Re: [PATCHv6 2/6] zram: add writeback batch size device attr

[Brian Geffon <bgeffon@google.com>]

On Sat, Nov 22, 2025 at 2:40 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> Introduce writeback_batch_size device attribute so that
> the maximum number of in-flight writeback bio requests
> can be configured at run-time per-device.  This essentially
> enables batched bio writeback.
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Reviewed-by: Brian Geffon <bgeffon@google.com>

> ---
>  drivers/block/zram/zram_drv.c | 46 ++++++++++++++++++++++++++++++-----
>  drivers/block/zram/zram_drv.h |  1 +
>  2 files changed, 41 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 06ea56f0a00f..5906ba061165 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -590,6 +590,40 @@ static ssize_t writeback_limit_show(struct device *dev,
>         return sysfs_emit(buf, "%llu\n", val);
>  }
>
> +static ssize_t writeback_batch_size_store(struct device *dev,
> +                                         struct device_attribute *attr,
> +                                         const char *buf, size_t len)
> +{
> +       struct zram *zram = dev_to_zram(dev);
> +       u32 val;
> +
> +       if (kstrtouint(buf, 10, &val))
> +               return -EINVAL;
> +
> +       if (!val)
> +               return -EINVAL;
> +
> +       down_write(&zram->init_lock);
> +       zram->wb_batch_size = val;
> +       up_write(&zram->init_lock);
> +
> +       return len;
> +}
> +
> +static ssize_t writeback_batch_size_show(struct device *dev,
> +                                        struct device_attribute *attr,
> +                                        char *buf)
> +{
> +       u32 val;
> +       struct zram *zram = dev_to_zram(dev);
> +
> +       down_read(&zram->init_lock);
> +       val = zram->wb_batch_size;
> +       up_read(&zram->init_lock);
> +
> +       return sysfs_emit(buf, "%u\n", val);
> +}
> +
>  static void reset_bdev(struct zram *zram)
>  {
>         if (!zram->backing_dev)
> @@ -781,10 +815,7 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
>         kfree(wb_ctl);
>  }
>
> -/* XXX: should be a per-device sysfs attr */
> -#define ZRAM_WB_REQ_CNT 32
> -
> -static struct zram_wb_ctl *init_wb_ctl(void)
> +static struct zram_wb_ctl *init_wb_ctl(struct zram *zram)
>  {
>         struct zram_wb_ctl *wb_ctl;
>         int i;
> @@ -799,7 +830,7 @@ static struct zram_wb_ctl *init_wb_ctl(void)
>         init_waitqueue_head(&wb_ctl->done_wait);
>         spin_lock_init(&wb_ctl->done_lock);
>
> -       for (i = 0; i < ZRAM_WB_REQ_CNT; i++) {
> +       for (i = 0; i < zram->wb_batch_size; i++) {
>                 struct zram_wb_req *req;
>
>                 /*
> @@ -1200,7 +1231,7 @@ static ssize_t writeback_store(struct device *dev,
>                 goto release_init_lock;
>         }
>
> -       wb_ctl = init_wb_ctl();
> +       wb_ctl = init_wb_ctl(zram);
>         if (!wb_ctl) {
>                 ret = -ENOMEM;
>                 goto release_init_lock;
> @@ -2843,6 +2874,7 @@ static DEVICE_ATTR_RW(backing_dev);
>  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);
>  #endif
>  #ifdef CONFIG_ZRAM_MULTI_COMP
>  static DEVICE_ATTR_RW(recomp_algorithm);
> @@ -2864,6 +2896,7 @@ static struct attribute *zram_disk_attrs[] = {
>         &dev_attr_writeback.attr,
>         &dev_attr_writeback_limit.attr,
>         &dev_attr_writeback_limit_enable.attr,
> +       &dev_attr_writeback_batch_size.attr,
>  #endif
>         &dev_attr_io_stat.attr,
>         &dev_attr_mm_stat.attr,
> @@ -2925,6 +2958,7 @@ static int zram_add(void)
>
>         init_rwsem(&zram->init_lock);
>  #ifdef CONFIG_ZRAM_WRITEBACK
> +       zram->wb_batch_size = 32;
>         spin_lock_init(&zram->wb_limit_lock);
>  #endif
>
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 6cee93f9c0d0..1a647f42c1a4 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -129,6 +129,7 @@ struct zram {
>         struct file *backing_dev;
>         spinlock_t wb_limit_lock;
>         bool wb_limit_enable;
> +       u32 wb_batch_size;
>         u64 bd_wb_limit;
>         struct block_device *bdev;
>         unsigned long *bitmap;
> --
> 2.52.0.460.gd25c4c69ec-goog
>