Re: [PATCH v3 06/13] memcg: kmem controller infrastructure

Hi, Glauber.

2012/9/18 Glauber Costa [off-list ref]:

+/*
+ * We need to verify if the allocation against current->mm->owner's memcg is
+ * possible for the given order. But the page is not allocated yet, so we'll
+ * need a further commit step to do the final arrangements.
+ *
+ * It is possible for the task to switch cgroups in this mean time, so at
+ * commit time, we can't rely on task conversion any longer.  We'll then use
+ * the handle argument to return to the caller which cgroup we should commit
+ * against. We could also return the memcg directly and avoid the pointer
+ * passing, but a boolean return value gives better semantics considering
+ * the compiled-out case as well.
+ *
+ * Returning true means the allocation is possible.
+ */
+bool
+__memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
+{
+       struct mem_cgroup *memcg;
+       bool ret;
+       struct task_struct *p;
+
+       *_memcg = NULL;
+       rcu_read_lock();
+       p = rcu_dereference(current->mm->owner);
+       memcg = mem_cgroup_from_task(p);
+       rcu_read_unlock();
+
+       if (!memcg_can_account_kmem(memcg))
+               return true;
+
+       mem_cgroup_get(memcg);
+
+       ret = memcg_charge_kmem(memcg, gfp, PAGE_SIZE << order) == 0;
+       if (ret)
+               *_memcg = memcg;
+       else
+               mem_cgroup_put(memcg);
+
+       return ret;
+}

"*_memcg = memcg" should be executed when "memcg_charge_kmem" is success.
"memcg_charge_kmem" return 0 if success in charging.
Therefore, I think this code is wrong.
If I am right, it is a serious bug that affect behavior of all the patchset.

+void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
+                             int order)
+{
+       struct page_cgroup *pc;
+
+       WARN_ON(mem_cgroup_is_root(memcg));
+
+       /* The page allocation failed. Revert */
+       if (!page) {
+               memcg_uncharge_kmem(memcg, PAGE_SIZE << order);
+               return;
+       }

In case of "!page ", mem_cgroup_put(memcg) is needed,
because we already call "mem_cgroup_get(memcg)" in
__memcg_kmem_newpage_charge().
I know that mem_cgroup_put()/get() will be removed in later patch, but
it is important that every patch works fine.

Thanks.