linux-kernel - Re: [RFC PATCHv5 2/6] zram: add writeback batch size device attr

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Message-ID: <CADyq12ypYai3Q2QntCjvp26U_J3xWpO4J_DSV+UwCqLFinkcGg@mail.gmail.com>
Date: Thu, 20 Nov 2025 10:57:39 -0500
From: Brian Geffon <bgeffon@...gle.com>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>, 
	Yuwen Chen <ywen.chen@...mail.com>, Richard Chang <richardycc@...gle.com>, 
	Fengyu Lian <licayy@...look.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-block@...r.kernel.org
Subject: Re: [RFC PATCHv5 2/6] zram: add writeback batch size device attr

On Thu, Nov 20, 2025 at 10:22 AM Sergey Senozhatsky
<senozhatsky@...omium.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@...omium.org>
> ---
>  drivers/block/zram/zram_drv.c | 48 ++++++++++++++++++++++++++++++-----
>  drivers/block/zram/zram_drv.h |  1 +
>  2 files changed, 43 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 37c1416ac902..7904159e9226 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -588,6 +588,42 @@ 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;
> +       ssize_t ret = -EINVAL;
> +
> +       if (kstrtouint(buf, 10, &val))
> +               return ret;
> +
> +       if (!val)
> +               val = 1;

IMO a value of 0 for the batch size doesn't make sense, -EINVAL?

> +
> +       down_read(&zram->init_lock);
> +       zram->wb_batch_size = val;
> +       up_read(&zram->init_lock);
> +       ret = len;
> +
> +       return ret;
> +}
> +
> +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)
> @@ -779,10 +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;
> @@ -797,7 +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;
>
>                 /*
> @@ -1197,7 +1230,7 @@ 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;
> @@ -2840,6 +2873,7 @@ 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);
> @@ -2861,6 +2895,7 @@ 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,
> @@ -2922,6 +2957,7 @@ 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.rc1.455.g30608eb744-goog
>