Re: [PATCH v3 16/28] memcg: kmem controller charge/uncharge infrastructure

[Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Fri, May 25, 2012 at 05:03:36PM +0400, Glauber Costa wrote:
> +struct kmem_cache *__mem_cgroup_get_kmem_cache(struct kmem_cache *cachep,
> +					     gfp_t gfp)
> +{
> +	struct mem_cgroup *memcg;
> +	int idx;
> +	struct task_struct *p;
> +
> +	gfp |=  cachep->allocflags;
> +
> +	if (cachep->memcg_params.memcg)
> +		return cachep;
> +
> +	idx = cachep->memcg_params.id;
> +	VM_BUG_ON(idx == -1);
> +
> +	p = rcu_dereference(current->mm->owner);
> +	memcg = mem_cgroup_from_task(p);
> +
> +	if (!mem_cgroup_kmem_enabled(memcg))
> +		return cachep;
> +
> +	if (memcg->slabs[idx] == NULL) {
> +		memcg_create_cache_enqueue(memcg, cachep);
> +		return cachep;
> +	}
> +
> +	return memcg->slabs[idx];
> +}
> +EXPORT_SYMBOL(__mem_cgroup_get_kmem_cache);
> +
> +bool __mem_cgroup_new_kmem_page(struct page *page, gfp_t gfp)
> +{
> +	struct mem_cgroup *memcg;
> +	struct page_cgroup *pc;
> +	bool ret = true;
> +	size_t size;
> +	struct task_struct *p;
> +
> +	if (!current->mm || in_interrupt())
> +		return true;
> +
> +	rcu_read_lock();
> +	p = rcu_dereference(current->mm->owner);
> +	memcg = mem_cgroup_from_task(p);
> +
> +	if (!mem_cgroup_kmem_enabled(memcg))
> +		goto out;

Do you think it's possible that this memcg can be destroyed (like ss->destroy())
concurrently?

Probably not because there is a synchronize_rcu() in cgroup_diput() so as long
as we are in rcu_read_lock() we are fine.

OTOH current->mm->owner can exit() right after we fetched its memcg and thus the css_set
can be freed concurrently? And then the cgroup itself after we call rcu_read_unlock()
due to cgroup_diput().
And yet we are doing the mem_cgroup_get() below unconditionally assuming it's
always fine to get a reference to it.

May be I'm missing something?


> +	mem_cgroup_get(memcg);
> +
> +	size = (1 << compound_order(page)) << PAGE_SHIFT;
> +
> +	ret = memcg_charge_kmem(memcg, gfp, size) == 0;
> +	if (!ret) {
> +		mem_cgroup_put(memcg);
> +		goto out;
> +	}
> +
> +	pc = lookup_page_cgroup(page);
> +	lock_page_cgroup(pc);
> +	pc->mem_cgroup = memcg;
> +	SetPageCgroupUsed(pc);
> +	unlock_page_cgroup(pc);
> +
> +out:
> +	rcu_read_unlock();
> +	return ret;
> +}
> +EXPORT_SYMBOL(__mem_cgroup_new_kmem_page);