mm, memcg: prevent memory.min load/store tearing

This can be set concurrently with reads, which may cause the wrong value
to be propagated.
Signed-off-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/e809b4e6b0c1626dac6945970de06409a180ee65.1584034301.git.chris@chrisdown.nameSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -6389,7 +6389,7 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 		return MEMCG_PROT_NONE;
 
 	if (parent == root) {
-		memcg->memory.emin = memcg->memory.min;
+		memcg->memory.emin = READ_ONCE(memcg->memory.min);
 		memcg->memory.elow = memcg->memory.low;
 		goto out;
 	}
@@ -6397,7 +6397,8 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 	parent_usage = page_counter_read(&parent->memory);
 
 	memcg->memory.emin = effective_protection(usage, parent_usage,
-			memcg->memory.min, READ_ONCE(parent->memory.emin),
+			READ_ONCE(memcg->memory.min),
+			READ_ONCE(parent->memory.emin),
 			atomic_long_read(&parent->memory.children_min_usage));
 
 	memcg->memory.elow = effective_protection(usage, parent_usage,

mm/page_counter.c

@@ -17,14 +17,15 @@ static void propagate_protected_usage(struct page_counter *c,
 				      unsigned long usage)
 {
 	unsigned long protected, old_protected;
-	unsigned long low;
+	unsigned long low, min;
 	long delta;
 
 	if (!c->parent)
 		return;
 
-	if (c->min || atomic_long_read(&c->min_usage)) {
-		protected = min(usage, c->min);
+	min = READ_ONCE(c->min);
+	if (min || atomic_long_read(&c->min_usage)) {
+		protected = min(usage, min);
 		old_protected = atomic_long_xchg(&c->min_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
@@ -207,7 +208,7 @@ void page_counter_set_min(struct page_counter *counter, unsigned long nr_pages)
 {
 	struct page_counter *c;
 
-	counter->min = nr_pages;
+	WRITE_ONCE(counter->min, nr_pages);
 
 	for (c = counter; c; c = c->parent)
 		propagate_protected_usage(c, atomic_long_read(&c->usage));