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Message-ID: <CALvZod5bxWQ8nwxyxv=ySN-75FEgtCKWussyNyFeDadURQ2XYw@mail.gmail.com>
Date: Wed, 18 Oct 2023 08:29:14 -0700
From: Shakeel Butt <shakeelb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Muchun Song <muchun.song@...ux.dev>,
Dennis Zhou <dennis@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task_struct
On Wed, Oct 18, 2023 at 2:52 AM Vlastimil Babka <vbabka@...e.cz> wrote:
[...]
> >
> > +static struct obj_cgroup *current_objcg_update(void)
> > +{
> > + struct mem_cgroup *memcg;
> > + struct obj_cgroup *old, *objcg = NULL;
> > +
> > + do {
> > + /* Atomically drop the update bit. */
> > + old = xchg(¤t->objcg, NULL);
> > + if (old) {
> > + old = (struct obj_cgroup *)
> > + ((unsigned long)old & ~CURRENT_OBJCG_UPDATE_FLAG);
> > + if (old)
> > + obj_cgroup_put(old);
> > +
> > + old = NULL;
> > + }
> > +
> > + /* Obtain the new objcg pointer. */
> > + rcu_read_lock();
> > + memcg = mem_cgroup_from_task(current);
> > + /*
> > + * The current task can be asynchronously moved to another
> > + * memcg and the previous memcg can be offlined. So let's
> > + * get the memcg pointer and try get a reference to objcg
> > + * under a rcu read lock.
> > + */
> > + for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
> > + objcg = rcu_dereference(memcg->objcg);
> > + if (likely(objcg && obj_cgroup_tryget(objcg)))
>
> So IIUC here we increase objcg refcount.
>
> > + break;
> > + objcg = NULL;
> > + }
> > + rcu_read_unlock();
> > +
> > + /*
> > + * Try set up a new objcg pointer atomically. If it
> > + * fails, it means the update flag was set concurrently, so
> > + * the whole procedure should be repeated.
> > + */
> > + } while (!try_cmpxchg(¤t->objcg, &old, objcg));
>
> And if this fails we throw objcg away and try again, but we should do
> obj_cgroup_put(objcg) first, as otherwise it would cause a leak?
>
Indeed there is a reference leak here.
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