[PATCH] mm, memcg: show memcg min setting in oom messages

[Yafang Shao <laoar.shao@xxxxxxxxx>]

A task running in a memcg may OOM because of the memory.min settings of his
slibing and parent. If this happens, the current oom messages can't show
why file page cache can't be reclaimed. So it is better to show the memcg
min settings.
Let's take an example.
      bar    bar/memory.max = 1200M memory.min=800M
     /  \
   barA barB barA/memory.min = 800M memory.current=1G (file page cache)
             barB/memory.min = 0 (process in this memcg is allocating page)

The process will do memcg reclaim if the bar/memory.max is reached. Once
the barA/memory.min is reached it will stop reclaiming file page caches in
barA, and if there is no reclaimable pages in bar and bar/barB it will
enter memcg OOM then.
After this pacch, bellow messages will be show then (only includeing the
relevant messages here). The lines begin with '#' are newly added info (the
'#' symbol is not in the original messages).
	memory: usage 1228800kB, limit 1228800kB, failcnt 18337
	...
	# Memory cgroup min setting:
	# /bar: min 819200KB emin 0KB
	# /bar/barA: min 819200KB emin 819200KB
	# /bar/barB: min 0KB emin 0KB
	...
	Memory cgroup stats for /bar:
	anon 418328576
	file 835756032
	...
	unevictable 0
	...
	oom-kill:constraint=CONSTRAINT_MEMCG..oom_memcg=/bar,task_memcg=/bar/barB

With the new added information, we can find the memory.min in bar/barA is
reached and the processes in bar/barB can't reclaim file page cache from
bar/barA any more. While without this new added information we don't know
why the file page cache in bar can't be reclaimed.

Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
---
 mm/memcontrol.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1c4c08b..97fdd93 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1519,16 +1519,27 @@ void mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct *
  */
 void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
 {
+	struct mem_cgroup *iter;
 	char *buf;
 
 	pr_info("memory: usage %llukB, limit %llukB, failcnt %lu\n",
 		K((u64)page_counter_read(&memcg->memory)),
 		K((u64)memcg->memory.max), memcg->memory.failcnt);
-	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
+	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);
-	else {
+
+		pr_info("Memory cgroup min setting:\n");
+		for_each_mem_cgroup_tree(iter, memcg) {
+			pr_cont_cgroup_path(iter->css.cgroup);
+			pr_cont(":");
+
+			pr_cont(" min %lluKB emin %lluKB\n",
+				K((u64)iter->memory.min),
+				K((u64)iter->memory.emin));
+		}
+	} else {
 		pr_info("memory+swap: usage %llukB, limit %llukB, failcnt %lu\n",
 			K((u64)page_counter_read(&memcg->memsw)),
 			K((u64)memcg->memsw.max), memcg->memsw.failcnt);
-- 
1.8.3.1