linux-toradex.git/Documentation/ABI/testing/sysfs-block-zram, branch master

zram: rename writeback_compressed device attr

2026-03-04T17:44:23+00:00

Rename writeback_compressed attr to compressed_writeback to avoid possible
confusion and have more natural naming.  writeback_compressed may look
like an alternative version of writeback while in fact
writeback_compressed only sets a writeback property.  Make this
distinction more clear with a new compressed_writeback name.

This updates a feature which is new in 7.0-rcX.

Link: https://lkml.kernel.org/r/20260226025429.1042083-1-senozhatsky@chromium.org
Fixes: 4c1d61389e8e ("zram: introduce writeback_compressed device attribute")
Signed-off-by: Sergey Senozhatsky 
Suggested-by: Minchan Kim 
Acked-by: Minchan Kim 
Cc: Brian Geffon 
Cc: Richard Chang 
Cc: Suren Baghdasaryan 
Cc: "Christoph Böhmwalder" 
Cc: Jens Axboe 
Cc: Jonathan Corbet 
Cc: Lars Ellenberg 
Cc: Philipp Reisner 
Cc: Shuah Khan 
Signed-off-by: Andrew Morton

zram: document writeback_batch_size

2026-01-21T03:24:36+00:00

Add missing writeback_batch_size documentation.

Link: https://lkml.kernel.org/r/20251201094754.4149975-4-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky 
Cc: Brian Geffon 
Cc: David Stevens 
Cc: Minchan Kim 
Cc: Richard Chang 
Signed-off-by: Andrew Morton

zram: introduce writeback_compressed device attribute

2026-01-21T03:24:36+00:00

Introduce witeback_compressed device attribute to toggle compressed
writeback (decompression on demand) feature.

[senozhatsky@chromium.org: rewrote original patch, added documentation]
Link: https://lkml.kernel.org/r/20251201094754.4149975-3-senozhatsky@chromium.org
Signed-off-by: Richard Chang 
Co-developed-by: Sergey Senozhatsky 
Signed-off-by: Sergey Senozhatsky 
Cc: Brian Geffon 
Cc: David Stevens 
Cc: Minchan Kim 
Signed-off-by: Andrew Morton

zram: remove max_comp_streams device attr

2025-03-17T05:06:33+00:00

max_comp_streams device attribute has been defunct since May 2016 when
zram switched to per-CPU compression streams, remove it.

Link: https://lkml.kernel.org/r/20250303022425.285971-5-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky 
Cc: Hillf Danton 
Cc: Kairui Song 
Cc: Minchan Kim 
Cc: Sebastian Andrzej Siewior 
Cc: Yosry Ahmed 
Signed-off-by: Andrew Morton

zram: introduce algorithm_params device attribute

2024-09-09T23:39:09+00:00

This attribute is used to setup compression algorithms' parameters, so we
can tweak algorithms' characteristics.  At this point only 'level' is
supported (to be extended in the future).

Each call sets up parameters for one particular algorithm, which should be
specified either by the algorithm's priority or algo name.  This is
expected to be called after corresponding algorithm is selected via
comp_algorithm or recomp_algorithm.

 echo "priority=0 level=1" > /sys/block/zram0/algorithm_params
or
 echo "algo=zstd level=1" > /sys/block/zram0/algorithm_params

Link: https://lkml.kernel.org/r/20240902105656.1383858-16-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky 
Cc: Minchan Kim 
Cc: Nick Terrell 
Signed-off-by: Andrew Morton

Docs/ABI/zram: document zram recompress sysfs knobs

2022-11-30T23:58:53+00:00

Document zram re-compression sysfs knobs.

Link: https://lkml.kernel.org/r/20221115020314.386235-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky 
Acked-by: Minchan Kim 
Cc: Nitin Gupta 
Signed-off-by: Andrew Morton

zram: idle writeback fixes and cleanup

2019-01-09T01:15:10+00:00

This patch includes some fixes and cleanup for idle-page writeback.

1. writeback_limit interface

Now writeback_limit interface is rather conusing.  For example, once
writeback limit budget is exausted, admin can see 0 from
/sys/block/zramX/writeback_limit which is same semantic with disable
writeback_limit at this moment.  IOW, admin cannot tell that zero came
from disable writeback limit or exausted writeback limit.

To make the interface clear, let's sepatate enable of writeback limit to
another knob - /sys/block/zram0/writeback_limit_enable

* before:
  while true :
    # to re-enable writeback limit once previous one is used up
    echo 0 > /sys/block/zram0/writeback_limit
    echo $((200<<20)) > /sys/block/zram0/writeback_limit
    ..
    .. # used up the writeback limit budget

* new
  # To enable writeback limit, from the beginning, admin should
  # enable it.
  echo $((200<<20)) > /sys/block/zram0/writeback_limit
  echo 1 > /sys/block/zram/0/writeback_limit_enable
  while true :
    echo $((200<<20)) > /sys/block/zram0/writeback_limit
    ..
    .. # used up the writeback limit budget

It's much strightforward.

2. fix condition check idle/huge writeback mode check

The mode in writeback_store is not bit opeartion any more so no need to
use bit operations.  Furthermore, current condition check is broken in
that it does writeback every pages regardless of huge/idle.

