[086/192] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm

Commit Message

Andrew Morton June 29, 2021, 2:37 a.m. UTC

From: Muchun Song <[e-mail]>
Subject: mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm

When mm is NULL, we do not need to hold rcu lock and call css_tryget for
the root memcg.  And we also do not need to check !mm in every loop of
while.  So bail out early when !mm.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muchun Song <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Reviewed-by: Shakeel Butt <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: Xiongchun Duan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/memcontrol.c |   25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

Patch

--- a/mm/memcontrol.c~mm-memcontrol-bail-out-early-when-mm-in-get_mem_cgroup_from_mm
+++ a/mm/memcontrol.c
@@ -919,20 +919,23 @@  struct mem_cgroup *get_mem_cgroup_from_m
 	if (mem_cgroup_disabled())
 		return NULL;
 
+	/*
+	 * Page cache insertions can happen without an
+	 * actual mm context, e.g. during disk probing
+	 * on boot, loopback IO, acct() writes etc.
+	 *
+	 * No need to css_get on root memcg as the reference
+	 * counting is disabled on the root level in the
+	 * cgroup core. See CSS_NO_REF.
+	 */
+	if (unlikely(!mm))
+		return root_mem_cgroup;
+
 	rcu_read_lock();
 	do {
-		/*
-		 * Page cache insertions can happen without an
-		 * actual mm context, e.g. during disk probing
-		 * on boot, loopback IO, acct() writes etc.
-		 */
-		if (unlikely(!mm))
+		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
+		if (unlikely(!memcg))
 			memcg = root_mem_cgroup;
-		else {
-			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
-			if (unlikely(!memcg))
-				memcg = root_mem_cgroup;
-		}
 	} while (!css_tryget(&memcg->css));
 	rcu_read_unlock();
 	return memcg;