Re: [PATCH v2 07/13] memcg: Slab accounting.

[Glauber Costa]

On 03/15/2012 02:04 AM, Suleiman Souhlal wrote:
> On Wed, Mar 14, 2012 at 3:47 AM, Glauber Costa<glommer@xxxxxxxxxxxxx>  wrote:
> > On 03/14/2012 02:50 AM, Suleiman Souhlal wrote:
> > > On Sun, Mar 11, 2012 at 3:25 AM, Glauber Costa<glommer@xxxxxxxxxxxxx>
> > >   wrote:
> > > > On 03/10/2012 12:39 AM, Suleiman Souhlal wrote:
> > > > > +static inline void
> > > > > +mem_cgroup_kmem_cache_prepare_sleep(struct kmem_cache *cachep)
> > > > > +{
> > > > > +       /*
> > > > > +        * Make sure the cache doesn't get freed while we have
> > > > > interrupts
> > > > > +        * enabled.
> > > > > +        */
> > > > > +       kmem_cache_get_ref(cachep);
> > > > > +       rcu_read_unlock();
> > > > > +}
> > > >
> > > >
> > > >
> > > > Is this really needed ? After this function call in slab.c, the slab code
> > > > itself accesses cachep a thousand times. If it could be freed, it would
> > > > already explode today for other reasons?
> > > > Am I missing something here?
> > >
> > >
> > > We need this because once we drop the rcu_read_lock and go to sleep,
> > > the memcg could get deleted, which could lead to the cachep from
> > > getting deleted as well.
> > >
> > > So, we need to grab a reference to the cache, to make sure that the
> > > cache doesn't disappear from under us.
> >
> >
> > Don't we grab a memcg reference when we fire the cache creation?
> > (I did that for slub, can't really recall from the top of my head if
> > you are doing it as well)
> >
> > That would prevent the memcg to go away, while relieving us from the
> > need to take a temporary reference for every page while sleeping.
>
> The problem isn't the memcg going away, but the cache going away.
I see the problem.

I still think there are ways to avoid getting a reference at every page,
but it might not be worth the complication...

> Keep in mind that this function is only called in workqueue context.
> (In the earlier revision of the patchset this function was called in
> the process context, but kmem_cache_create() would ignore memory
> limits, because of __GFP_NOACCOUNT.)

ok, fair.

> When mem_cgroup_get_kmem_cache() returns a memcg cache, that cache has
> already been created.
>
> The memcg pointer is not stable between alloc and free: It can become
> NULL when the cgroup gets deleted, at which point the accounting has
> been "moved to root" (uncharged from the cgroup it was charged in).
> When that has happened, we don't want to uncharge it again.
> I think the current code already handles this situation.

Okay, convinced.
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