Miklos Szeredi [off-list ref] wrote:

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.

It's optimised for shortness of path and trades memory for performance.  It's
currently implemented using an xarray, so memory usage is dependent on the
sparseness of the tree.  Each node in the tree is 576 bytes and in the worst
case, each one node will contain one mount - and then you have to backfill the
ancestry, though for lower memory costs.

Systemd makes life more interesting since it sets up a whole load of
propagations.  Each mount you make may cause several others to be created, but
that would likely make the tree more efficient.

David