Re: [PATCH] mm: workingset: fix NULL ptr in count_shadow_nodes

[Johannes Weiner]

On Thu, Dec 01, 2016 at 02:21:56PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 4.6+
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks Michal