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 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.

Link: http://lkml.kernel.org/r/1563385526-20805-1-git-send-email-yang.shi@linux.alibaba.com
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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 44df66a98f2a..dbdc46a84f63 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -699,7 +699,14 @@ 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))