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Subject[PATCHv5 10/10] zram: add dynamic device add/remove functionality
Date
We currently don't support on-demand device creation.  The one and only
way to have N zram devices is to specify num_devices module parameter
(default value: 1). IOW if, for some reason, at some point, user wants to
have N + 1 devies he/she must umount all the existing devices, unload the
module, load the module passing num_devices equals to N + 1. And do this
again, if needed.

This patch introduces zram control sysfs class, which has two sysfs
attrs:
- hot_add -- add a new zram device
- hot_remove -- remove a specific (device_id) zram device

hot_add sysfs attr is read-only and has only automatic device id
assignment mode (as requested by Minchan Kim). read operation performed
on this attr creates a new zram device and returns back its device_id or
error status.

Usage example:
# add a new specific zram device
cat /sys/class/zram-control/hot_add
2

# remove a specific zram device
echo 4 > /sys/class/zram-control/hot_remove

Returning zram_add() error code back to user (-ENOMEM in this case)

cat /sys/class/zram-control/hot_add
cat: /sys/class/zram-control/hot_add: Cannot allocate memory

NOTE, there might be users who already depend on the fact that at least
zram0 device gets always created by zram_init(). Preserve this behavior.

[minchan@kernel.org: use zram->claim to avoid lockdep splat]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
Documentation/ABI/testing/sysfs-class-zram | 24 +++++++
Documentation/blockdev/zram.txt | 23 ++++++-
drivers/block/zram/zram_drv.c | 100 ++++++++++++++++++++++++++++-
3 files changed, 141 insertions(+), 6 deletions(-)
create mode 100644 Documentation/ABI/testing/sysfs-class-zram

diff --git a/Documentation/ABI/testing/sysfs-class-zram b/Documentation/ABI/testing/sysfs-class-zram
new file mode 100644
index 0000000..48ddacb
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-zram
@@ -0,0 +1,24 @@
+What: /sys/class/zram-control/
+Date: August 2015
+KernelVersion: 4.2
+Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+ The zram-control/ class sub-directory belongs to zram
+ device class
+
+What: /sys/class/zram-control/hot_add
+Date: August 2015
+KernelVersion: 4.2
+Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+ RO attribute. Read operation will cause zram to add a new
+ device and return its device id back to user (so one can
+ use /dev/zram<id>), or error code.
+
+What: /sys/class/zram-control/hot_remove
+Date: August 2015
+KernelVersion: 4.2
+Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+ WO attribute. Remove a specific /dev/zramX device, where X
+ is a device_id provided by user.
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 65e9430..c4de576 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -99,7 +99,24 @@ size of the disk when not in use so a huge zram is wasteful.
mkfs.ext4 /dev/zram1
mount /dev/zram1 /tmp

-7) Stats:
+7) Add/remove zram devices
+
+zram provides a control interface, which enables dynamic (on-demand) device
+addition and removal.
+
+In order to add a new /dev/zramX device, perform read operation on hot_add
+attribute. This will return either new device's device id (meaning that you
+can use /dev/zram<id>) or error code.
+
+Example:
+ cat /sys/class/zram-control/hot_add
+ 1
+
+To remove the existing /dev/zramX device (where X is a device id)
+execute
+ echo X > /sys/class/zram-control/hot_remove
+
+8) Stats:
Per-device statistics are exported as various nodes under /sys/block/zram<id>/

A brief description of exported device attritbutes. For more details please
@@ -174,11 +191,11 @@ line of text and contains the following stats separated by whitespace:
zero_pages
num_migrated

-8) Deactivate:
+9) Deactivate:
swapoff /dev/zram0
umount /dev/zram1

-9) Reset:
+10) Reset:
Write any positive value to 'reset' sysfs node
echo 1 > /sys/block/zram0/reset
echo 1 > /sys/block/zram1/reset
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index b3541df..f401883 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -29,10 +29,14 @@
#include <linux/vmalloc.h>
#include <linux/err.h>
#include <linux/idr.h>
+#include <linux/sysfs.h>

#include "zram_drv.h"

static DEFINE_IDR(zram_index_idr);
+/* idr index must be protected */
+static DEFINE_MUTEX(zram_index_mutex);
+
static int zram_major;
static const char *default_compressor = "lzo";

