On Wed, Aug 5, 2020 at 4:14 PM David Howells [off-list ref] wrote:

However, looking up that identifier requires some sort of structure for doing
this and it's kind of worst case for the IDR tree as the keys are gradually
going to spread out, causing it to eat more memory.  It may be a tradeoff
worth making, and the memory consumption might not be that bad - or we could
use some other data structure such as an rbtree.

idr_alloc_cyclic() seems to be a good template for doing the lower
32bit allocation, and we can add code to increment the high 32bit on
wraparound.

Lots of code uses idr_alloc_cyclic() so I guess it shouldn't be too
bad in terms of memory use or performance.

Thanks,
Miklos