The problem happens because parent_mem_cgroup() returns a NULL pointer,
which is dereferenced later without a check.
As cgroup v1 has no memory guarantee support, let's make
mem_cgroup_protected() immediately return MEMCG_PROT_NONE, if the given
cgroup has no parent (non-hierarchical mode is used).
Link: http://lkml.kernel.org/r/20180611175418.7007-2-guro@fb.com
Fixes: bf8d5d52ffe8 ("memcg: introduce memory.min") Signed-off-by: Roman Gushchin <guro@fb.com> Reported-by: Shakeel Butt <shakeelb@google.com> Tested-by: Shakeel Butt <shakeelb@google.com> Tested-by: John Stultz <john.stultz@linaro.org> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Michal Hocko <mhocko@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>