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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 18 Apr 2019 07:05:24 -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>,
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        Christoph Lameter <cl@linux.com>,
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On Wed, Apr 17, 2019 at 8:07 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Wed, Apr 17, 2019 at 06:55:12PM -0700, Shakeel Butt wrote:
> > 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().
>
> Yes, you're right and I'm writing a nonsense: css_tryget_online()
> can't prevent the cgroup from being offlined.
>

The reason I knew about that race is because I tried something similar
but for different use-case:

https://lkml.org/lkml/2018/3/26/472

> So, the problem with getting a reference in memcg_kmem_get_cache()
> is that it's an atomic operation on the hot path, something I'd like
> to avoid.
>
> I can make the refcounter percpu, but it'll add some complexity and size
> to the kmem_cache object. Still an option, of course.
>

I kind of prefer this option.

> I wonder if we can use rcu_read_lock() instead, and bump the refcounter
> only if we're going into reclaim.
>
> What do you think?

Should it be just reclaim or anything that can reschedule the current thread?

I can tell how we resolve the similar issue for our
eager-kmem_cache-deletion use-case. Our solution (hack) works only for
CONFIG_SLAB (we only use SLAB) and non-preemptible kernel. The
underlying motivation was to reduce the overhead of slab reaper of
traversing thousands of empty offlined kmem caches. CONFIG_SLAB
disables interrupts before accessing the per-cpu caches and reenables
the interrupts if it has to fallback to the page allocation. We use
this window to call memcg_kmem_get_cache() and only increment the
refcnt of kmem_cache if going to the fallback. Thus no need to do
atomic operation on the hot path.

Anyways, I think having percpu refcounter for each memcg kmem_cache is
not that costy for CONFIG_MEMCG_KMEM users and to me that seems like
the most simple solution.

Shakeel