[PATCH v3 RESEND] zram: auto add new devices on demand

[Timofey Titovets <nefelim4ag@gmail.com>]

From: Timofey Titovets <nefelim4ag@gmail.com>

This add supporting of auto allocate new zram devices on demand, like 
loop devices

This working by following rules:
	- Pre create zram devices specified by num_device
	- if all device already in use -> add new free device

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

 From v2 -> v3
Delete logic to destroy not used devices, for avoid concurrency issues
Code refactoring for made patch small and clean as possible
Patch can pass the test:

#!/bin/sh
modprobe zram
while true; do
                 echo 10485760 > /sys/block/zram0/disksize&
                 echo 1 > /sys/block/zram0/reset&
done

Can be pulled 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..a090ac7 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -21,6 +21,9 @@ Following shows a typical sequence of steps for using 
zram.
  	This creates 4 devices: /dev/zram{0,1,2,3}
  	(num_devices parameter is optional. Default: 1)

+	If all device in use kernel will create new zram device
+	(between num_devices and 31)
+
  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..cc78779 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -43,6 +43,8 @@ static const char *default_compressor = "lzo";
  /* Module params (documentation at end) */
  static unsigned int num_devices = 1;

+static inline int zram_add_dev(void);
+
  #define ZRAM_ATTR_RO(name)						\
  static ssize_t zram_attr_##name##_show(struct device *d,		\
  				struct device_attribute *attr, char *b)	\
@@ -168,6 +170,7 @@ 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 +242,7 @@ 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 +378,7 @@ 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 +612,7 @@ 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;

@@ -667,6 +673,7 @@ static ssize_t disksize_store(struct device *dev,
  	zram->disksize = disksize;
  	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
  	revalidate_disk(zram->disk);
+	zram_add_dev();
  	up_write(&zram->init_lock);
  	return len;

@@ -954,6 +961,34 @@ static void destroy_device(struct zram *zram)
  	blk_cleanup_queue(zram->queue);
  }

+/*
+ * Automatically add empty zram device,
+ * if all created devices already initialized
+ */
+static inline int zram_add_dev(void)
+{
+	int dev_id;
+
+	for (dev_id = 0; dev_id < num_devices; dev_id++) {
+		if (!zram_devices[dev_id].disksize)
+			return 0;
+	}
+
+	if (max_num_devices <= num_devices) {
+		pr_warn("Can't add zram%u, max device number reached\n", num_devices);
+		return 1;
+	}
+
+	if (create_device(&zram_devices[num_devices], num_devices)) {
+		destroy_device(&zram_devices[num_devices]);
+		return 1;
+	}
+	pr_info("Created zram%u device\n", num_devices);
+	num_devices++;
+
+	return 0;
+}
+
  static int __init zram_init(void)
  {
  	int ret, dev_id;
@@ -972,13 +1007,14 @@ 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)
@@ -1025,7 +1061,7 @@ 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>");