linux-kernel - Re: [PATCH v4] zram: Implement multi-page write-back

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Message-ID: <mc5ww2wi6islokdc4kwu2pw4a7l5ufwigsfw4b626okq5uk2ic@f5x2otsnarws>
Date: Thu, 13 Nov 2025 14:10:07 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yuwen Chen <ywen.chen@...mail.com>
Cc: axboe@...nel.dk, akpm@...ux-foundation.org, bgeffon@...gle.com, 
	licayy@...look.com, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, liumartin@...gle.com, minchan@...nel.org, richardycc@...gle.com, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v4] zram: Implement multi-page write-back

On (25/11/13 11:04), Sergey Senozhatsky wrote:
> On (25/11/06 09:49), Yuwen Chen wrote:
> > +
> > +#define ZRAM_WB_REQ_CNT (32)
> > +
> 
> How was this number chosen?  Did you try lower/higher values?
> I think we might want this to be runtime tunable via sysfs, e.g.
> writeback_batch_size attr, with min value of 1.

So I think something like this should work:

---
 drivers/block/zram/zram_drv.c | 50 ++++++++++++++++++++++++++++++-----
 drivers/block/zram/zram_drv.h |  1 +
 2 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 10b6e57603a0..cf92d4e8ca9b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -570,6 +570,44 @@ 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;
+
+	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);
+	spin_lock(&zram->wb_limit_lock);
+	val = zram->wb_batch_size;
+	spin_unlock(&zram->wb_limit_lock);
+	up_read(&zram->init_lock);
+
+	return sysfs_emit(buf, "%u\n", val);
+}
+
 static void reset_bdev(struct zram *zram)
 {
 	if (!zram->backing_dev)
@@ -776,10 +814,7 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
 	kfree(wb_ctl);
 }
 
-/* should be a module param */
-#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;
@@ -793,7 +828,7 @@ static struct zram_wb_ctl *init_wb_ctl(void)
 	atomic_set(&wb_ctl->num_inflight, 0);
 	init_completion(&wb_ctl->done);
 
-	for (i = 0; i < ZRAM_WB_REQ_CNT; i++) {
+	for (i = 0; i < zram->wb_batch_size; i++) {
 		struct zram_wb_req *req;
 
 		req = kmalloc(sizeof(*req), GFP_KERNEL);
@@ -1145,7 +1180,7 @@ 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;
@@ -2786,6 +2821,7 @@ 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);
@@ -2807,6 +2843,7 @@ 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,
@@ -2868,6 +2905,7 @@ static int zram_add(void)
 
 	init_rwsem(&zram->init_lock);
 #ifdef CONFIG_ZRAM_WRITEBACK
+	zram->wb_batch_size = 1;
 	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;
-- 
2.51.2.1041.gc1ab5b90ca-goog