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

Andrew Morton
2019-08-07T19:00:41Z

On Wed, 7 Aug 2019 11:31:51 -0700 Tejun Heo <tj@kernel.org> wrote:

> Hello,
>
> On Tue, Aug 06, 2019 at 04:01:02PM -0700, Andrew Morton wrote:
> > On Sat, 3 Aug 2019 07:01:53 -0700 Tejun Heo <tj@kernel.org> 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.
> >
> > Why is the id non-recycling? Presumably to address some
> > lifetime/lookup issues, but what are they?
>
> The ID by itself is used to point to the bdi from cgroup and idr
> recycles really aggressively. Combined with, for example, loop device
> based containers, stale pointing can become pretty common. We're
> having similar issues with cgroup IDs.

OK, but why is recycling a problem? For example, file descriptors
recycle as aggressively as is possible, and that doesn't cause any
trouble. Presumably recycling is a problem with cgroups because of
some sort of stale reference problem?