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

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Date:   Wed, 7 Aug 2019 12:00:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, jack@...e.cz, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, guro@...com
Subject: Re: [PATCH 2/4] bdi: Add bdi->id

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