Date: Thu, 1 Dec 2016 16:07:15 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Marek =?iso-8859-1?Q?Marczykowski-G=F3recki?= 
        <marmarek@mimuw.edu.pl>,
        Balbir Singh <bsingharora@gmail.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow_nodes
Message-ID: <20161201210715.GA21302@cmpxchg.org>
References: <20161201132156.21450-1-mhocko@kernel.org>
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On Thu, Dec 01, 2016 at 02:21:56PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg aware")
> has made the workingset shadow nodes shrinker memcg aware. The
> implementation is not correct though because memcg_kmem_enabled() might
> become true while we are doing a global reclaim when the sc->memcg might
> be NULL which is exactly what Marek has seen:

[...]

> This patch fixes the issue by checking sc->memcg rather than memcg_kmem_enabled()
> which is sufficient because shrink_slab makes sure that only memcg aware shrinkers
> will get non-NULL memcgs and only if memcg_kmem_enabled is true.
> 
> Fixes: 0a6b76dd23fa ("mm: workingset: make shadow node shrinker memcg aware")
> Reported-and-tested-by: Marek Marczykowski-G?recki <marmarek@mimuw.edu.pl>
> Cc: stable@vger.kernel.org # 4.6+
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks Michal