[PATCH v2] zram: auto add/del devices on demand

[Timofey Titovets]

From: Timofey Titovets <nefelim4ag@xxxxxxxxx>

This add supporting of autochange count of zram devices on demand, like 
loop devices;
This working by following rules:
	- Always save minimum devices count specified by num_device (can be 
specified while kernel module loading)
	- if last device already using add new device;
	- if last and prelast devices is free - delete last zram device;

Signed-off-by: Timofey Titovets <nefelim4ag@xxxxxxxxx>

From v1 -> v2:
Delete useless variable 'ret', added documentation for explain new zram 
behaviour

Please pull from:
https://github.com/Nefelim4ag/linux.git
---
Tested on 3.15.5-2-ARCH, can be applied on any kernel version
after this patch 'zram: add LZ4 compression support' - 
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=6e76668e415adf799839f0ab205142ad7002d260
---
diff --git a/Documentation/blockdev/zram.txt 
b/Documentation/blockdev/zram.txt
index 0595c3f..7f5c921 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -18,9 +18,19 @@ Following shows a typical sequence of steps for using 
zram.

 1) Load Module:
 	modprobe zram num_devices=4
-	This creates 4 devices: /dev/zram{0,1,2,3}
+	This pre creates 4 devices: /dev/zram{0,1,2,3}
 	(num_devices parameter is optional. Default: 1)

+	Kernel dynamically changes number of zram devices by demand
+	(between num_devices and 31)
+	If set disk size(4) for last device
+	kernel automatically adds new zram device
+	If last two devices have zero disk size after reset(8),
+	kernel will destroy last device
+
+	Summing up all features, comes one simple rule:
+	"The last zram device is always free for use"
+
 2) Set max number of compression streams
 	Compression backend may use up to max_comp_streams compression streams,
 	thus allowing up to max_comp_streams concurrent compression operations.
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 089e72c..9b2fc89 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -42,6 +42,10 @@ static const char *default_compressor = "lzo";

 /* Module params (documentation at end) */
 static unsigned int num_devices = 1;
+static unsigned int last_created_dev = 1;
+
+static void zram_add_dev(void);
+static void zram_del_dev(void);

 #define ZRAM_ATTR_RO(name)						\
 static ssize_t zram_attr_##name##_show(struct device *d,		\
@@ -168,6 +172,7 @@ static ssize_t comp_algorithm_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
+
 	down_write(&zram->init_lock);
 	if (init_done(zram)) {
 		up_write(&zram->init_lock);
@@ -239,6 +244,7 @@ static struct zram_meta *zram_meta_alloc(u64 disksize)
 {
 	size_t num_pages;
 	struct zram_meta *meta = kmalloc(sizeof(*meta), GFP_KERNEL);
+
 	if (!meta)
 		goto out;

@@ -374,6 +380,7 @@ static int zram_bvec_read(struct zram *zram, struct 
bio_vec *bvec,
 	struct page *page;
 	unsigned char *user_mem, *uncmem = NULL;
 	struct zram_meta *meta = zram->meta;
+
 	page = bvec->bv_page;

 	read_lock(&meta->tb_lock);
@@ -607,6 +614,7 @@ static void zram_reset_device(struct zram *zram, 
bool reset_capacity)
 	/* Free all pages that are still in this zram device */
 	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
 		unsigned long handle = meta->table[index].handle;
+
 		if (!handle)
 			continue;

@@ -668,6 +676,7 @@ static ssize_t disksize_store(struct device *dev,
 	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
 	revalidate_disk(zram->disk);
 	up_write(&zram->init_lock);
+	zram_add_dev();
 	return len;

 out_destroy_comp:
@@ -712,6 +721,7 @@ static ssize_t reset_store(struct device *dev,
 	bdput(bdev);

 	zram_reset_device(zram, true);
+	zram_del_dev();
 	return len;

 out:
@@ -954,6 +964,41 @@ static void destroy_device(struct zram *zram)
 	blk_cleanup_queue(zram->queue);
 }

+/* remove last free device, if last and prelast dev a free */
+static void zram_del_dev(void)
+{
+	if (last_created_dev < num_devices)
+		return;
+
+	if (zram_devices[last_created_dev].disksize == 0 &&
+		zram_devices[last_created_dev-1].disksize == 0
+		) {
+		destroy_device(&zram_devices[last_created_dev]);
+		last_created_dev--;
+		pr_info("Deleted zram%u device\n", last_created_dev);
+	}
+}
+
+/* Auto add empty zram device, if last device in use */
+static void zram_add_dev(void)
+{
+	if (last_created_dev+1 > max_num_devices) {
+		pr_warn("Can't add zram%u, max device number reached\n",
+			num_devices);
+		return;
+	}
+
+	if (&zram_devices[last_created_dev].disksize > 0) {
+		last_created_dev++;
+		if (create_device(&zram_devices[last_created_dev], last_created_dev)) {
+			destroy_device(&zram_devices[last_created_dev]);
+			last_created_dev--;
+			return;
+		}
+		pr_info("Created zram%u device\n", last_created_dev);
+	}
+}
+
 static int __init zram_init(void)
 {
 	int ret, dev_id;
@@ -972,18 +1017,20 @@ static int __init zram_init(void)
 		goto out;
 	}

-	/* Allocate the device array and initialize each one */
-	zram_devices = kzalloc(num_devices * sizeof(struct zram), GFP_KERNEL);
+	/* Allocate the device array */
+	zram_devices = kcalloc(max_num_devices, sizeof(struct zram), GFP_KERNEL);
 	if (!zram_devices) {
 		ret = -ENOMEM;
 		goto unregister;
 	}

+	/* Initialise zram{0..num_devices} */
 	for (dev_id = 0; dev_id < num_devices; dev_id++) {
 		ret = create_device(&zram_devices[dev_id], dev_id);
 		if (ret)
 			goto free_devices;
 	}
+	last_created_dev = num_devices-1;

 	pr_info("Created %u device(s) ...\n", num_devices);

@@ -1004,7 +1051,7 @@ static void __exit zram_exit(void)
 	int i;
 	struct zram *zram;

-	for (i = 0; i < num_devices; i++) {
+	for (i = 0; i < last_created_dev+1; i++) {
 		zram = &zram_devices[i];

 		destroy_device(zram);
@@ -1025,7 +1072,7 @@ 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@xxxxxxxxxx>");
--
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