[PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

[Matthew Wilcox]

On Wed, Aug 05, 2020 at 04:30:10PM +0100, David Howells wrote:
> Miklos Szeredi <miklos at szeredi.hu> 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.

I would recommend using xa_alloc and ignoring the ID assigned from
xa_alloc.  Looking up by unique ID is then a matter of iterating every
mount (xa_for_each()) looking for a matching unique ID in the mount
struct.  That's O(n) search, but it's faster than a linked list, and we
don't have that many mounts in a system.

The maple tree will handle this case more effectively, but I can't
recommend waiting for that to be ready.