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    SubjectRe: [Lsf] [RFC] writeback and cgroup
    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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