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zram: introduce writeback_compressed device attribute

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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 <richardycc@google.com>
Co-developed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Sergey 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: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Richard Chang 2025-12-01 18:47:49 +09:00 • committed by Andrew Morton
parent d38fab605c
commit 4c1d61389e
3 changed files with 58 additions and 0 deletions
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7
Documentation/ABI/testing/sysfs-block-zram
View file

@ -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.

13
Documentation/admin-guide/blockdev/zram.rst
View file

@ -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.

38
drivers/block/zram/zram_drv.c
View file

@ -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,