Commit 6566d1a3 authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Linus Torvalds

zram: add dynamic device add/remove functionality

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
- 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

	# 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.

[ use zram->claim to avoid lockdep splat]
Signed-off-by: default avatarSergey Senozhatsky <>
Cc: Minchan Kim <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent f405c445
What: /sys/class/zram-control/
Date: August 2015
KernelVersion: 4.2
Contact: Sergey Senozhatsky <>
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 <>
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 <>
WO attribute. Remove a specific /dev/zramX device, where X
is a device_id provided by user.
......@@ -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.
cat /sys/class/zram-control/hot_add
To remove the existing /dev/zramX device (where X is a device id)
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:
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
......@@ -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";
......@@ -1273,24 +1277,104 @@ static int zram_add(void)
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;
if (bdev->bd_openers || zram->claim) {
return -EBUSY;
zram->claim = true;
* 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.
/* Make sure all the pending I/O are finished */
pr_info("Removed device: %s\n", zram->disk->disk_name);
idr_remove(&zram_index_idr, zram->disk->first_minor);
return 0;
/* zram-control sysfs attributes */
static ssize_t hot_add_show(struct class *class,
struct class_attribute *attr,
char *buf)
int ret;
ret = zram_add();
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;
zram = idr_find(&zram_index_idr, dev_id);
if (zram)
ret = zram_remove(zram);
ret = -ENODEV;
return ret ? ret : count;
static struct class_attribute zram_control_class_attrs[] = {
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)
......@@ -1299,6 +1383,7 @@ static int zram_remove_cb(int id, void *ptr, void *data)
static void destroy_devices(void)
idr_for_each(&zram_index_idr, &zram_remove_cb, NULL);
unregister_blkdev(zram_major, "zram");
......@@ -1308,14 +1393,23 @@ 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");
return -EBUSY;
while (num_devices != 0) {
ret = zram_add();
if (ret < 0)
goto out_error;
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