mm/memcg: simplify mem_cgroup_get_max()

mem_cgroup_get_max() used to get memory+swap max from both the v1 memsw
and v2 memory+swap page counters & return the maximum of these 2 values.
This is redundant and it is more efficient to just get either the v1 or
the v2 values depending on which one is currently in use.

[[email protected]: v4]
  Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Chris Down <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Yafang Shao <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -1633,17 +1633,19 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
  */
 unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 {
-	unsigned long max;
-
-	max = READ_ONCE(memcg->memory.max);
-	if (mem_cgroup_swappiness(memcg)) {
-		unsigned long memsw_max;
-		unsigned long swap_max;
-
-		memsw_max = memcg->memsw.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);
+	unsigned long max = READ_ONCE(memcg->memory.max);
+
+	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
+		if (mem_cgroup_swappiness(memcg))
+			max += min(READ_ONCE(memcg->swap.max),
+				   (unsigned long)total_swap_pages);
+	} else { /* v1 */
+		if (mem_cgroup_swappiness(memcg)) {
+			/* Calculate swap excess capacity from memsw limit */
+			unsigned long swap = READ_ONCE(memcg->memsw.max) - max;
+
+			max += min(swap, (unsigned long)total_swap_pages);
+		}
 	}
 	return max;
 }