Hi, Glauber.
2012/9/18 Glauber Costa <glommer@xxxxxxxxxxxxx>:
> +/*
> + * 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.
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