Re: [patch] remove artificial software max_loop limit

[Ken Chen]

On 3/30/07, Ken Chen <kenchen@xxxxxxxxxx> wrote:
> Remove artificial maximum 256 loop device that can be created due to a
> legacy device number limit.  Searching through lkml archive, there are
> several instances where users complained about the artificial limit
> that the loop driver impose.  There is no reason to have such limit.
>
> This patch rid the limit entirely and make loop device and associated
> block queue instantiation on demand.  With on-demand instantiation, it
> also gives the benefit of not wasting memory if these devices are not
> in use (compare to current implementation that always create 8 loop
> devices), a net improvement in both areas.  This version is both
> tested with creation of large number of loop devices and is compatible
> with existing losetup/mount user land tools.
>
> Signed-off-by: Ken Chen <kenchen@xxxxxxxxxx>

Spotted one bug: the sequence of loop device lookup, instantiation,
and then insert into global loop_devices_list better be in one
critical section, otherwise smp race ensues.  Fix that up and also use
mutex lock instead of spin_lock for loop_devices_list.

Signed-off-by: Ken Chen <kenchen@xxxxxxxxxx>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 6b5b642..1e87aea 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -77,9 +77,8 @@

#include <asm/uaccess.h>

-static int max_loop = 8;
-static struct loop_device *loop_dev;
-static struct gendisk **disks;
+static LIST_HEAD(loop_devices);
+static DEFINE_MUTEX(loop_devices_mutex);

/*
 * Transfer functions
@@ -183,7 +182,7 @@ figure_loop_size(struct loop_device *lo)
	if (unlikely((loff_t)x != size))
		return -EFBIG;

-	set_capacity(disks[lo->lo_number], x);
+	set_capacity(lo->lo_disk, x);
	return 0;					
}

@@ -812,7 +811,7 @@ static int loop_set_fd
	lo->lo_queue->queuedata = lo;
	lo->lo_queue->unplug_fn = loop_unplug;

-	set_capacity(disks[lo->lo_number], size);
+	set_capacity(lo->lo_disk, size);
	bd_set_size(bdev, size << 9);

	set_blocksize(bdev, lo_blocksize);
@@ -832,7 +831,7 @@ out_clr:
	lo->lo_device = NULL;
	lo->lo_backing_file = NULL;
	lo->lo_flags = 0;
-	set_capacity(disks[lo->lo_number], 0);
+	set_capacity(lo->lo_disk, 0);
	invalidate_bdev(bdev, 0);
	bd_set_size(bdev, 0);
	mapping_set_gfp_mask(mapping, lo->old_gfp_mask);
@@ -918,7 +917,7 @@ static int loop_clr_fd
	memset(lo->lo_crypt_name, 0, LO_NAME_SIZE);
	memset(lo->lo_file_name, 0, LO_NAME_SIZE);
	invalidate_bdev(bdev, 0);
-	set_capacity(disks[lo->lo_number], 0);
+	set_capacity(lo->lo_disk, 0);
	bd_set_size(bdev, 0);
	mapping_set_gfp_mask(filp->f_mapping, gfp);
	lo->lo_state = Lo_unbound;
@@ -1322,6 +1321,18 @@ static long lo_compat_ioctl
}
#endif

+static struct loop_device *loop_find_dev(int number)
+{
+	struct loop_device *lo;
+
+	list_for_each_entry(lo, &loop_devices, lo_list) {
+		if (lo->lo_number == number)
+			return lo;
+	}
+	return NULL;
+}
+
+static struct loop_device *loop_init_one(int i);
static int lo_open(struct inode *inode, struct file *file)
{
	struct loop_device *lo = inode->i_bdev->bd_disk->private_data;
@@ -1330,6 +1341,11 @@ static int lo_open
	lo->lo_refcnt++;
	mutex_unlock(&lo->lo_ctl_mutex);

+	mutex_lock(&loop_device_mutex);
+	if (!loop_find_dev(lo->lo_number + 1))
+		loop_init_one(lo->lo_number + 1);
+	mutex_unlock(&loop_device_mutex);
+
	return 0;
}

@@ -1357,8 +1373,6 @@ static struct block_device_operations lo_fops
/*
 * And now the modules code and kernel interface.
 */
-module_param(max_loop, int, 0);
-MODULE_PARM_DESC(max_loop, "Maximum number of loop devices (1-256)");
MODULE_LICENSE("GPL");
MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);

@@ -1383,7 +1397,7 @@ int loop_unregister_transfer(int number)

	xfer_funcs[n] = NULL;

-	for (lo = &loop_dev[0]; lo < &loop_dev[max_loop]; lo++) {
+	list_for_each_entry(lo, &loop_devices, lo_list) {
		mutex_lock(&lo->lo_ctl_mutex);

		if (lo->lo_encryption == xfer)
@@ -1398,102 +1412,106 @@ int loop_unregister_transfer(int number)
EXPORT_SYMBOL(loop_register_transfer);
EXPORT_SYMBOL(loop_unregister_transfer);

-static int __init loop_init(void)
+static struct loop_device *loop_init_one(int i)
{
-	int	i;
+	struct loop_device *lo;
+	struct gendisk *disk;

-	if (max_loop < 1 || max_loop > 256) {
-		printk(KERN_WARNING "loop: invalid max_loop (must be between"
-				    " 1 and 256), using default (8)\n");
-		max_loop = 8;
-	}
+	lo = kzalloc(sizeof(*lo), GFP_KERNEL);
+	if (!lo)
+		goto out;

