writes
+ invalid_io       The number of non-page-size-aligned I/O requests
+ notify_free      Depending on device usage scenario it may account
+
+                  a) the number of pages freed because of swap slot free
+                     notifications
+                  b) the number of pages freed because of
+                     REQ_OP_DISCARD requests sent by bio. The former ones are
+                     sent to a swap block device when a swap slot is freed,
+                     which implies that this disk is being used as a swap disk.
+
+                  The latter ones are sent by filesystem mounted with
+                  discard option, whenever some data blocks are getting
+                  discarded.
+ =============    =============================================================
+
+File /sys/block/zram<id>/mm_stat
+
+The mm_stat file represents the device's mm statistics. It consists of a single
+line of text and contains the following stats separated by whitespace:
+
+ ================ =============================================================
+ orig_data_size   uncompressed size of data stored in this disk.
+                  Unit: bytes
+ compr_data_size  compressed size of data stored in this disk
+ mem_used_total   the amount of memory allocated for this disk. This
+                  includes allocator fragmentation and metadata overhead,
+                  allocated for this disk. So, allocator space efficiency
+                  can be calculated using compr_data_size and this statistic.
+                  Unit: bytes
+ mem_limit        the maximum amount of memory ZRAM can use to store
+                  the compressed data
+ mem_used_max     the maximum amount of memory zram has consumed to
+                  store the data
+ same_pages       the number of same element filled pages written to this disk.
+                  No memory is allocated for such pages.
+ pages_compacted  the number of pages freed during compaction
+ huge_pages	  the number of incompressible pages
+ ================ =============================================================
+
+File /sys/block/zram<id>/bd_stat
+
+The bd_stat file represents a 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
+
+10) Reset
+=========
+
+	Write any positive value to 'reset' sysfs node::
+
+		echo 1 > /sys/block/zram0/reset
+		echo 1 > /sys/block/zram1/reset
+
+	This frees all the memory allocated for the given device and
+	resets the disksize to zero. You must set the disksize again
+	before reusing the device.
+
+Optional Feature
+================
+
+writeback
+---------
+
+With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
+to backing storage rather than keeping it in memory.
+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 wants to use incompressible page writeback, they could do via::
+
+	echo huge > /sys/block/zramX/writeback
+
+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 requests 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 wants 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 sets 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 meaningless.)
+
+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 admins want to allow further write again once the bugdet is exhausted,
+he could do it like below::
+
+	$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
+		/sys/block/zram0/writeback_limit
+
+If admin wants to see remaining writeback budget since last 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 wants to measure writeback count in a certain period, he could
+know it via /sys/block/zram0/bd_stat's 3rd column.
+
+memory tracking
+===============
+
+With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the
+zram block. It could be useful to catch cold or incompressible
+pages of the process with*pagemap.
+
+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 ..hi
+
+First column
+	zram's block index.
+Second column
+	access time since the system was booted
+Third column
+	state of the block:
+
+	s:
+		same page
+	w:
+		written page to backing store
+	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
+storage. It's a debugging feature so anyone shouldn't rely on it to work
+properly.
+
+Nitin Gupta
+ngupta@vflare.org