Re: [PATCH] mm: vmscan: check if mem cgroup is disabled or not before calling memcg slab shrinker

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 17.07.2019 20:45, Yang Shi wrote:
> Shakeel Butt reported premature oom on kernel with
> "cgroup_disable=memory" since mem_cgroup_is_root() returns false even
> though memcg is actually NULL.  The drop_caches is also broken.
> 
> It is because commit aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab()
> calls in shrink_node()") removed the !memcg check before
> !mem_cgroup_is_root().  And, surprisingly root memcg is allocated even
> though memory cgroup is disabled by kernel boot parameter.
> 
> Add mem_cgroup_disabled() check to make reclaimer work as expected.
> 
> Fixes: aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab() calls in shrink_node()")
> Reported-by: Shakeel Butt <shakeelb@google.com>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Qian Cai <cai@lca.pw>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: stable@vger.kernel.org  4.19+
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

Surprise really.

We have mem_cgroup as not early inited, so all of these boundary
cases and checks has to be supported. But it looks like it's not
possible to avoid that in any way.

> ---
>  mm/vmscan.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f8e3dcd..c10dc02 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -684,7 +684,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  	unsigned long ret, freed = 0;
>  	struct shrinker *shrinker;
>  
> -	if (!mem_cgroup_is_root(memcg))
> +	/*
> +	 * The root memcg might be allocated even though memcg is disabled
> +	 * via "cgroup_disable=memory" boot parameter.  This could make
> +	 * mem_cgroup_is_root() return false, then just run memcg slab
> +	 * shrink, but skip global shrink.  This may result in premature
> +	 * oom.
> +	 */
> +	if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>  		return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>  
>  	if (!down_read_trylock(&shrinker_rwsem))
>