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>

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 36c57de0a10ae5bb85866b5f1601e69249d97fe1..ed10c2e4b5c293abd2c65416fc54c14e2ebb1a85 100644 (file)
--- 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 3e273c1bb749d1f483ddb5b930375d2f8b2af81d..9547e4e959791c26e950c5d0c1bd6444a028881f 100644 (file)
--- 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 6263d300312e7b2021a09ca6b7104b16212cfca7..3cc03c3f7389f9a76e7904704961f78392bfed35 100644 (file)
--- 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,