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    SubjectRe: [PATCH 0/4] io-controller: Use names rather than major:minor
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    On Fri, Mar 26, 2010 at 8:20 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
    > On Fri, Mar 26, 2010 at 09:31:41AM +0800, Gui Jianfeng wrote:
    >> +int blk_lookup_devname(dev_t devt, char *name)
    >> +{

    [ snip... loop through all block devices for devt ...snip ]

    >> So we can keep dev_t in blkio layer, and export to user a device name by calling
    >> this function. Also, we retrive device number by calling blk_lookup_devt().
    >> This change might keep things much simple. Jens, do you have any thoughts?
    >>
    > I agree with Gui that lets keep the dev_t the core in blkio layer. Keeping
    > a pointer to gendisk in request queue is becoming little messy.

    Agreed on leaving gendisk pointer out of request_queue. In doing
    further investigation, I've found that it's up to the driver to
    maintain the association between gendisk and request_queue, and some
    drivers put multiple gendisk behind a single request_queue, so the
    back pointer would be ill-specified.

    > But if that does not work for you, then I would also like to keep things
    > simple and translate dev_t to diskname during read routine. Similiarly,
    > while somebody is putting policy, use blk_lookup_devt().

    I like the simplicity of blk_lookup_devt(), but I don't like the idea
    of iterating through all block devices on every lookup of the name.
    Perhaps we could cache the name somewhere?

    Actually, the name is the name of the *queue* (or the key in
    blk-cgroup), because as I mentioned above there can be a many to one
    relationship between disks and queues in general.

    The more I think about it, the more it seems to make sense to extend
    blkio_policy_ops to include a function to get the name of the key.
    blk-cgroup makes no current use of the dev, except to invent a name
    for the request_queue whose policy is being set or printed. It could
    be argued that the thing being scheduled has a better idea of the name
    of that thing.

    > But this will lead to issue of how do you now display both device number
    > and disk name in the output. May be following.
    >
    > major:minor  diskname  data
    >
    > I am not sure if people are fond of multiple values in a single file. At
    > the same time for setting the rules or deleting the rules, it will make
    > syntax complicated/confusing. Also will require breaking ABI for existing
    > blkio.time, blkio.sectors, blkio.dequeue files.

    I don't like this, either. It breaks ABI and is more confusing for users.

    > So I would prefer to keep the major/minor number based interface for
    > follwing reasons.
    >
    > - Chaning it now breaks ABI.
    > - Other cgroup controller "device" is also using major/minor number based
    >  interface for device access policy. So it is consistent with other
    >  controller.

    Which controllers are these?

    > - Displaying both device major/minor and diskname is an option but that
    >  makes the file format syntax little complicated and new rule setting
    >  or removoal confusing.

    A few messages back you mentioned that you preferred device names
    because they would be better for users of the system. If there was a
    simple implementation, would you still be behind a new name-based
    interface? We could go that direction and maintain ABI by deprecating
    current interface and making a new interface with names.

    If you can't tell, I'm a big fan of using the name! :) It's *much*
    more consistent with the interfaces in /sys.

    Chad
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