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

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

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