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SubjectRe: [PATCH stable-4.4 1/3] mm: memcontrol: fix cgroup creation failure after many small jobs
On Mon 15-08-16 09:37:48, Johannes Weiner wrote:
> On Mon, Aug 15, 2016 at 02:46:19PM +0200, Michal Hocko wrote:
> > On Mon 15-08-16 08:34:07, Johannes Weiner wrote:
> > > Hi Michal, thanks for doing this. There is only one issue I can see:
> > >
> > > On Fri, Aug 12, 2016 at 11:56:17AM +0200, Michal Hocko wrote:
> > > > @@ -4171,17 +4211,27 @@ 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 out_free;
> > > > +
> > > > memcg->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
> > > > if (!memcg->stat)
> > > > - goto out_free;
> > > > + goto out_idr;
> > > >
> > > > if (memcg_wb_domain_init(memcg, GFP_KERNEL))
> > > > goto out_free_stat;
> > > >
> > > > + idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> > >
> > > This publishes the memcg object too early. Before 4.5, the memcg is
> > > not fully initialized in mem_cgroup_alloc(). You have to move the
> > > idr_replace() down to that function (and idr_remove() on free_out).
> >
> > You are right. I am just wondering whether it matters. Nobody should see
> > the id so nobody will be looking it up, no?
>
> Page cache shadow entries refer to these IDs weakly. It's possible to
> refault with a recently recycled memcg ID and crash. That's why we do
> the whole alloc(NULL) -> replace(memcg) dance in the first place.

Ahh, OK, you are right. So I have moved the idr_replace into
mem_cgroup_css_alloc. Does the following incremental diff looks better?
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 41fb6a0d2d03..7d6ac40efa81 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4239,12 +4239,6 @@ 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 out_free;
-
memcg->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
if (!memcg->stat)
goto out_idr;
@@ -4252,13 +4246,16 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
if (memcg_wb_domain_init(memcg, GFP_KERNEL))
goto out_free_stat;

- idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
+ memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
+ 1, MEM_CGROUP_ID_MAX,
+ GFP_KERNEL);
+ if (memcg->id.id < 0)
+ goto out_free_stat;
+
return memcg;

out_free_stat:
free_percpu(memcg->stat);
-out_idr:
- idr_remove(&mem_cgroup_idr, memcg->id.id);
out_free:
kfree(memcg);
return NULL;
@@ -4340,9 +4337,11 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
#ifdef CONFIG_CGROUP_WRITEBACK
INIT_LIST_HEAD(&memcg->cgwb_list);
#endif
+ idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
return &memcg->css;

free_out:
+ idr_remove(&mem_cgroup_idr, memcg->id.id);
__mem_cgroup_free(memcg);
return ERR_PTR(error);
}
--
Michal Hocko
SUSE Labs

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