linux-kernel - Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow

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Date:   Thu, 1 Dec 2016 16:07:15 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...uw.edu.pl>, Balbir Singh <bsingharora@...il.com>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow_nodes

On Thu, Dec 01, 2016 at 02:21:56PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.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-Górecki <marmarek@...uw.edu.pl>
> Cc: stable@...r.kernel.org # 4.6+
> Signed-off-by: Michal Hocko <mhocko@...e.com>

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

Thanks Michal