3. clean up idle_store

No need to use goto.

[minchan@kernel.org: missed spin_lock_init]
  Link: http://lkml.kernel.org/r/20190103001601.GA255139@google.com
Link: http://lkml.kernel.org/r/20181224033529.19450-1-minchan@kernel.org
Signed-off-by: Minchan Kim 
Suggested-by: John Dias 
Cc: Sergey Senozhatsky 
Cc: John Dias 
Cc: Srinivas Paladugu 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

zram: writeback throttle

2018-12-28T20:11:49+00:00

If there are lots of write IO with flash device, it could have a
wearout problem of storage. To overcome the problem, admin needs
to design write limitation to guarantee flash health
for entire product life.

This patch creates a new knob "writeback_limit" for zram.

writeback_limit's default value is 0 so that it doesn't limit
any writeback. If admin want to measure writeback count in a
certain period, he could know it via /sys/block/zram0/bd_stat's
3rd column.

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

	MB_SHIFT=20
	4K_SHIFT=12
	echo $((400<>4K_SHIFT)) > \
		/sys/block/zram0/writeback_limit.

If admin want to allow further write again, he could do it like below

	echo 0 > /sys/block/zram0/writeback_limit

If admin want to see remaining writeback budget,

	cat /sys/block/zram0/writeback_limit

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.

[minchan@kernel.org: v4]
  Link: http://lkml.kernel.org/r/20181203024045.153534-8-minchan@kernel.org
Link: http://lkml.kernel.org/r/20181127055429.251614-8-minchan@kernel.org
Signed-off-by: Minchan Kim 
Reviewed-by: Sergey Senozhatsky 
Cc: Joey Pabalinas 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

zram: add bd_stat statistics

2018-12-28T20:11:49+00:00

bd_stat represents things that happened in the backing device.  Currently
it supports bd_counts, bd_reads and bd_writes which are helpful to
understand wearout of flash and memory saving.

[minchan@kernel.org: v4]
  Link: http://lkml.kernel.org/r/20181203024045.153534-7-minchan@kernel.org
Link: http://lkml.kernel.org/r/20181127055429.251614-7-minchan@kernel.org
Signed-off-by: Minchan Kim 
Reviewed-by: Sergey Senozhatsky 
Cc: Joey Pabalinas 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

zram: support idle/huge page writeback

2018-12-28T20:11:49+00:00

Add a new feature "zram idle/huge page writeback".  In the zram-swap use
case, zram usually has many idle/huge swap pages.  It's pointless to keep
them in memory (ie, zram).

To solve this problem, this feature introduces idle/huge page writeback to
the backing device so the goal is to save more memory space on embedded
systems.

Normal sequence to use idle/huge page writeback feature is as follows,

while (1) {
        # mark allocated zram slot to idle
        echo all > /sys/block/zram0/idle
        # leave system working for several hours
        # Unless there is no access for some blocks on zram,
	# they are still IDLE marked pages.

        echo "idle" > /sys/block/zram0/writeback
	or/and
	echo "huge" > /sys/block/zram0/writeback
        # write the IDLE or/and huge marked slot into backing device
	# and free the memory.
}

Per the discussion at
https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,

This patch removes direct incommpressibe page writeback feature
(d2afd25114f4 ("zram: write incompressible pages to backing device")).

Below concerns from Sergey:
== &< ==

"IDLE writeback" is superior to "incompressible writeback".

"incompressible writeback" is completely unpredictable and uncontrollable;
it depens on data patterns and compression algorithms.  While "IDLE
writeback" is predictable.

I even suspect, that, *ideally*, we can remove "incompressible writeback".
"IDLE pages" is a super set which also includes "incompressible" pages.
So, technically, we still can do "incompressible writeback" from "IDLE
writeback" path; but a much more reasonable one, based on a page idling
period.

I understand that you want to keep "direct incompressible writeback"
around.  ZRAM is especially popular on devices which do suffer from flash
wearout, so I can see "incompressible writeback" path becoming a dead
code, long term.

== &< ==

Below concerns from Minchan:
== &< ==

My concern is if we enable CONFIG_ZRAM_WRITEBACK in this implementation,
both hugepage/idlepage writeck will turn on.  However someuser want to
enable only idlepage writeback so we need to introduce turn on/off knob
for hugepage or new CONFIG_ZRAM_IDLEPAGE_WRITEBACK for those usecase.  I
don't want to make it complicated *if possible*.

Long term, I imagine we need to make VM aware of new swap hierarchy a
little bit different with as-is.  For example, first high priority swap
can return -EIO or -ENOCOMP, swap try to fallback to next lower priority
swap device.  With that, hugepage writeback will work tranparently.

So we could regard it as regression because incompressible pages doesn't
go to backing storage automatically.  Instead, user should do it via "echo
huge" > /sys/block/zram/writeback" manually.

== &< ==

Link: http://lkml.kernel.org/r/20181127055429.251614-6-minchan@kernel.org
Signed-off-by: Minchan Kim 
Reviewed-by: Joey Pabalinas 
Reviewed-by: Sergey Senozhatsky 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds