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SubjectRe: [PATCH] User-configurable HDIO_GETGEO for dm volumes
On Wed, Feb 15, 2006 at 12:22:27PM -0800, Darrick J. Wong wrote:
> - Store a hd_geometry structure with each dm_table entry.

> I chose to attach the
> hd_geometry structure to dm_table because it seemed like a convenient
> place to attach config data.

Given the current device-mapper code structure, I don't think
that's a good place to attach it. Did you consider 'struct
mapped_device' instead? In your patch, the geometry will
disappear every time the mapped device's table is reloaded.
Userspace device-mapper applications are free to reload tables
whenever they wish, so this patch is of little value unless every
userspace device-mapper application you use is updated to support
geometries, or you write a userspace daemon to monitor the devices
for reloads and reset your preferred geometry each time...

That said, it might then be an idea for __bind() to clear
the geometry iff a non-zero device size changes.

+static int dm_blk_getgeo(struct block_device *bdev, struct hd_geometry *geo)
+ struct mapped_device *md = bdev->bd_disk->private_data;
+ return dm_table_get_geometry(md->map, geo);

And if md->map is NULL?
See above and side-step this issue by avoiding dm_table* completely?

+ {DM_DEV_SETGEO_CMD, dev_setgeo}

Naming inconsistency here:

Currently you have a DM_TABLE_* implementation - the data
is stored per-table not per-device. I'm suggesting you leave
this as DM_DEV_* but fix the implementation to match the name.

Also, every time a new ioctl is added upstream you need to increment
the minor number (and date):

- #define DM_VERSION_MINOR 5
+ #define DM_VERSION_MINOR 6
- #define DM_VERSION_EXTRA "-ioctl (2005-10-04)"
+ #define DM_VERSION_EXTRA "-ioctl (2006-02-17)"

And please document the new ioctl briefly within dm-ioctl.h like the
existing ones.

As for how to pass the binary data, passing it as a string in the
same manner as DM_DEV_RENAME should be OK if you prefer not to
define a new binary structure for it.

+ /*
+ * Grab our input buffer.
+ */
+ tmsg = get_result_buffer(param, param_size, &len);

'result' here means *output* not *input*. Calling that function here
might change the pointer that says where your input data starts
before you've processed it!

+ if (x != 4)
+ goto out2;

+ if (indata[0] > 65535 || indata[1] > 255
+ || indata[2] > 255 || indata[3] > ULONG_MAX)
+ goto out2;

Please issue error messages with DMWARN().

+ tbl = dm_get_table(md);
+ r = dm_table_set_geometry(tbl, &dgm);

Most device-mapper ioctls populate the 'status' fields on
return. In this case, it doesn't make much difference because
the ioctl isn't changing any of them. Regardless, *every* function
in _ioctls[] must set param->data_size correctly before returning.
(Set to zero in this particular case.)

[I also find it helpful if variable names are consistent between
functions e.g. 'table' rather than 'tbl']

+#define DM_DEV_SETGEO_32 _IOWR(DM_IOCTL, DM_DEV_SETGEO_CMD, ioctl_struct)

And include/linux/compat_ioctl.h?

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