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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 17 Apr 2019 18:55:12 -0700
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Subject: Re: [PATCH 4/5] mm: rework non-root kmem_cache lifecycle management
To:     Roman Gushchin <guro@fb.com>
Cc:     Roman Gushchin <guroan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
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        Rik van Riel <riel@surriel.com>,
        "david@fromorbit.com" <david@fromorbit.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
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On Wed, Apr 17, 2019 at 5:39 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Wed, Apr 17, 2019 at 04:41:01PM -0700, Shakeel Butt wrote:
> > On Wed, Apr 17, 2019 at 2:55 PM Roman Gushchin <guroan@gmail.com> wrote:
> > >
> > > This commit makes several important changes in the lifecycle
> > > of a non-root kmem_cache, which also affect the lifecycle
> > > of a memory cgroup.
> > >
> > > Currently each charged slab page has a page->mem_cgroup pointer
> > > to the memory cgroup and holds a reference to it.
> > > Kmem_caches are held by the cgroup. On offlining empty kmem_caches
> > > are freed, all other are freed on cgroup release.
> >
> > No, they are not freed (i.e. destroyed) on offlining, only
> > deactivated. All memcg kmem_caches are freed/destroyed on memcg's
> > css_free.
>
> You're right, my bad. I was thinking about the corresponding sysfs entry
> when was writing it. We try to free it from the deactivation path too.
>
> >
> > >
> > > So the current scheme can be illustrated as:
> > > page->mem_cgroup->kmem_cache.
> > >
> > > To implement the slab memory reparenting we need to invert the scheme
> > > into: page->kmem_cache->mem_cgroup.
> > >
> > > Let's make every page to hold a reference to the kmem_cache (we
> > > already have a stable pointer), and make kmem_caches to hold a single
> > > reference to the memory cgroup.
> >
> > What about memcg_kmem_get_cache()? That function assumes that by
> > taking reference on memcg, it's kmem_caches will stay. I think you
> > need to get reference on the kmem_cache in memcg_kmem_get_cache()
> > within the rcu lock where you get the memcg through css_tryget_online.
>
> Yeah, a very good question.
>
> I believe it's safe because css_tryget_online() guarantees that
> the cgroup is online and won't go offline before css_free() in
> slab_post_alloc_hook(). I do initialize kmem_cache's refcount to 1
> and drop it on offlining, so it protects the online kmem_cache.
>

Let's suppose a thread doing a remote charging calls
memcg_kmem_get_cache() and gets an empty kmem_cache of the remote
memcg having refcnt equal to 1. That thread got a reference on the
remote memcg but no reference on the kmem_cache. Let's suppose that
thread got stuck in the reclaim and scheduled away. In the meantime
that remote memcg got offlined and decremented the refcnt of all of
its kmem_caches. The empty kmem_cache which the thread stuck in
reclaim have pointer to can get deleted and may be using an already
destroyed kmem_cache after coming back from reclaim.

I think the above situation is possible unless the thread gets the
reference on the kmem_cache in memcg_kmem_get_cache().

Shakeel