mm, memcg: do not high throttle allocators based on wraparound

If a cgroup violates its memory.high constraints, we may end up unduly
penalising it.  For example, for the following hierarchy:

  A:   max high, 20 usage
  A/B: 9 high, 10 usage
  A/C: max high, 10 usage

We would end up doing the following calculation below when calculating
high delay for A/B:

  A/B: 10 - 9 = 1...
  A:   20 - PAGE_COUNTER_MAX = 21, so set max_overage to 21.

This gets worse with higher disparities in usage in the parent.

I have no idea how this disappeared from the final version of the patch,
but it is certainly Not Good(tm).  This wasn't obvious in testing because,
for a simple cgroup hierarchy with only one child, the result is usually
roughly the same.  It's only in more complex hierarchies that things go
really awry (although still, the effects are limited to a maximum of 2
seconds in schedule_timeout_killable at a maximum).

[chris@chrisdown.name: changelog]
Fixes: e26733e0 ("mm, memcg: throttle allocators based on ancestral memory.high")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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: <stable@vger.kernel.org>	[5.4.x]
Link: http://lkml.kernel.org/r/20200331152424.GA1019937@chrisdown.nameSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2336,6 +2336,9 @@ static unsigned long calculate_high_delay(struct mem_cgroup *memcg,
 		usage = page_counter_read(&memcg->memory);
 		high = READ_ONCE(memcg->high);
 
+		if (usage <= high)
+			continue;
+
 		/*
 		 * Prevent division by 0 in overage calculation by acting as if
 		 * it was a threshold of 1 page