Linux Virtualization - Jan 2008 - [PATCH] virtio_blk: Dont waste major numbers

Rusty,

currently virtio_blk uses one major number per device. While this works
quite well on most systems it is wasteful and will exhaust major numbers
on larger installations.

This patch allocates a major number on init and will use 16 minor numbers
for each disk. That will allow ~64k virtio_blk disks.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 drivers/block/virtio_blk.c |   28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

Index: kvm/drivers/block/virtio_blk.c
==================================================================---
kvm.orig/drivers/block/virtio_blk.c
+++ kvm/drivers/block/virtio_blk.c
@@ -7,10 +7,13 @@
 #include <linux/scatterlist.h>
 
 #define VIRTIO_MAX_SG	(3+MAX_PHYS_SEGMENTS)
+#define PART_BITS 4
 
 MODULE_LICENSE("GPL");
 
 static unsigned char virtblk_index = 'a';
+static int major, minor;
+
 struct virtio_blk
 {
 	spinlock_t lock;
@@ -173,10 +176,13 @@ static struct block_device_operations vi
 static int virtblk_probe(struct virtio_device *vdev)
 {
 	struct virtio_blk *vblk;
-	int err, major;
+	int err;
 	u64 cap;
 	u32 v;
 
+	if (minor >= 1 << MINORBITS)
+		return -ENOSPC;
+
 	vdev->priv = vblk = kmalloc(sizeof(*vblk), GFP_KERNEL);
 	if (!vblk) {
 		err = -ENOMEM;
@@ -200,17 +206,11 @@ static int virtblk_probe(struct virtio_d
 		goto out_free_vq;
 	}
 
-	major = register_blkdev(0, "virtblk");
-	if (major < 0) {
-		err = major;
-		goto out_mempool;
-	}
-
 	/* FIXME: How many partitions?  How long is a piece of string? */
-	vblk->disk = alloc_disk(1 << 4);
+	vblk->disk = alloc_disk(1 << PART_BITS);
 	if (!vblk->disk) {
 		err = -ENOMEM;
-		goto out_unregister_blkdev;
+		goto out_mempool;
 	}
 
 	vblk->disk->queue = blk_init_queue(do_virtblk_request,
&vblk->lock);
@@ -221,10 +221,12 @@ static int virtblk_probe(struct virtio_d
 
 	sprintf(vblk->disk->disk_name, "vd%c", virtblk_index++);
 	vblk->disk->major = major;
-	vblk->disk->first_minor = 0;
+	vblk->disk->first_minor = minor;
 	vblk->disk->private_data = vblk;
 	vblk->disk->fops = &virtblk_fops;
 
+	minor += 1 << PART_BITS;
+
 	/* If barriers are supported, tell block layer that queue is ordered */
 	if (vdev->config->feature(vdev, VIRTIO_BLK_F_BARRIER))
 		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL);
@@ -260,8 +262,6 @@ static int virtblk_probe(struct virtio_d
 
 out_put_disk:
 	put_disk(vblk->disk);
-out_unregister_blkdev:
-	unregister_blkdev(major, "virtblk");
 out_mempool:
 	mempool_destroy(vblk->pool);
 out_free_vq:
@@ -302,11 +302,15 @@ static struct virtio_driver virtio_blk  
 static int __init init(void)
 {
+	major = register_blkdev(0, "virtblk");
+	if (major < 0)
+		return major;
 	return register_virtio_driver(&virtio_blk);
 }
 
 static void __exit fini(void)
 {
+	unregister_blkdev(major, "virtblk");
 	unregister_virtio_driver(&virtio_blk);
 }
 module_init(init);

Christian Borntraeger wrote:
> Rusty,
> 
> currently virtio_blk uses one major number per device. While this works
> quite well on most systems it is wasteful and will exhaust major numbers
> on larger installations.
> 
> This patch allocates a major number on init and will use 16 minor numbers
> for each disk. That will allow ~64k virtio_blk disks.
>
There's are some other limitations to the number of virtio block 
devices.  For instances...
>  	sprintf(vblk->disk->disk_name, "vd%c", virtblk_index++);
This gets bogus after 64 disks.  We also have a hard limit for 
virtio-pci based on the number of PCI slots available.  One thing I was 
considering was whether we should try to support multiple disks per 
virtio device.

Otherwise, this patch looks good to me.

Regards,

Anthony Liguori

Am Donnerstag, 31. Januar 2008 schrieb Anthony Liguori:
> There's are some other limitations to the number of virtio block 
> devices.  For instances...
> 
> >  	sprintf(vblk->disk->disk_name, "vd%c",
virtblk_index++);
> 
> This gets bogus after 64 disks.  
Right. I will fix that with an additional patch.
> We also have a hard limit for  
> virtio-pci based on the number of PCI slots available.  One thing I was 
> considering was whether we should try to support multiple disks per 
> virtio device.
What is the hard limit in the PCI name space?


Christian

Christian Borntraeger wrote:
> Rusty,
> 
> currently virtio_blk uses one major number per device. While this works
> quite well on most systems it is wasteful and will exhaust major numbers
> on larger installations.
> 
> This patch allocates a major number on init and will use 16 minor numbers
> for each disk. That will allow ~64k virtio_blk disks.
> 
Would it be too much to allow 64 minors (63 partitions)?  I have run out 
of 16, myself, but never 64.

	-hpa

Anthony Liguori wrote:
> Christian Borntraeger wrote:
>> Rusty,
>>
>> currently virtio_blk uses one major number per device. While this works
>> quite well on most systems it is wasteful and will exhaust major
numbers
>> on larger installations.
>>
>> This patch allocates a major number on init and will use 16 minor
numbers
>> for each disk. That will allow ~64k virtio_blk disks.
>>
> 
> There's are some other limitations to the number of virtio block 
> devices.  For instances...
> 
>>      sprintf(vblk->disk->disk_name, "vd%c",
virtblk_index++);
> 
> This gets bogus after 64 disks.  We also have a hard limit for 
> virtio-pci based on the number of PCI slots available.  One thing I was 
> considering was whether we should try to support multiple disks per 
> virtio device.
> 
I would much rather prefer a /dev/vd/dXpY naming scheme, similar to 
cciss and other large disk installations.

Unfortunately yet another side effect of people not habitually 
registering major numbers is that the namespace is not as well maintained.

	-hpa