[PATCH 4/8] zram: add dynamic device add/remove functionality

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

Introduce /dev/zram-control interface which lets user-space to add a new zram
device (ZRAM_CTL_ADD ioctl) or remove the existing one (ZRAM_CTL_REMOVE ioctl).

This patch adds only two ioctl operations: add device and remove device. There
is no support for `automatic' device_id generation in this patch (will come up
later), so correct device_id must be provided.

Corresponding user-space visible defines (ZRAM_CTL_ADD/ZRAM_CTL_REMOVE) are
located in a new header file: include/uapi/linux/zram.h. Well, we have `private'
zram_drv.h and `public' zram.h headers from now on. I don't really want to make
zram_drv.h public, that would require moving of all private defines and structs
to zram_drv.c (100+ lines of code) and zram_drv.c is already big and a bit
complicated. On the other hand, zram uses struct block_device_operations that
has a per-device ->ioctl callback (not used by zram as of now). Ideally, we can
drop some of the existing zram_stats sysfs device ATTRs and use ioctl
(f.e. ZRAM_CTL_INFO) to copy out zram->stats to user-space. That will require
zram_info structure being visible to user-space via zram.h, so this new header
file can be of use.

Interface usage example (pretty much like LOOP ctl interface):

	#include <linux/zram.h>

	cfd = open("/dev/zram-control", O_RDWR);

	/* add a new specific zram device */
	err = ioctl(cfd, ZRAM_CTL_ADD, devnr);

	/* remove a specific zram device */
	err = ioctl(cfd, ZRAM_CTL_REMOVE, devnr);

NOTE, zram provides /dev/zram-control alias and zram module will be loaded
automatically when /dev/zram-control is access the first time. However, there
might be users who already depend on the fact that at least zram0 device gets
always created by zram_init(). Thus, due to compatibility reasons, along with
requested device_id (f.e. ZRAM_CTL_ADD 5) zram0 will also be created.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 drivers/block/zram/zram_drv.c | 99 ++++++++++++++++++++++++++++++++++++++++++-
 include/linux/miscdevice.h    |  1 +
 include/uapi/linux/Kbuild     |  1 +
 include/uapi/linux/zram.h     | 17 ++++++++
 4 files changed, 117 insertions(+), 1 deletion(-)
 create mode 100644 include/uapi/linux/zram.h

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 92087da..4f87d1e 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -33,10 +33,15 @@
 #include <linux/vmalloc.h>
 #include <linux/err.h>
 #include <linux/idr.h>
+#include <linux/miscdevice.h>
+#include <linux/zram.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";
 
@@ -1060,6 +1065,19 @@ static struct attribute_group zram_disk_attr_group = {
 	.attrs = zram_disk_attrs,
 };
 
+/* lookup if there is any device pointer that match to 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);
+}
+
 static int zram_add(int device_id)
 {
 	struct zram *zram;
@@ -1174,6 +1192,76 @@ static void zram_remove(struct zram *zram)
 	kfree(zram);
 }
 
