[PATCH v4 2/6] cgroup/dmem: Add dmem_cgroup_common_ancestor helper

[Natalie Vock <natalie.vock@gmx.de>]

This helps to find a common subtree of two resources, which is important
when determining whether it's helpful to evict one resource in favor of
another.

Signed-off-by: Natalie Vock <natalie.vock@gmx.de>
---
 include/linux/cgroup_dmem.h |  9 +++++++++
 kernel/cgroup/dmem.c        | 25 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/cgroup_dmem.h b/include/linux/cgroup_dmem.h
index 1a88cd0c9eb00..444b84f4c253a 100644
--- a/include/linux/cgroup_dmem.h
+++ b/include/linux/cgroup_dmem.h
@@ -28,6 +28,8 @@ bool dmem_cgroup_below_min(struct dmem_cgroup_pool_state *root,
 			   struct dmem_cgroup_pool_state *test);
 bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state *root,
 			   struct dmem_cgroup_pool_state *test);
+struct dmem_cgroup_pool_state *dmem_cgroup_common_ancestor(struct dmem_cgroup_pool_state *a,
+							   struct dmem_cgroup_pool_state *b);
 
 void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool);
 #else
@@ -75,6 +77,13 @@ static inline bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state *root,
 	return false;
 }
 
+static inline
+struct dmem_cgroup_pool_state *dmem_cgroup_common_ancestor(struct dmem_cgroup_pool_state *a,
+							   struct dmem_cgroup_pool_state *b)
+{
+	return NULL;
+}
+
 static inline void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool)
 { }
 
diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 28227405f7cfe..26e794400c5c7 100644
--- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -756,6 +756,31 @@ bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state *root,
 }
 EXPORT_SYMBOL_GPL(dmem_cgroup_below_low);
 
+/**
+ * dmem_cgroup_common_ancestor(): Find the first common ancestor of two pools.
+ * @a: First pool to find the common ancestor of.
+ * @b: First pool to find the common ancestor of.
+ *
+ * Return: The first pool that is a parent of both @a and @b, or NULL if either @a or @b are NULL.
+ */
+struct dmem_cgroup_pool_state *dmem_cgroup_common_ancestor(struct dmem_cgroup_pool_state *a,
+							   struct dmem_cgroup_pool_state *b)
+{
+	struct dmem_cgroup_pool_state *parent;
+
+	while (a) {
+		parent = b;
+		while (parent) {
+			if (a == parent)
+				return a;
+			parent = pool_parent(parent);
+		}
+		a = pool_parent(a);
+	}
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(dmem_cgroup_common_ancestor);
+
 static int dmem_cgroup_region_capacity_show(struct seq_file *sf, void *v)
 {
 	struct dmem_cgroup_region *region;

-- 
2.53.0