From: Yang Shi yang.shi@linux.alibaba.com Subject: mm: vmscan: check if mem cgroup is disabled or not before calling memcg slab shrinker

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 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.

Link: http://lkml.kernel.org/r/1563385526-20805-1-git-send-email-yang.shi@linux.al... Fixes: aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab() calls in shrink_node()") Signed-off-by: Yang Shi yang.shi@linux.alibaba.com Reported-by: Shakeel Butt shakeelb@google.com Reviewed-by: Shakeel Butt shakeelb@google.com Reviewed-by: Kirill Tkhai ktkhai@virtuozzo.com Acked-by: Michal Hocko mhocko@suse.com Cc: Jan Hadrava had@kam.mff.cuni.cz Cc: Vladimir Davydov vdavydov.dev@gmail.com Cc: Johannes Weiner hannes@cmpxchg.org 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: Andrew Morton akpm@linux-foundation.org ---

mm/vmscan.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/mm/vmscan.c~mm-vmscan-check-if-mem-cgroup-is-disabled-or-not-before-calling-memcg-slab-shrinker +++ a/mm/vmscan.c @@ -699,7 +699,14 @@ static unsigned long shrink_slab(gfp_t g 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)) _