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[dm-devel] [PATCH] Support HDIO_GETGEO on device-mapper volumes



Hi again,

I'm trying to install grub on a device-mapper RAID1 array that I set up via dmraid (in other words, a bootable software fakeraid). Since dm doesn't implement the HDIO_GETGEO ioctl, grub assumes that the CHS geometry is 620/128/63, which makes it impossible to configure it to boot a filesystem that lives beyond the 2GB mark, even if the system BIOS supports that.

The attached patch implements a simple ioctl handler that supplies a compatible geometry when HDIO_GETGEO is called against a device-mapper device. Its behavior is somewhat similar to what sd_mod does if the scsi controller doesn't provide its own geometry. Granted, the notion of cylinders, heads and sectors is silly on a RAID array, but with this patch, interested programs can obtain CHS data that's somewhat close to correct; this seems to be a better option than having each program make up its own potentially different geometry, or making an arbitrary guess. Assuming that all of the programs that need to know CHS geometry will query the kernel via HDIO_GETGEO, this patch makes it so that all of those programs end up using the same geometry.

The patch applies cleanly against 2.6.15.3; if there aren't any objections then I'm submitting this for upstream. However, I'm all ears for suggestions.

Signed-off-by: Darrick J. Wong <djwong us ibm com>

--Darrick
diff -Naurp a/drivers/md/dm.c b/drivers/md/dm.c
--- a/drivers/md/dm.c	2006-01-14 22:16:02.000000000 -0800
+++ b/drivers/md/dm.c	2006-02-09 14:05:18.000000000 -0800
@@ -17,6 +17,7 @@
 #include <linux/mempool.h>
 #include <linux/slab.h>
 #include <linux/idr.h>
+#include <linux/hdreg.h>
 
 static const char *_name = DM_NAME;
 
@@ -223,6 +224,47 @@ static int dm_blk_close(struct inode *in
 	return 0;
 }
 
+static int dm_blk_ioctl(struct inode * inode, struct file * filp,
+			unsigned int cmd, unsigned long arg)
+{
+	struct block_device *bdev = inode->i_bdev;
+	struct hd_geometry __user *loc = (void __user *)arg;
+	struct mapped_device *md;
+	int diskinfo[4];
+
+	if (cmd == HDIO_GETGEO) {
+		if (!arg)
+			return -EINVAL;
+
+		/* Make up some fake geometry. */
+		md = bdev->bd_disk->private_data;
+		diskinfo[0] = 0x40;	/* 1 << 6 */
+		diskinfo[1] = 0x20;	/* 1 << 5 */
+		diskinfo[2] = md->disk->capacity >> 11;
+		diskinfo[3] = 0;
+
+		/* cylinder count too big? */
+		if (diskinfo[2] > 65536) {
+			diskinfo[0] = 255;
+			diskinfo[1] = 63;
+			diskinfo[2] = md->disk->capacity / 16065; /* 255*63 */
+		}
+
+		if (put_user(diskinfo[0], &loc->heads))
+			return -EFAULT;
+		if (put_user(diskinfo[1], &loc->sectors))
+			return -EFAULT;
+		if (put_user(diskinfo[2], &loc->cylinders))
+			return -EFAULT;
+		if (put_user(diskinfo[3], &loc->start))
+			return -EFAULT;
+
+		return 0;
+	}
+
+	return -ENOTTY;
+}
+
 static inline struct dm_io *alloc_io(struct mapped_device *md)
 {
 	return mempool_alloc(md->io_pool, GFP_NOIO);
@@ -1179,6 +1221,7 @@ int dm_suspended(struct mapped_device *m
 static struct block_device_operations dm_blk_dops = {
 	.open = dm_blk_open,
 	.release = dm_blk_close,
+	.ioctl = dm_blk_ioctl,
 	.owner = THIS_MODULE
 };
 

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