mm, memcg: prevent memory.swap.max load tearing

The write side of this is xchg()/smp_mb(), so that's all good.  Just a few
sites missing a READ_ONCE.

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/bbec2c3d822217334855c8877a9d28b2a6d395fb.1584034301.git.chris@chrisdown.name
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1525,7 +1525,7 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		pr_info("swap: usage %llukB, limit %llukB, failcnt %lu\n",
 			K((u64)page_counter_read(&memcg->swap)),
-			K((u64)memcg->swap.max), memcg->swap.failcnt);
+			K((u64)READ_ONCE(memcg->swap.max)), memcg->swap.failcnt);
 	else {
 		pr_info("memory+swap: usage %llukB, limit %llukB, failcnt %lu\n",
 			K((u64)page_counter_read(&memcg->memsw)),
@@ -1558,7 +1558,7 @@ unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 		unsigned long swap_max;
 
 		memsw_max = memcg->memsw.max;
-		swap_max = memcg->swap.max;
+		swap_max = READ_ONCE(memcg->swap.max);
 		swap_max = min(swap_max, (unsigned long)total_swap_pages);
 		max = min(max + swap_max, memsw_max);
 	}
@@ -7117,7 +7117,8 @@ bool mem_cgroup_swap_full(struct page *page)
 		return false;
 
 	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg))
-		if (page_counter_read(&memcg->swap) * 2 >= memcg->swap.max)
+		if (page_counter_read(&memcg->swap) * 2 >=
+		    READ_ONCE(memcg->swap.max))
 			return true;
 
 	return false;