mm, memcg: Prevent memory.oom.group load/store tearing

Patch series "mm, memcg: cgroup v1 and v2 tunable load/store tearing
fixes", v2.

This patch series helps to prevent load/store tearing in
several cgroup knobs.

As kindly pointed out by Michal Hocko and Roman Gushchin
, the changelog has been rephrased.

Besides, more knobs were checked, according to kind suggestions
from Shakeel Butt and Muchun Song.


This patch (of 4):

The knob for cgroup v2 memory controller: memory.oom.group
is not protected by any locking so it can be modified while it is used.
This is not an actual problem because races are unlikely (the knob is
usually configured long before any workloads hits actual memcg oom)
but it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
doing anything funky.

The access of memcg->oom_group is lockless, so it can be
concurrently set at the same time as we are trying to read it.

Link: https://lkml.kernel.org/r/20230306154138.3775-1-findns94@gmail.com
Link: https://lkml.kernel.org/r/20230306154138.3775-2-findns94@gmail.comSigned-off-by: Yue Zhao <findns94@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Tang Yizhou <tangyeechou@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -2067,7 +2067,7 @@ struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
 	 * highest-level memory cgroup with oom.group set.
 	 */
 	for (; memcg; memcg = parent_mem_cgroup(memcg)) {
-		if (memcg->oom_group)
+		if (READ_ONCE(memcg->oom_group))
 			oom_group = memcg;
 
 		if (memcg == oom_domain)
@@ -6623,7 +6623,7 @@ static int memory_oom_group_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
 
-	seq_printf(m, "%d\n", memcg->oom_group);
+	seq_printf(m, "%d\n", READ_ONCE(memcg->oom_group));
 
 	return 0;
 }
@@ -6645,7 +6645,7 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
 	if (oom_group != 0 && oom_group != 1)
 		return -EINVAL;
 
-	memcg->oom_group = oom_group;
+	WRITE_ONCE(memcg->oom_group, oom_group);
 
 	return nbytes;
 }