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: 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>

3 files changed, 58 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 36c57de0a10a..ed10c2e4b5c2 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -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.
diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 3e273c1bb749..9547e4e95979 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -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.
 
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 6263d300312e..3cc03c3f7389 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -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,