[dm-devel] Re: [PATCH] 1/1: Device-Mapper: Remove 1024 devices limitation

[Andrew Morton]

Kevin Corry <kevcorry at us.ibm.com> wrote:
>
> > Yes, idr is the one to use.  That linear search you have in there becomes
>  > logarithmic.  Will speed up the registration of 100,000 minors no end ;)
> 
>  I've got a patch that switches from a bit-set to an IDR structure. The only
>  thing I'm slightly uncertain about is the case where we're trying to create
>  a device with a specific minor number (when creating a DM device, you have
>  the choice to specify a minor number or have DM find the first available
>  one). To do this, I call idr_find() with the desired minor. If that returns
>  NULL (meaning it's not already in use), then I call idr_get_new_above() with
>  that same desired minor. In the cases I've tested, this always chooses the
>  desired minor, but can we depend on that behavior?

Yes, that has to work - you're holding the lock throughout.

It would be sensible to make that a part of the idr API though.