Disable partition scan

For some setups (multipath or dmraid) the in-kernel partition scan
is pointless as the (block) partitions won't be used anywhere.
Worse, it might trigger I/O errors as the partition table might not
be accessible (eg for the passive path of a multipath device) or
even invalid (eg for RAID0 dmraid).
This patch allows to switch off the in-kernel partition scan by adding
'no_partition_scan' to the kernel commandline. Partitions scan can be
allowed for individual disk by echoing a positive number into
/sys/block/XXX/range and rescan the disk.

Signed-off-by: Hannes Reinecke <hare@suse.de>

diff --git a/block/genhd.c b/block/genhd.c
index 656c2c7..3d6a53b 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -173,6 +173,18 @@ static int exact_lock(dev_t devt, void *data)
 	return 0;
 }
 
+static int __read_mostly no_partition_scan;
+
+static int __init no_partition_scan_setup(char *str)
+{
+	no_partition_scan = 1;
+	printk(KERN_INFO "genhd: omit partition scan.\n");
+
+	return 1;
+}
+
+__setup("no_partition_scan", no_partition_scan_setup);
+
 /**
  * add_disk - add partitioning information to kernel list
  * @disk: per-device partitioning information
@@ -186,6 +198,8 @@ void add_disk(struct gendisk *disk)
 	int retval;
 
 	disk->flags |= GENHD_FL_UP;
+	if (no_partition_scan)
+		disk->flags |= GENHD_FL_NO_PARTITION_SCAN;
 	blk_register_region(MKDEV(disk->major, disk->first_minor),
 			    disk->minors, NULL, exact_match, exact_lock, disk);
 	register_disk(disk);
@@ -419,7 +433,27 @@ static ssize_t disk_range_show(struct device *dev,
 {
 	struct gendisk *disk = dev_to_disk(dev);
 
-	return sprintf(buf, "%d\n", disk->minors);
+	return sprintf(buf, "%d\n",
+		       (disk->flags & GENHD_FL_NO_PARTITION_SCAN ? 0 : disk->minors));
+}
+
+static ssize_t disk_range_store(struct device *dev,
+				struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct gendisk *disk = dev_to_disk(dev);
+	int i;
+
+	if (count > 0 && sscanf(buf, "%d", &i) > 0) {
+		if (i == 0)
+			disk->flags |= GENHD_FL_NO_PARTITION_SCAN;
+		else if (i <= disk->minors)
+			disk->flags &= ~GENHD_FL_NO_PARTITION_SCAN;
+		else
+			count = -EINVAL;
+	}
+
+	return count;
 }
 
 static ssize_t disk_removable_show(struct device *dev,
@@ -509,7 +543,7 @@ static ssize_t disk_fail_store(struct device *dev,
 
 #endif
 
-static DEVICE_ATTR(range, S_IRUGO, disk_range_show, NULL);
+static DEVICE_ATTR(range, S_IRUGO|S_IWUSR, disk_range_show, disk_range_store);
 static DEVICE_ATTR(removable, S_IRUGO, disk_removable_show, NULL);
 static DEVICE_ATTR(ro, S_IRUGO, disk_ro_show, NULL);
 static DEVICE_ATTR(size, S_IRUGO, disk_size_show, NULL);
diff --git a/fs/partitions/check.c b/fs/partitions/check.c
index 7d6b34e..a73f6dc 100644
--- a/fs/partitions/check.c
+++ b/fs/partitions/check.c
@@ -485,6 +485,8 @@ int rescan_partitions(struct gendisk *disk, struct block_device *bdev)
 		delete_partition(disk, p);
 	if (disk->fops->revalidate_disk)
 		disk->fops->revalidate_disk(disk);
+	if (disk->flags & GENHD_FL_NO_PARTITION_SCAN)
+		return 0;
 	if (!get_capacity(disk) || !(state = check_partition(disk, bdev)))
 		return 0;
 	if (IS_ERR(state))	/* I/O error reading the partition table */
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 118216f..87c45ad 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -109,6 +109,7 @@ struct hd_struct {
 #define GENHD_FL_UP				16
 #define GENHD_FL_SUPPRESS_PARTITION_INFO	32
 #define GENHD_FL_FAIL				64
+#define GENHD_FL_NO_PARTITION_SCAN		128
 
 #define BLK_SCSI_MAX_CMDS	(256)
 #define BLK_SCSI_CMD_PER_LONG	(BLK_SCSI_MAX_CMDS / (sizeof(long) * 8))
diff --git a/include/linux/virtio_blk.h b/include/linux/virtio_blk.h