[Lsf] [RFC] writeback and cgroup
Fengguang Wu
fengguang.wu at intel.com
Tue Apr 17 02:14:24 UTC 2012
On Mon, Apr 16, 2012 at 11:52:07AM -0400, Vivek Goyal wrote:
> On Mon, Apr 16, 2012 at 09:07:07PM +0800, Fengguang Wu wrote:
>
> [..]
> > Vivek, I noticed these lines in cfq code
> >
> > sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
> >
> > Why not use bdi->dev->devt? The problem is that dev_name() will
> > return "btrfs-X" for btrfs rather than "major:minor".
>
> Isn't bdi->dev->devt 0? I see following code.
>
> add_disk()
> bdi_register_dev()
> bdi_register()
> device_create_vargs(MKDEV(0,0))
> dev->devt = devt = MKDEV(0,0);
>
> So for normal block devices, I think bdi->dev->devt will be zero, that's
> why probably we don't use it.
Yes indeed. I can confirm this with tracing. There are two main cases
- some filesystems do not have a real device for the bdi.
- add_disk() calls bdi_register_dev() with the devt, however this
information is not passed down for some reason.
device_create_vargs() will try to create a sysfs dev file if the
devt is not MKDEV(0,0).
Thanks,
Fengguang
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