[RFC 2/4] mm, memcg: allow OOM if no memcg is eligible during direct reclaim

[Michal Hocko <mhocko@suse.cz>]

If there is no memcg eligible for reclaim then the global direct
reclaim would end up in the endless loop because zones in the zonelists
are not considered unreclaimable (as per all_unreclaimable) and so the
OOM killer would never fire and direct reclaim would be triggered
without no chance to reclaim anything.

Memcg reclaim doesn't suffer from this because the OOM killer is
triggered after few unsuccessful attempts of the reclaim.

Fix this by checking the number of scanned pages which is obviously 0 if
nobody is eligible and also check that the whole tree hierarchy is not
eligible and tell OOM it can go ahead.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/memcontrol.h |  6 ++++++
 mm/memcontrol.c            | 10 ++++++++++
 mm/vmscan.c                |  7 +++++++
 3 files changed, 23 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6841e591718d..4ae6a9838a26 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -94,6 +94,7 @@ bool task_in_mem_cgroup(struct task_struct *task,
 
 extern bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
 		struct mem_cgroup *root);
+extern bool mem_cgroup_reclaim_no_eligible(struct mem_cgroup *root);
 extern struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page);
 extern struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
 extern struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm);
@@ -297,6 +298,11 @@ static inline bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
 	return true;
 }
 
+static bool mem_cgroup_reclaim_no_eligible(struct mem_cgroup *root)
+{
+	return false;
+}
+
 static inline struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
 {
 	return NULL;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a1cfee4491bf..102e2da9ec8d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2874,6 +2874,16 @@ bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
 	return true;
 }
 
+bool mem_cgroup_reclaim_no_eligible(struct mem_cgroup *root)
+{
+	struct mem_cgroup *iter;
+
+	for_each_mem_cgroup_tree(iter, root)
+		if (mem_cgroup_reclaim_eligible(iter, root))
+			return false;
+	return true;
+}
+
 struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
 {
 	struct mem_cgroup *memcg = NULL;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c9ce5f97872..234d1690563a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2481,6 +2481,13 @@ out:
 	if (aborted_reclaim)
 		return 1;
 
+	/*
+	 * If the target memcg is not eligible for reclaim then we have no opetion
+	 * but OOM
+	 */
+	if (!sc->nr_scanned && mem_cgroup_reclaim_no_eligible(sc->target_mem_cgroup))
+		return 0;
+
 	/* top priority shrink_zones still had more to do? don't OOM, then */
 	if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
 		return 1;
-- 
1.8.4.4

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