+static long zram_control_ioctl(struct file *file, unsigned int cmd,
+			       unsigned long parm)
+{
+	struct zram *zram;
+	int ret = -ENOSYS, err;
+
+	mutex_lock(&zram_index_mutex);
+	zram = zram_lookup(parm);
+
+	switch (cmd) {
+	case ZRAM_CTL_ADD:
+		if (!IS_ERR(zram)) {
+			ret = -EEXIST;
+			break;
+		}
+		ret = zram_add(parm);
+		break;
+	case ZRAM_CTL_REMOVE:
+		if (IS_ERR(zram)) {
+			ret = PTR_ERR(zram);
+			break;
+		}
+
+		/* First, make ->disksize device attr RO, closing
+		 * ZRAM_CTL_REMOVE vs disksize_store() race window */
+		ret = sysfs_chmod_file(&disk_to_dev(zram->disk)->kobj,
+				&dev_attr_disksize.attr, S_IRUGO);
+		if (ret)
+			break;
+
+		ret = zram_reset_device(zram);
+		if (ret == 0) {
+			/* ->disksize is RO and there are no ->bd_openers */
+			zram_remove(zram);
+			break;
+		}
+
+		/* 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;
+		break;
+	}
+	mutex_unlock(&zram_index_mutex);
+
+	return ret;
+}
+
+static const struct file_operations zram_ctl_fops = {
+	.open		= nonseekable_open,
+	.unlocked_ioctl	= zram_control_ioctl,
+	.compat_ioctl	= zram_control_ioctl,
+	.owner		= THIS_MODULE,
+	.llseek		= noop_llseek,
+};
+
+static struct miscdevice zram_misc = {
+	.minor		= ZRAM_CTRL_MINOR,
+	.name		= "zram-control",
+	.fops		= &zram_ctl_fops,
+};
+
+MODULE_ALIAS_MISCDEV(ZRAM_CTRL_MINOR);
+MODULE_ALIAS("devname:zram-control");
+
 static int zram_exit_cb(int id, void *ptr, void *data)
 {
 	zram_remove(ptr);
@@ -1182,6 +1270,7 @@ static int zram_exit_cb(int id, void *ptr, void *data)
 
 static void destroy_devices(void)
 {
+	misc_deregister(&zram_misc);
 	idr_for_each(&zram_index_idr, &zram_exit_cb, NULL);
 	idr_destroy(&zram_index_idr);
 	unregister_blkdev(zram_major, "zram");
@@ -1198,6 +1287,12 @@ static int __init zram_init(void)
 		return -EINVAL;
 	}
 
+	ret = misc_register(&zram_misc);
+	if (ret < 0) {
+		pr_warn("Unable to register zram-control\n");
+		return ret;
+	}
+
 	zram_major = register_blkdev(0, "zram");
 	if (zram_major <= 0) {
 		pr_warn("Unable to get major number\n");
@@ -1205,7 +1300,9 @@ static int __init zram_init(void)
 	}
 
 	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;
 	}
@@ -1227,7 +1324,7 @@ module_init(zram_init);
 module_exit(zram_exit);
 
 module_param(num_devices, uint, 0);
-MODULE_PARM_DESC(num_devices, "Number of zram devices");
+MODULE_PARM_DESC(num_devices, "Number of pre-created zram devices");
 
 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index ee80dd7..11a14e3 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -49,6 +49,7 @@
 #define LOOP_CTRL_MINOR		237
 #define VHOST_NET_MINOR		238
 #define UHID_MINOR		239
+#define ZRAM_CTRL_MINOR		240
 #define MISC_DYNAMIC_MINOR	255
 
 struct device;
diff --git a/include/uapi/linux/Kbuild b/include/uapi/linux/Kbuild
index 68ceb97..9cabde9 100644
--- a/include/uapi/linux/Kbuild
+++ b/include/uapi/linux/Kbuild
@@ -448,3 +448,4 @@ header-y += xattr.h
 header-y += xfrm.h
 header-y += zorro.h
 header-y += zorro_ids.h
+header-y += zram.h
diff --git a/include/uapi/linux/zram.h b/include/uapi/linux/zram.h
new file mode 100644
index 0000000..7b84532
--- /dev/null
+++ b/include/uapi/linux/zram.h
@@ -0,0 +1,17 @@
+/*
+ * include/linux/zram.h
+ *
+ * Copyright (C) 2015 Sergey Senozhatsky.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#ifndef _UAPI_LINUX_ZRAM_H
+#define _UAPI_LINUX_ZRAM_H
+
+/* /dev/zram-control interface */
+#define ZRAM_CTL_ADD		0x5C80
+#define ZRAM_CTL_REMOVE		0x5C81
+
+#endif /* _UAPI_LINUX_ZRAM_H */
-- 
2.3.1.167.g7f4ba4b