Commit 4c1d6138 authored by Richard Chang's avatar Richard Chang Committed by Andrew Morton
Browse files

zram: introduce writeback_compressed device attribute 

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: default avatarRichard Chang <richardycc@google.com>
Co-developed-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: default avatarSergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Stevens <stevensd@google.com>
Cc: Minchan Kim <minchan@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent d38fab60

Documentation/ABI/testing/sysfs-block-zram

+7 −0
Original line number Diff line number Diff line
@@ -150,3 +150,10 @@ Contact: Sergey Senozhatsky <senozhatsky@chromium.org> 
Description:

		The algorithm_params file is write-only and is used to setup

		compression algorithm parameters.



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

Date:		Decemeber 2025

Contact:	Richard Chang <richardycc@google.com>

Description:

		The writeback_compressed device atrribute toggles compressed

		writeback feature.

Documentation/admin-guide/blockdev/zram.rst

+13 −0
Original line number Diff line number Diff line
@@ -214,6 +214,7 @@ mem_limit WO specifies the maximum amount of memory ZRAM can 
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

writeback_compressed	RW	show and set compressed writeback feature

comp_algorithm    	RW	show and change the compression algorithm

algorithm_params	WO	setup compression algorithm parameters

compact           	WO	trigger memory compaction
@@ -434,6 +435,18 @@ 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.



By default zram stores written back pages in decompressed (raw) form, which

means that writeback operation involves decompression of the page before

writing it to the backing device.  This behavior can be changed by enabling

`writeback_compressed` feature, which causes zram to write compressed pages

to the backing device, thus avoiding decompression overhead.  To enable

this feature, execute::



	$ echo yes > /sys/block/zramX/writeback_compressed



Note that this feature should be configured before the `zramX` device is

initialized.



If admin wants to measure writeback count in a certain period, they could

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

drivers/block/zram/zram_drv.c

+38 −0
Original line number Diff line number Diff line
@@ -539,6 +539,42 @@ struct zram_rb_req { 
	u32 index;

};



static ssize_t writeback_compressed_store(struct device *dev,

					  struct device_attribute *attr,

					  const char *buf, size_t len)

{

	struct zram *zram = dev_to_zram(dev);

	bool val;



	if (kstrtobool(buf, &val))

		return -EINVAL;



	down_write(&zram->init_lock);

	if (init_done(zram)) {

		up_write(&zram->init_lock);

		return -EBUSY;

	}



	zram->wb_compressed = val;

	up_write(&zram->init_lock);



	return len;

}



static ssize_t writeback_compressed_show(struct device *dev,

					 struct device_attribute *attr,

					 char *buf)

{

	bool val;

	struct zram *zram = dev_to_zram(dev);



	down_read(&zram->init_lock);

	val = zram->wb_compressed;

	up_read(&zram->init_lock);



	return sysfs_emit(buf, "%d\n", val);

}



static ssize_t writeback_limit_enable_store(struct device *dev,

					    struct device_attribute *attr,

					    const char *buf, size_t len)
@@ -3048,6 +3084,7 @@ static DEVICE_ATTR_WO(writeback); 
static DEVICE_ATTR_RW(writeback_limit);

static DEVICE_ATTR_RW(writeback_limit_enable);

static DEVICE_ATTR_RW(writeback_batch_size);

static DEVICE_ATTR_RW(writeback_compressed);

#endif

#ifdef CONFIG_ZRAM_MULTI_COMP

static DEVICE_ATTR_RW(recomp_algorithm);
@@ -3070,6 +3107,7 @@ static struct attribute *zram_disk_attrs[] = { 
	&dev_attr_writeback_limit.attr,

	&dev_attr_writeback_limit_enable.attr,

	&dev_attr_writeback_batch_size.attr,

	&dev_attr_writeback_compressed.attr,

#endif

	&dev_attr_io_stat.attr,

	&dev_attr_mm_stat.attr,