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

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Message-Id: <1563385526-20805-1-git-send-email-yang.shi@linux.alibaba.com>
Date:   Thu, 18 Jul 2019 01:45:26 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     shakeelb@...gle.com, vdavydov.dev@...il.com, hannes@...xchg.org,
        mhocko@...e.com, ktkhai@...tuozzo.com, guro@...com,
        hughd@...gle.com, cai@....pw, kirill.shutemov@...ux.intel.com,
        akpm@...ux-foundation.org
Cc:     yang.shi@...ux.alibaba.com, stable@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH] 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.

Fixes: aeed1d325d42 ("mm/vmscan.c: generalize shrink_slab() calls in shrink_node()")
Reported-by: Shakeel Butt <shakeelb@...gle.com>
Cc: Vladimir Davydov <vdavydov.dev@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Kirill Tkhai <ktkhai@...tuozzo.com>
Cc: Roman Gushchin <guro@...com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Qian Cai <cai@....pw>
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: stable@...r.kernel.org  4.19+
Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
---
 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))
-- 
1.8.3.1