Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

Vladimir Davydov
2018-08-01T16:16:33Z

On Mon, Jul 30, 2018 at 11:31:13AM -0400, Johannes Weiner wrote:
> On Sun, Jul 29, 2018 at 10:26:21PM +0300, Vladimir Davydov wrote:
> > On Fri, Jul 27, 2018 at 03:31:34PM -0400, Johannes Weiner wrote:
> > > That said, the lifetime of the root reference on the ID is the online
> > > state, we put that in css_offline. Is there a reason we need to have
> > > the ID ready and the memcg in the IDR before onlining it?
> >
> > I fail to see any reason for this in the code.
>
> Me neither, thanks for double checking.
>
> The patch also survives stress testing cgroup creation and destruction
> with the script from 73f576c04b94 ("mm: memcontrol: fix cgroup
> creation failure after many small jobs").
>
> > > Can we do something like this and not mess with the alloc/free
> > > sequence at all?
> >
> > I guess so, and this definitely looks better to me.
>
> Cool, then I think we should merge Kirill's patch as the fix and mine
> as a follow-up cleanup.
>
> ---
>
> From b4106ea1f163479da805eceada60c942bd66e524 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Mon, 30 Jul 2018 11:03:55 -0400
> Subject: [PATCH] mm: memcontrol: simplify memcg idr allocation and error
> unwinding
>
> The memcg ID is allocated early in the multi-step memcg creation
> process, which needs 2-step ID allocation and IDR publishing, as well
> as two separate IDR cleanup/unwind sites on error.
>
> Defer the IDR allocation until the last second during onlining to
> eliminate all this complexity. There is no requirement to have the ID
> and IDR entry earlier than that. And the root reference to the ID is
> put in the offline path, so this matches nicely.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>