Re: [PATCH 01/15] dm: support ioctls on mapped devices

[Andrew Morton <akpm@osdl.org>]

On Thu, 22 Jun 2006 10:15:30 +0200
Milan Broz <mbroz@redhat.com> wrote:

> Andrew Morton wrote:
> > On Wed, 21 Jun 2006 20:31:21 +0100
> > Alasdair G Kergon <agk@redhat.com> wrote:
> > 
> >> From: Milan Broz <mbroz@redhat.com>
> >>  
> >> Extend the core device-mapper infrastructure to accept arbitrary ioctls
> >> on a mapped device provided that it has exactly one target and it is 
> >> capable of supporting ioctls.
> > 
> > I don't understand that.  We're taking an ioctl against a dm device and
> > we're passing it through to an underlying device?  Or something else?
> 
> Solving this situation: logical volume (say /dev/mapper/lv1) mapped in dm 
> to single device (/dev/sda):
> 
> If there is need to send ioctl you must know that /dev/mapper/lv1
> is mapped to /dev/sda (and use /dev/sda for ioctl).
> This is dm work -  so send ioctl to /dev/mapper/lv1 directly
> and let dm decide what to do.

OK.  I do think dm needs to remember /dev/sda's file* to get this right
though.  That's where the ->ioctl methods are.

> This is supported only for single mapping. If there are more than one target
> it will return -ENOTTY.
> 
> >> [We can't use unlocked_ioctl because we need 'inode': 'file' might be NULL.
> >> Is it worth changing this?]
> > 
> > It _should_ be possible to use unlocked_ioctl() - unlocked_ioctl() would be
> > pretty useless if someone was passing it a NULL file*.  More details?
> 
> yes, 
> (I prefer change block code to not pass NULL and use unlocked_ioctl,
> - Alasdair ?)
> 
> see
> 
> drivers/char/raw.c:
> 126: return blkdev_ioctl(bdev->bd_inode, *NULL*, command, arg);

Oh dear.  raw_open() doesn't have a file* for the device.

> and block/ioctl.c: [file = NULL here]
> 206: if (disk->fops->unlocked_ioctl)
> 207:	return disk->fops->unlocked_ioctl(*file*, cmd, arg);