[Devel] [PATCH rh7] mm: vmscan: check if mem cgroup is disabled or not before calling memcg slab shrinker

[Kirill Tkhai]

From: Yang Shi <yang.shi at linux.alibaba.com>

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 at google.com>
Cc: Vladimir Davydov <vdavydov.dev at gmail.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Kirill Tkhai <ktkhai at virtuozzo.com>
Cc: Roman Gushchin <guro at fb.com>
Cc: Hugh Dickins <hughd at google.com>
Cc: Qian Cai <cai at lca.pw>
Cc: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Cc: stable at vger.kernel.org  4.19+
Reviewed-by: Shakeel Butt <shakeelb at google.com>
Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 mm/vmscan.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 74d3e8c61cf7..539b171c7073 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -544,8 +544,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 
 	if (unlikely(test_tsk_thread_flag(current, TIF_MEMDIE)))
 		return 0;
-
-	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)) {