linux-kernel - [PATCH] mm/memcontrol: Don't increase effective low/min if no protection needed

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Date:   Tue, 11 Oct 2022 10:30:15 -0400
From:   Waiman Long <longman@...hat.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Tejun Heo <tj@...nel.org>,
        Chris Down <chris@...isdown.name>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] mm/memcontrol: Don't increase effective low/min if no protection needed

Since commit bc50bcc6e00b ("mm: memcontrol: clean up and document
effective low/min calculations"), the effective low/min protections can
be non-zero even if the corresponding memory.low/min values are 0. That
can surprise users to see MEMCG_LOW events even when the memory.low
value is not set. One example is the LTP's memcontrol04 test which fails
because it detects some MEMCG_LOW events for a cgroup with a memory.min
value of 0.

Fix this by updating effective_protection() to not returning a non-zero
low/min protection values if the corresponding memory.low/min values
or those of its parent are 0.

Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 mm/memcontrol.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c9ced5..893d4d5e518a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6660,6 +6660,9 @@ static unsigned long effective_protection(unsigned long usage,
 	unsigned long protected;
 	unsigned long ep;

+	if (!setting || !parent_effective)
+		return 0UL;	/* No protection is needed */
+
 	protected = min(usage, setting);
 	/*
 	 * If all cgroups at this level combined claim and use more
-- 
2.31.1