Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

[Michal Hocko]

On Thu 21-03-13 10:08:49, Michal Hocko wrote:
> On Thu 21-03-13 09:22:21, Li Zefan wrote:
> > As cgroup supports rename, it's unsafe to dereference dentry->d_name
> > without proper vfs locks. Fix this by using cgroup_name().
> > 
> > Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
> > ---
> > 
> > This patch depends on "cgroup: fix cgroup_path() vs rename() race",
> > which has been queued for 3.10.
> > 
> > ---
> >  mm/memcontrol.c | 15 +++++++--------
> >  1 file changed, 7 insertions(+), 8 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 53b8201..72be5c9 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3217,17 +3217,16 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
> >  static char *memcg_cache_name(struct mem_cgroup *memcg, struct kmem_cache *s)
> >  {
> >  	char *name;
> > -	struct dentry *dentry;
> > +
> > +	name = (char *)__get_free_page(GFP_TEMPORARY);
> 
> Ouch. Can we use a static temporary buffer instead?

> This is called from workqueue context so we do not have to be afraid
> of the deep call chain.

Bahh, I was thinking about two things at the same time and that is how
it ends... I meant a temporary buffer on the stack. But a separate
allocation sounds even easier.

> It is also not a hot path AFAICS.
> 
> Even GFP_ATOMIC for kasprintf would be an improvement IMO.

What about the following (not even compile tested because I do not have
cgroup_name in my tree yet):
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f608546..ede0382 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3370,13 +3370,18 @@ static char *memcg_cache_name(struct mem_cgroup *memcg, struct kmem_cache *s)
 	struct dentry *dentry;
 
 	rcu_read_lock();
-	dentry = rcu_dereference(memcg->css.cgroup->dentry);
+	name = kasprintf(GFP_ATOMIC, "%s(%d:%s)", s->name,
+			 memcg_cache_id(memcg), cgroup_name(memcg->css.cgroup));
 	rcu_read_unlock();
 
-	BUG_ON(dentry == NULL);
-
-	name = kasprintf(GFP_KERNEL, "%s(%d:%s)", s->name,
-			 memcg_cache_id(memcg), dentry->d_name.name);
+	if (!name) {
+		name = kmalloc(PAGE_SIZE, GFP_KERNEL);
+		rcu_read_lock();
+		name = snprintf(name, PAGE_SIZE, "%s(%d:%s)", s->name,
+				memcg_cache_id(memcg),
+				cgroup_name(memcg->css.cgroup));
+		rcu_read_unlock();
+	}
 
 	return name;
 }
-- 
Michal Hocko
SUSE Labs
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