[PATCH v5] mm/vmscan: add a fatal signals check in drop_slab_node

[zangchunxin@bytedance.com]

From: Chunxin Zang <zangchunxin@bytedance.com>

On our server, there are about 10k memcg in one machine. They use memory
very frequently. We have observed that drop_caches can take a
considerable amount of time, and can't stop it.

There are two reasons:
1. There is somebody constantly generating more objects to reclaim
  on drop_caches, result the 'freed' always bigger than 10.
2. The process has no chance to process signals.

We can get the following info through 'ps':

 root:~# ps -aux | grep drop
 root  357956 ... R    Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
 root 1771385 ... R    Aug16 21146421:17 echo 3 > /proc/sys/vm/drop_caches

Add a bail out on the fatal signals in the main loop so that the
operation can be terminated by userspace.

Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
	changelogs in v5:
	1) v4 patch used wrong branch, very apologies about that.

	changelogs in v4:
    changelogs in v3:
    1) Fix some descriptive problems pointed out by Michal Hocko.
        v2 named: mm/vmscan: fix infinite loop in drop_slab_node

    changelogs in v2:
    1) via check fatal signal break loop.

 mm/vmscan.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b6d84326bdf2..c3ed8b45d264 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -704,6 +704,9 @@ void drop_slab_node(int nid)
 	do {
 		struct mem_cgroup *memcg = NULL;
 
+		if (fatal_signal_pending(current))
+			return;
+
 		freed = 0;
 		memcg = mem_cgroup_iter(NULL, NULL, NULL);
 		do {
-- 
2.11.0