Merge branch 'akpm' (patches from Andrew)

Merge more updates from Andrew Morton:

 - most of the rest of MM

 - KASAN updates

 - lib/ updates

 - checkpatch updates

 - some binfmt_elf changes

 - various misc bits

* emailed patches from Andrew Morton <akpm@linux-foundation.org>: (115 commits)
  kernel/exit.c: avoid undefined behaviour when calling wait4()
  kernel/signal.c: avoid undefined behaviour in kill_something_info
  binfmt_elf: safely increment argv pointers
  s390: reduce ELF_ET_DYN_BASE
  powerpc: move ELF_ET_DYN_BASE to 4GB / 4MB
  arm64: move ELF_ET_DYN_BASE to 4GB / 4MB
  arm: move ELF_ET_DYN_BASE to 4MB
  binfmt_elf: use ELF_ET_DYN_BASE only for PIE
  fs, epoll: short circuit fetching events if thread has been killed
  checkpatch: improve multi-line alignment test
  checkpatch: improve macro reuse test
  checkpatch: change format of --color argument to --color[=WHEN]
  checkpatch: silence perl 5.26.0 unescaped left brace warnings
  checkpatch: improve tests for multiple line function definitions
  checkpatch: remove false warning for commit reference
  checkpatch: fix stepping through statements with $stat and ctx_statement_block
  checkpatch: [HLP]LIST_HEAD is also declaration
  checkpatch: warn when a MAINTAINERS entry isn't [A-Z]:\t
  checkpatch: improve the unnecessary OOM message test
  lib/bsearch.c: micro-optimize pivot position calculation
  ...

1 files changed, 33 insertions, 19 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 425aa0caa712..3df3c04d73ab 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -631,7 +631,7 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
 	val = __this_cpu_read(memcg->stat->nr_page_events);
 	next = __this_cpu_read(memcg->stat->targets[target]);
 	/* from time_after() in jiffies.h */
-	if ((long)next - (long)val < 0) {
+	if ((long)(next - val) < 0) {
 		switch (target) {
 		case MEM_CGROUP_TARGET_THRESH:
 			next = val + THRESHOLDS_EVENTS_TARGET;
@@ -5317,38 +5317,52 @@ struct cgroup_subsys memory_cgrp_subsys = {
 
 /**
  * mem_cgroup_low - check if memory consumption is below the normal range
- * @root: the highest ancestor to consider
+ * @root: the top ancestor of the sub-tree being checked
  * @memcg: the memory cgroup to check
  *
  * Returns %true if memory consumption of @memcg, and that of all
- * configurable ancestors up to @root, is below the normal range.
+ * ancestors up to (but not including) @root, is below the normal range.
+ *
+ * @root is exclusive; it is never low when looked at directly and isn't
+ * checked when traversing the hierarchy.
+ *
+ * Excluding @root enables using memory.low to prioritize memory usage
+ * between cgroups within a subtree of the hierarchy that is limited by
+ * memory.high or memory.max.
+ *
+ * For example, given cgroup A with children B and C:
+ *
+ *    A
+ *   / \
+ *  B   C
+ *
+ * and
+ *
+ *  1. A/memory.current > A/memory.high
+ *  2. A/B/memory.current < A/B/memory.low
+ *  3. A/C/memory.current >= A/C/memory.low
+ *
+ * As 'A' is high, i.e. triggers reclaim from 'A', and 'B' is low, we
+ * should reclaim from 'C' until 'A' is no longer high or until we can
+ * no longer reclaim from 'C'.  If 'A', i.e. @root, isn't excluded by
+ * mem_cgroup_low when reclaming from 'A', then 'B' won't be considered
+ * low and we will reclaim indiscriminately from both 'B' and 'C'.
  */
 bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_disabled())
 		return false;
 
-	/*
-	 * The toplevel group doesn't have a configurable range, so
-	 * it's never low when looked at directly, and it is not
-	 * considered an ancestor when assessing the hierarchy.
-	 */
-
-	if (memcg == root_mem_cgroup)
-		return false;
-
-	if (page_counter_read(&memcg->memory) >= memcg->low)
+	if (!root)
+		root = root_mem_cgroup;
+	if (memcg == root)
 		return false;
 
-	while (memcg != root) {
-		memcg = parent_mem_cgroup(memcg);
-
-		if (memcg == root_mem_cgroup)
-			break;
-
+	for (; memcg != root; memcg = parent_mem_cgroup(memcg)) {
 		if (page_counter_read(&memcg->memory) >= memcg->low)
 			return false;
 	}
+
 	return true;
 }