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Date:   Tue, 8 Jan 2019 21:44:32 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kernel-team@...com,
        Linux MM <linux-mm@...ck.org>, stable@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] mm,slab,memcg: call memcg kmem put cache with same
 condition as get

On Tue, Jan 8, 2019 at 9:36 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Tue, Jan 8, 2019 at 8:01 PM Rik van Riel <riel@...riel.com> wrote:
> >
> > There is an imbalance between when slab_pre_alloc_hook calls
> > memcg_kmem_get_cache and when slab_post_alloc_hook calls
> > memcg_kmem_put_cache.
> >
>
> Can you explain how there is an imbalance? If the returned kmem cache
> from memcg_kmem_get_cache() is the memcg kmem cache then the refcnt of
> memcg is elevated and the memcg_kmem_put_cache() will correctly
> decrement the refcnt of the memcg.
>
> > This can cause a memcg kmem cache to be destroyed right as
> > an object from that cache is being allocated,

Also please note that the memcg kmem caches are destroyed (if empty)
on memcg offline. The css_tryget_online() within
memcg_kmem_get_cache() will fail.

See kernel/cgroup/cgroup.c
* 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
 *    and thus css_tryget_online() is guaranteed to fail, the css can be
 *    offlined by invoking offline_css().  After offlining, the base ref is
 *    put.  Implemented in css_killed_work_fn().

> > which is probably
> > not good. It could lead to things like a memcg allocating new
> > kmalloc slabs instead of using freed space in old ones, maybe
> > memory leaks, and maybe oopses as a memcg kmalloc slab is getting
> > destroyed on one CPU while another CPU is trying to do an allocation
> > from that same memcg.
> >
> > The obvious fix would be to use the same condition for calling
> > memcg_kmem_put_cache that we also use to decide whether to call
> > memcg_kmem_get_cache.
> >
> > I am not sure how long this bug has been around, since the last
> > changeset to touch that code - 452647784b2f ("mm: memcontrol: cleanup
> >  kmem charge functions") - merely moved the bug from one location to
> > another. I am still tagging that changeset, because the fix should
> > automatically apply that far back.
> >
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > Fixes: 452647784b2f ("mm: memcontrol: cleanup kmem charge functions")
> > Cc: kernel-team@...com
> > Cc: linux-mm@...ck.org
> > Cc: stable@...r.kernel.org
> > Cc: Alexey Dobriyan <adobriyan@...il.com>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Pekka Enberg <penberg@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: David Rientjes <rientjes@...gle.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Tejun Heo <tj@...nel.org>
> > ---
> >  mm/slab.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 4190c24ef0e9..ab3d95bef8a0 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -444,7 +444,8 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
> >                 p[i] = kasan_slab_alloc(s, object, flags);
> >         }
> >
> > -       if (memcg_kmem_enabled())
> > +       if (memcg_kmem_enabled() &&
> > +           ((flags & __GFP_ACCOUNT) || (s->flags & SLAB_ACCOUNT)))
>
> I don't think these extra checks are needed. They are safe but not needed.
>
> >                 memcg_kmem_put_cache(s);
> >  }
> >
> > --
> > 2.17.1
> >
>
> thanks,
> Shakeel

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