Re: [PATCH 2/4] bdi: Add bdi->id

Tejun Heo
2019-08-03T15:53:54Z

Hey, Matthew.

On Sat, Aug 03, 2019 at 08:39:08AM -0700, Matthew Wilcox wrote:
> On Sat, Aug 03, 2019 at 07:01:53AM -0700, Tejun Heo wrote:
> > There currently is no way to universally identify and lookup a bdi
> > without holding a reference and pointer to it. This patch adds an
> > non-recycling bdi->id and implements bdi_get_by_id() which looks up
> > bdis by their ids. This will be used by memcg foreign inode flushing.
> >
> > I left bdi_list alone for simplicity and because while rb_tree does
> > support rcu assignment it doesn't seem to guarantee lossless walk when
> > walk is racing aginst tree rebalance operations.
>
> This would seem like the perfect use for an allocating xarray. That
> does guarantee lossless walk under the RCU lock. You could get rid of the
> bdi_list too.

It definitely came to mind but there's a bunch of downsides to
recycling IDs or using radix tree for non-compacting allocations.

Thanks.

--
tejun