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

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jul 26, 2018 at 04:25:12PM -0700, Andrew Morton wrote:
> On Fri, 13 Apr 2018 14:51:01 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Fri 13-04-18 14:14:33, Michal Hocko wrote:
> > [...]
> > > Well, this is probably a matter of taste. I will not argue. I will not
> > > object if Johannes is OK with your patch. But the whole thing confused
> > > hell out of me so I would rather un-clutter it...
> > 
> > In other words, this
> > 
> 
> This discussion has rather petered out.  afaict we're waiting for
> hannes to offer an opinion?
> 
> 
> From: Kirill Tkhai <ktkhai@virtuozzo.com>
> Subject: memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure
> 
> In case of memcg_online_kmem() failure, memcg_cgroup::id remains hashed in
> mem_cgroup_idr even after memcg memory is freed.  This leads to leak of ID
> in mem_cgroup_idr.
> 
> This patch adds removal into mem_cgroup_css_alloc(), which fixes the
> problem.  For better readability, it adds a generic helper which is used
> in mem_cgroup_alloc() and mem_cgroup_id_put_many() as well.
> 
> Link: http://lkml.kernel.org/r/152354470916.22460.14397070748001974638.stgit@localhost.localdomain
> Fixes 73f576c04b94 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

I also do wonder if we can do it cleaner, but since it's a fix I don't
want that discussion to hold things up:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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? Can we do
something like this and not mess with the alloc/free sequence at all?

Michal, Vladimir, am I missing something?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c59519d600ea..865e6d41d3d1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4144,12 +4144,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 	if (!memcg)
 		return NULL;
 
-	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
-				 1, MEM_CGROUP_ID_MAX,
-				 GFP_KERNEL);
-	if (memcg->id.id < 0)
-		goto fail;
-
 	memcg->stat_cpu = alloc_percpu(struct mem_cgroup_stat_cpu);
 	if (!memcg->stat_cpu)
 		goto fail;
@@ -4176,11 +4170,8 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 #ifdef CONFIG_CGROUP_WRITEBACK
 	INIT_LIST_HEAD(&memcg->cgwb_list);
 #endif
-	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
 	return memcg;
 fail:
-	if (memcg->id.id > 0)
-		idr_remove(&mem_cgroup_idr, memcg->id.id);
 	__mem_cgroup_free(memcg);
 	return NULL;
 }
@@ -4246,10 +4237,17 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	int i;
+
+	i = idr_alloc(&mem_cgroup_idr, memcg, 1, MEM_CGROUP_ID_MAX, GFP_KERNEL);
+	if (i < 0)
+		return i;
 
 	/* Online state pins memcg ID, memcg ID pins CSS */
+	memcg->id.id = i;
 	atomic_set(&memcg->id.ref, 1);
 	css_get(css);
+
 	return 0;
 }