Re: [RFC][PATCH 2/7] memcg: don't use mem_cgroup_get() when creating a kmemcg cache

Michal Hocko
2013-04-05T13:45:57Z

On Fri 05-04-13 14:28:12, Glauber Costa wrote:
> On 04/03/2013 07:31 PM, Michal Hocko wrote:
> > On Wed 03-04-13 17:12:21, Li Zefan wrote:
> >> Use css_get()/css_put() instead of mem_cgroup_get()/mem_cgroup_put().
> >>
> >> Signed-off-by: Li Zefan <lizefan@huawei.com>
> >> ---
> >> mm/memcontrol.c | 10 +++++-----
> >> 1 file changed, 5 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 43ca91d..dafacb8 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -3191,7 +3191,7 @@ void memcg_release_cache(struct kmem_cache *s)
> >> list_del(&s->memcg_params->list);
> >> mutex_unlock(&memcg->slab_caches_mutex);
> >>
> >> - mem_cgroup_put(memcg);
> >> + css_put(&memcg->css);
> >> out:
> >> kfree(s->memcg_params);
> >> }
> >> @@ -3350,16 +3350,18 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
> >>
> >> mutex_lock(&memcg_cache_mutex);
> >> new_cachep = cachep->memcg_params->memcg_caches[idx];
> >> - if (new_cachep)
> >> + if (new_cachep) {
> >> + css_put(&memcg->css);
> >> goto out;
> >> + }
> >>
> >> new_cachep = kmem_cache_dup(memcg, cachep);
> >> if (new_cachep == NULL) {
> >> new_cachep = cachep;
> >> + css_put(&memcg->css);
> >> goto out;
> >> }
> >>
> >> - mem_cgroup_get(memcg);
> >> atomic_set(&new_cachep->memcg_params->nr_pages , 0);
> >>
> >> cachep->memcg_params->memcg_caches[idx] = new_cachep;
> >> @@ -3449,8 +3451,6 @@ static void memcg_create_cache_work_func(struct work_struct *w)
> >>
> >> cw = container_of(w, struct create_work, work);
> >> memcg_create_kmem_cache(cw->memcg, cw->cachep);
> >> - /* Drop the reference gotten when we enqueued. */
> >> - css_put(&cw->memcg->css);
> >> kfree(cw);
> >> }
> >
> > You are putting references but I do not see any single css_{try}get
> > here. /me puzzled.
> >
>
> There are two things being done in this code:
> First, we acquired a css_ref to make sure that the underlying cgroup
> would not go away. That is a short lived reference, and it is put as
> soon as the cache is created.
> At this point, we acquire a long-lived per-cache memcg reference count
> to guarantee that the memcg will still be alive.
>
> so it is:
>
> enqueue: css_get
> create : memcg_get, css_put
> destroy: css_put
>
> If I understand Li's patch correctly, he is not touching the first
> css_get, only turning that into the long lived reference (which was not
> possible before, since that would prevent rmdir).
>
> Then he only needs to get rid of the memcg_get, change the memcg_put to
> css_put, and get rid of the now extra css_put.
>
> He is issuing extra css_puts in memcg_create_kmem_cache, but only in
> failure paths. So the code reads as:
> * css_get on enqueue (already done, so not shown in patch)
> * if it fails, css_put
> * if it succeeds, don't do anything. This is already the long-lived
> reference count. put it at release time.

OK, this makes more sense now. It is __memcg_create_cache_enqueue which
takes the reference and it is not put after this because it replaced
mem_cgroup reference counting.
Li, please put something along these lines into the changelog. This is
really tricky and easy to get misunderstand.

You can put my Acked-by then.

> The code looks correct, and of course, extremely simpler due to the
> use of a single reference.
>
> Li, am I right in my understanding that this is your intention?
>
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--
Michal Hocko
SUSE Labs