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

[Andrew Morton]

On Wed, 7 Aug 2019 11:31:51 -0700 Tejun Heo <tj@xxxxxxxxxx> 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@xxxxxxxxxx> 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?