-	if (register_blkdev(LOOP_MAJOR, "loop"))
-		return -EIO;
+	lo->lo_queue = blk_alloc_queue(GFP_KERNEL);
+	if (!lo->lo_queue)
+		goto out_free_dev;
+
+	disk = lo->lo_disk = alloc_disk(1);
+	if (!disk)
+		goto out_free_queue;
+
+	mutex_init(&lo->lo_ctl_mutex);
+	lo->lo_number		= i;
+	lo->lo_thread		= NULL;
+	init_waitqueue_head(&lo->lo_event);
+	spin_lock_init(&lo->lo_lock);
+	disk->major		= LOOP_MAJOR;
+	disk->first_minor	= i;
+	disk->fops		= &lo_fops;
+	disk->private_data	= lo;
+	disk->queue		= lo->lo_queue;
+	sprintf(disk->disk_name, "loop%d", i);
+	add_disk(disk);
+	list_add_tail(&lo->lo_list, &loop_devices);
+	return lo;
+
+out_free_queue:
+	blk_cleanup_queue(lo->lo_queue);
+out_free_dev:
+	kfree(lo);
+out:
+	return ERR_PTR(-ENOMEM);
+}

-	loop_dev = kmalloc(max_loop * sizeof(struct loop_device), GFP_KERNEL);
-	if (!loop_dev)
-		goto out_mem1;
-	memset(loop_dev, 0, max_loop * sizeof(struct loop_device));
+static void loop_del_one(struct loop_device *lo)
+{
+	del_gendisk(lo->lo_disk);
+	blk_cleanup_queue(lo->lo_queue);
+	put_disk(lo->lo_disk);
+	list_del(&lo->lo_list);
+	kfree(lo);
+}

-	disks = kmalloc(max_loop * sizeof(struct gendisk *), GFP_KERNEL);
-	if (!disks)
-		goto out_mem2;
+static struct kobject *loop_probe(dev_t dev, int *part, void *data)
+{
+	unsigned int number = dev & MINORMASK;
+	struct loop_device *lo;

-	for (i = 0; i < max_loop; i++) {
-		disks[i] = alloc_disk(1);
-		if (!disks[i])
-			goto out_mem3;
-	}
+	mutex_lock(&loop_device_mutex);
+	lo = loop_find_dev(number);
+	if (lo == NULL)
+		lo = loop_init_one(number);
+	mutex_unlock(&loop_device_mutex);

-	for (i = 0; i < max_loop; i++) {
-		struct loop_device *lo = &loop_dev[i];
-		struct gendisk *disk = disks[i];
-
-		memset(lo, 0, sizeof(*lo));
-		lo->lo_queue = blk_alloc_queue(GFP_KERNEL);
-		if (!lo->lo_queue)
-			goto out_mem4;
-		mutex_init(&lo->lo_ctl_mutex);
-		lo->lo_number = i;
-		lo->lo_thread = NULL;
-		init_waitqueue_head(&lo->lo_event);
-		spin_lock_init(&lo->lo_lock);
-		disk->major = LOOP_MAJOR;
-		disk->first_minor = i;
-		disk->fops = &lo_fops;
-		sprintf(disk->disk_name, "loop%d", i);
-		disk->private_data = lo;
-		disk->queue = lo->lo_queue;
-	}
+	*part = 0;
+	if (IS_ERR(lo))
+		return (void *)lo;
+	else
+		return &lo->lo_disk->kobj;
+}
+
+static int __init loop_init(void)
+{
+	struct loop_device *lo;
+
+	if (register_blkdev(LOOP_MAJOR, "loop"))
+		return -EIO;
+	blk_register_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS,
+				  THIS_MODULE, loop_probe, NULL, NULL);

-	/* We cannot fail after we call this, so another loop!*/
-	for (i = 0; i < max_loop; i++)
-		add_disk(disks[i]);
-	printk(KERN_INFO "loop: loaded (max %d devices)\n", max_loop);
+	lo = loop_init_one(0);
+	if (IS_ERR(lo))
+		goto out_free;
+
+	printk(KERN_INFO "loop: loaded");
	return 0;

-out_mem4:
-	while (i--)
-		blk_cleanup_queue(loop_dev[i].lo_queue);
-	i = max_loop;
-out_mem3:
-	while (i--)
-		put_disk(disks[i]);
-	kfree(disks);
-out_mem2:
-	kfree(loop_dev);
-out_mem1:
+out_free:
	unregister_blkdev(LOOP_MAJOR, "loop");
	printk(KERN_ERR "loop: ran out of memory\n");
	return -ENOMEM;
}

-static void loop_exit(void)
+static void __exit loop_exit(void)
{
-	int i;
+	struct loop_device *lo, *next;

-	for (i = 0; i < max_loop; i++) {
-		del_gendisk(disks[i]);
-		blk_cleanup_queue(loop_dev[i].lo_queue);
-		put_disk(disks[i]);
-	}
+	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
+		loop_del_one(lo);
+
+	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS);
	if (unregister_blkdev(LOOP_MAJOR, "loop"))
		printk(KERN_WARNING "loop: cannot unregister blkdev\n");
-
-	kfree(disks);
-	kfree(loop_dev);
}

module_init(loop_init);
module_exit(loop_exit);
-
-#ifndef MODULE
-static int __init max_loop_setup(char *str)
-{
-	max_loop = simple_strtol(str, NULL, 0);
-	return 1;
-}
-
-__setup("max_loop=", max_loop_setup);
-#endif
diff --git a/include/linux/loop.h b/include/linux/loop.h
index 191a595..0b99b31 100644
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -64,6 +64,8 @@ struct loop_device {
	wait_queue_head_t	lo_event;

	request_queue_t		*lo_queue;
+	struct gendisk		*lo_disk;
+	struct list_head	lo_list;
};

#endif /* __KERNEL__ */
-
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