On Wed, 2020-08-05 at 20:33 +0100, Matthew Wilcox wrote: > On Wed, Aug 05, 2020 at 04:30:10PM +0100, David Howells wrote: > > Miklos Szeredi <miklos@xxxxxxxxxx> 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. How many is not many, 5000, 10000, I agree that 30000 plus is fairly rare, even for the autofs direct mount case I hope the implementation here will help to fix. Ian
