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

[Michal Hocko]

On Mon 01-10-12 15:51:20, Glauber Costa wrote:
> On 10/01/2012 03:51 PM, Michal Hocko wrote:
> > On Mon 01-10-12 14:09:09, Glauber Costa wrote:
> >> On 10/01/2012 01:48 PM, Michal Hocko wrote:
> >>> On Fri 28-09-12 15:34:19, Glauber Costa wrote:
> >>>> On 09/27/2012 05:44 PM, Michal Hocko wrote:
> >>>>>>> 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.
-- 
Michal Hocko
SUSE Labs