@@ -1271,24 +1275,104 @@ out_free_dev:
return ret;
}

-static void zram_remove(struct zram *zram)
+static int zram_remove(struct zram *zram)
{
- pr_info("Removed device: %s\n", zram->disk->disk_name);
+ struct block_device *bdev;
+
+ bdev = bdget_disk(zram->disk, 0);
+ if (!bdev)
+ return -ENOMEM;
+
+ mutex_lock(&bdev->bd_mutex);
+ if (bdev->bd_openers || zram->claim) {
+ mutex_unlock(&bdev->bd_mutex);
+ bdput(bdev);
+ return -EBUSY;
+ }
+
+ zram->claim = true;
+ mutex_unlock(&bdev->bd_mutex);
+
/*
* Remove sysfs first, so no one will perform a disksize
- * store while we destroy the devices
+ * store while we destroy the devices. This also helps during
+ * hot_remove -- zram_reset_device() is the last holder of
+ * ->init_lock, no later/concurrent disksize_store() or any
+ * other sysfs handlers are possible.
*/
sysfs_remove_group(&disk_to_dev(zram->disk)->kobj,
&zram_disk_attr_group);

+ /* Make sure all the pending I/O are finished */
+ fsync_bdev(bdev);
zram_reset_device(zram);
+ bdput(bdev);
+
+ pr_info("Removed device: %s\n", zram->disk->disk_name);
+
idr_remove(&zram_index_idr, zram->disk->first_minor);
blk_cleanup_queue(zram->disk->queue);
del_gendisk(zram->disk);
put_disk(zram->disk);
kfree(zram);
+ return 0;
+}
+
+/* zram-control sysfs attributes */
+static ssize_t hot_add_show(struct class *class,
+ struct class_attribute *attr,
+ char *buf)
+{
+ int ret;
+
+ mutex_lock(&zram_index_mutex);
+ ret = zram_add();
+ mutex_unlock(&zram_index_mutex);
+
+ if (ret < 0)
+ return ret;
+ return scnprintf(buf, PAGE_SIZE, "%d\n", ret);
+}
+
+static ssize_t hot_remove_store(struct class *class,
+ struct class_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ struct zram *zram;
+ int ret, dev_id;
+
+ /* dev_id is gendisk->first_minor, which is `int' */
+ ret = kstrtoint(buf, 10, &dev_id);
+ if (ret)
+ return ret;
+ if (dev_id < 0)
+ return -EINVAL;
+
+ mutex_lock(&zram_index_mutex);
+
+ zram = idr_find(&zram_index_idr, dev_id);
+ if (zram)
+ ret = zram_remove(zram);
+ else
+ ret = -ENODEV;
+
+ mutex_unlock(&zram_index_mutex);
+ return ret ? ret : count;
}

+static struct class_attribute zram_control_class_attrs[] = {
+ __ATTR_RO(hot_add),
+ __ATTR_WO(hot_remove),
+ __ATTR_NULL,
+};
+
+static struct class zram_control_class = {
+ .name = "zram-control",
+ .owner = THIS_MODULE,
+ .class_attrs = zram_control_class_attrs,
+};
+
static int zram_remove_cb(int id, void *ptr, void *data)
{
zram_remove(ptr);
@@ -1297,6 +1381,7 @@ static int zram_remove_cb(int id, void *ptr, void *data)

static void destroy_devices(void)
{
+ class_unregister(&zram_control_class);
idr_for_each(&zram_index_idr, &zram_remove_cb, NULL);
idr_destroy(&zram_index_idr);
unregister_blkdev(zram_major, "zram");
@@ -1306,14 +1391,23 @@ static int __init zram_init(void)
{
int ret;

+ ret = class_register(&zram_control_class);
+ if (ret) {
+ pr_warn("Unable to register zram-control class\n");
+ return ret;
+ }
+
zram_major = register_blkdev(0, "zram");
if (zram_major <= 0) {
pr_warn("Unable to get major number\n");
+ class_unregister(&zram_control_class);
return -EBUSY;
}

while (num_devices != 0) {
+ mutex_lock(&zram_index_mutex);
ret = zram_add();
+ mutex_unlock(&zram_index_mutex);
if (ret < 0)
goto out_error;
num_devices--;
--
2.4.0


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