+ zram-add-writeback-batch-size-device-attr.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: zram: add writeback batch size device attr
has been added to the -mm mm-unstable branch.  Its filename is
     zram-add-writeback-batch-size-device-attr.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-add-writeback-batch-size-device-attr.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: zram: add writeback batch size device attr
Date: Sat, 15 Nov 2025 11:34:45 +0900

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.

Link: https://lkml.kernel.org/r/20251115023447.495417-3-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Yuwen Chen <ywen.chen@foxmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 drivers/block/zram/zram_drv.c |   48 ++++++++++++++++++++++++++++----
 drivers/block/zram/zram_drv.h |    1 
 2 files changed, 43 insertions(+), 6 deletions(-)

--- a/drivers/block/zram/zram_drv.c~zram-add-writeback-batch-size-device-attr
+++ a/drivers/block/zram/zram_drv.c
@@ -588,6 +588,42 @@ static ssize_t writeback_limit_show(stru
 	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);
+	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)
@@ -775,10 +811,7 @@ static void release_wb_ctl(struct zram_w
 	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;
@@ -792,7 +825,7 @@ static struct zram_wb_ctl *init_wb_ctl(v
 	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;
 
 		/*
@@ -1180,7 +1213,7 @@ static ssize_t writeback_store(struct de
 		goto release_init_lock;
 	}
 
-	wb_ctl = init_wb_ctl();
+	wb_ctl = init_wb_ctl(zram);
 	if (!wb_ctl) {
 		ret = -ENOMEM;
 		goto release_init_lock;
@@ -2821,6 +2854,7 @@ 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);
@@ -2842,6 +2876,7 @@ 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,
@@ -2903,6 +2938,7 @@ 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
 
--- a/drivers/block/zram/zram_drv.h~zram-add-writeback-batch-size-device-attr
+++ a/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;
_

Patches currently in -mm which might be from senozhatsky@chromium.org are

zram-add-writeback-batch-size-device-attr.patch
zram-take-write-lock-in-wb-limit-store-handlers.patch
zram-drop-wb_limit_lock.patch