[PATCHv2 10/10] zram: add dynamic device add/remove functionality

Sergey Senozhatsky
2015-04-16T11:56:52Z

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). That means that 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:
- zram_add -- add a new zram device
- zram_remove -- remove a specific (device_id) zram device

zram_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/zram_add
2

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

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

cat /sys/class/zram-control/zram_add
cat: /sys/class/zram-control/zram_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.

[Andrew Morton <akpm@linux-foundation.org>: fix comment layout]
Reported-by: Minchan Kim <minchan@kernel.org>
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 | 124 ++++++++++++++++++++++++++++-
3 files changed, 166 insertions(+), 5 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..6f62ef5
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-zram
@@ -0,0 +1,24 @@
+What: /sys/class/zram-control/
+Date: August 2015
+KernelVersion: 4.1
+Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+ The zram-control/ class sub-directory belongs to zram
+ device class
+
+What: /sys/class/zram-control/zram_add
+Date: August 2015
+KernelVersion: 4.1
+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/zram_add
+Date: August 2015
+KernelVersion: 4.1
+Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
+Description:
+ 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 2ccc741..44b1a77 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 zram_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/zram_add
+ 1
+
+To remove the existing /dev/zramX device (where X is a device id)
+execute
+ echo X > /sys/class/zram-control/zram_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
@@ -178,11 +195,11 @@ line of text and contains the following stats separated by whitespace:
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 2c2e7cc..848222a 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -33,10 +33,14 @@
#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";

@@ -1168,8 +1172,15 @@ static int zram_add(int device_id)
if (!zram)
return -ENOMEM;

- ret = idr_alloc(&zram_index_idr, zram, device_id,
- device_id + 1, GFP_KERNEL);
+ if (device_id < 0) {
+ /* generate new device_id */
+ ret = idr_alloc(&zram_index_idr, zram, 0, 0, GFP_KERNEL);
+ device_id = ret;
+ } else {
+ /* use provided device_id */
+ ret = idr_alloc(&zram_index_idr, zram, device_id,
+ device_id + 1, GFP_KERNEL);
+ }
if (ret < 0)
goto out_free_dev;

@@ -1278,6 +1289,105 @@ static void zram_remove(struct zram *zram)
kfree(zram);
}

+/*
+ * Lookup if there is any device pointer that match the given device_id.
+ * return device pointer if so, or ERR_PTR() otherwise.
+ */
+static struct zram *zram_lookup(int dev_id)
+{
+ struct zram *zram;
+
+ zram = idr_find(&zram_index_idr, dev_id);
+ if (zram)
+ return zram;
+ return ERR_PTR(-ENODEV);
+}
+
+/* zram module control sysfs attributes */
+static ssize_t zram_add_show(struct class *class,
+ struct class_attribute *attr,
+ char *buf)
+{
+ int ret;
+
+ mutex_lock(&zram_index_mutex);
+ /* pick up available device_id */
+ ret = zram_add(-1);
+ mutex_unlock(&zram_index_mutex);
+
+ if (ret < 0)
+ return ret;
+ return scnprintf(buf, PAGE_SIZE, "%d\n", ret);
+}
+
+static ssize_t zram_remove_store(struct class *class,
+ struct class_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ struct zram *zram;
+ int ret, err, dev_id;
+
+ mutex_lock(&zram_index_mutex);
+
+ /* dev_id is gendisk->first_minor, which is `int' */
+ ret = kstrtoint(buf, 10, &dev_id);
+ if (ret || dev_id < 0) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ zram = zram_lookup(dev_id);
+ if (IS_ERR(zram)) {
+ ret = PTR_ERR(zram);
+ goto out;
+ }
+
+ /*
+ * First, make ->disksize device attr RO, closing
+ * zram_remove() vs disksize_store() race window
+ */
+ ret = sysfs_chmod_file(&disk_to_dev(zram->disk)->kobj,
+ &dev_attr_disksize.attr, S_IRUGO);
+ if (ret)
+ goto out;
+
+ ret = zram_reset_device(zram);
+ if (ret == 0) {
+ /* ->disksize is RO and there are no ->bd_openers */
+ zram_remove(zram);
+ goto out;
+ }
+
+ /*
+ * If there are still device bd_openers, try to make ->disksize
+ * RW again and return. even if we fail to make ->disksize RW,
+ * user still has RW ->reset attr. so it's possible to destroy
+ * that device.
+ */
+ err = sysfs_chmod_file(&disk_to_dev(zram->disk)->kobj,
+ &dev_attr_disksize.attr,
+ S_IWUSR | S_IRUGO);
+ if (err)
+ ret = err;
+
+out:
+ mutex_unlock(&zram_index_mutex);
+ return ret ? ret : count;
+}
+
+static struct class_attribute zram_control_class_attrs[] = {
+ __ATTR_RO(zram_add),
+ __ATTR_WO(zram_remove),
+ __ATTR_NULL,
+};
+
+static struct class zram_control_class = {
+ .name = "zram-control",
+ .owner = THIS_MODULE,
+ .class_attrs = zram_control_class_attrs,
+};
+
static int zram_exit_cb(int id, void *ptr, void *data)
{
zram_remove(ptr);
@@ -1286,6 +1396,7 @@ static int zram_exit_cb(int id, void *ptr, void *data)

static void destroy_devices(void)
{
+ class_unregister(&zram_control_class);
idr_for_each(&zram_index_idr, &zram_exit_cb, NULL);
idr_destroy(&zram_index_idr);
unregister_blkdev(zram_major, "zram");
@@ -1295,14 +1406,23 @@ static int __init zram_init(void)
{
int ret, dev_id;

+ 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;
}

for (dev_id = 0; dev_id < num_devices; dev_id++) {
+ mutex_lock(&zram_index_mutex);
ret = zram_add(dev_id);
+ mutex_unlock(&zram_index_mutex);
if (ret < 0)
goto out_error;
}
--
2.4.0.rc2