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

[Ian Kent]

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