quoted

Thanks for identifying and fixing this.

Li is right. The cache name will live long, but this is because the
slab/slub caches will strdup it internally. So the actual memcg
allocation is short lived.

OK, I have totally missed that. Sorry about the confusion. Then all the
churn around the allocation is pointless, no?
What about:
---

quoted

From 7ed7f53bb597e8cb40d9ac91ce16142fb60f1e93 Mon Sep 17 00:00:00 2001

From: Michal Hocko <redacted>
Date: Fri, 22 Mar 2013 10:22:54 +0100
Subject: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

As cgroup supports rename, it's unsafe to dereference dentry->d_name
without proper vfs locks. Fix this by using cgroup_name() rather than
dentry directly.

Also open code memcg_cache_name because it is called only from
kmem_cache_dup which frees the returned name right after
kmem_cache_create_memcg makes a copy of it. Such a short-lived
allocation doesn't make too much sense. So replace it by a static
buffer as kmem_cache_dup is called with memcg_cache_mutex.

I doubt it's a win to add 4K to kernel text size instead of adding
a few extra lines of code... but it's up to you.

Signed-off-by: Li Zefan <redacted>
Signed-off-by: Michal Hocko <redacted>
---
 mm/memcontrol.c |   33 +++++++++++----------------------
 1 file changed, 11 insertions(+), 22 deletions(-)

...

 static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
 					 struct kmem_cache *s)
 {
 	char *name;
 	struct kmem_cache *new;
+	static char tmp_name[PAGE_SIZE];
 
-	name = memcg_cache_name(memcg, s);
-	if (!name)
-		return NULL;
+	lockdep_assert_held(&memcg_cache_mutex);
+
+	rcu_read_lock();
+	tmp_name = snprintf(tmp_name, sizeof(tmp_name), "%s(%d:%s)", s->name,
+			 memcg_cache_id(memcg), cgroup_name(memcg->css.cgroup));

I guess you didn't turn on CONFIG_MEMCG_KMEM?

snprintf() returns a int value.

+	rcu_read_unlock();
 
-	new = kmem_cache_create_memcg(memcg, name, s->object_size, s->align,
+	new = kmem_cache_create_memcg(memcg, tmp_name, s->object_size, s->align,
 				      (s->flags & ~SLAB_PANIC), s->ctor, s);
 
 	if (new)
 		new->allocflags |= __GFP_KMEMCG;
 
-	kfree(name);
 	return new;
 }
 

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