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

On 10/01/2012 03:58 PM, Michal Hocko wrote:

On Mon 01-10-12 15:51:20, Glauber Costa wrote:

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On 10/01/2012 03:51 PM, Michal Hocko wrote:

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On Mon 01-10-12 14:09:09, Glauber Costa wrote:

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On 10/01/2012 01:48 PM, Michal Hocko wrote:

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On Fri 28-09-12 15:34:19, Glauber Costa wrote:

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On 09/27/2012 05:44 PM, Michal Hocko wrote:

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the reference count aquired by mem_cgroup_get will still prevent the
memcg from going away, no?

Yes but you are outside of the rcu now and we usually do css_get before
we rcu_unlock. mem_cgroup_get just makes sure the group doesn't get
deallocated but it could be gone before you call it. Or I am just
confused - these 2 levels of ref counting is really not nice.

Anyway, I have just noticed that __mem_cgroup_try_charge does
VM_BUG_ON(css_is_removed(&memcg->css)) on a given memcg so you should
keep css ref count up as well.

IIRC, css_get will prevent the cgroup directory from being removed.
Because some allocations are expected to outlive the cgroup, we
specifically don't want that.

Yes, but how do you guarantee that the above VM_BUG_ON doesn't trigger?
Task could have been moved to another group between mem_cgroup_from_task
and mem_cgroup_get, no?

Ok, after reading this again (and again), you seem to be right. It
concerns me, however, that simply getting the css would lead us to a
double get/put pair, since try_charge will have to do it anyway.

That happens only for !*ptr case and you provide a memcg here, don't
you.

        if (*ptr) { /* css should be a valid one */
                memcg = *ptr;
                VM_BUG_ON(css_is_removed(&memcg->css));
                if (mem_cgroup_is_root(memcg))
                        goto done;
                if (consume_stock(memcg, nr_pages))
                        goto done;
                css_get(&memcg->css);


The way I read this, this will still issue a css_get here, unless
consume_stock suceeds (assuming non-root)

So we'd still have to have a wrapping get/put pair outside the charge.

That is correct but it assumes that the css is valid so somebody upwards
made sure css will not go away. This would suggest css_get is not
necessary here but I guess the primary intention here is to make the
code easier so that we do not have to check whether we took css
reference on the return path.

In any case, umem would also suffer from double reference, so I'm fine
taking it here as well, since a solution for that is orthogonal.

I still need mem_cgroup_get() to make sure the data structure stays
around, but we only need to do it once at first charge.